Periodical of S.V.A.T. Astatine Volume 5 Number 2 November Black holes 30. UAV: Helicopter. Droplets on fibres. Injection Moulding

Periodical of S.V.A.T. Astatine Volume 5 | Number 2 | November 2010

Black holes

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10

UAV: Helicopter

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Droplets on fibres

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Injection Moulding

Colofon VAN DEN BOSCH & FIKKERT DRUKKERS SINDS 1932

The “ATtentie” is the periodical of S.V.A.T. Astatine, which is issued five times a year. The ATtentie is distributed among members of Astatine and employees connected to Advanced Technology at the University of Twente. Volume: 5 Number: 2 Issue: 20 Copies: 415 Date of issue: November 2010

Editorial staff: Pim Muilwijk Geert Folkertsma Jeroen van den Berg Daan in den Berken Monique Parfitt

Editor in chief, Layout Editor, Layout Editor Editor Board member

Address: S.V.A.T. Astatine t.a.v. ATtentie Postoffice box 217 7500 AE Enschede Tel. 053-489 4450 Bank: 1475.73.769 (Rabobank) [email protected] http://www.astatine.utwente.nl Printer Drukkerij Van den Bosch & Fikkert B.V. - http://www.druk-bosfik.nl With thanks: Ellen Norde, Enzo Meijer, Geert Folkertsma, Jaap Flokstra, Jelmer Boter, Jeroen van den Berg, Jochem Giesbers, Herman Hemmes, Pim Muilwijk. Copy can be delivered to the addresses mentioned above, in .doc(x) or .txt formats. Any figures or pictures can be bundled with the text in a .zip or .rar file. The deadline for the next ATtentie: 27 December 2010 © S.V.A.T. Astatine 2010, all rights reserved. Authors remain responsible for the contents of their works. The editors preserve the right to modify or reject received articles.

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Editorial

You may have noticed that the printing issue was delayed somewhat and it arrived a little later than it should have. This has several reasons. First of all, the ATtenCie lost two of its members: Jochem Giesbers and Auke Been. We want to thank them both for their efforts and the good times we had and wish them the best with their new activities. Jochem was kind enough to still write something for us. Luckily we also gained a new member: Daan in den Berken-Kolmes. Despite this, we lost a significant amount of content producing manpower and combined with the relatively low amount of activities that took place and few adverts we came up short. However, with some creative thinking (or should I say advanced thinking) we thought up new topics to write about and set to work. The result, of course, is this issue, which features two (!) Bachelor’s assignments, two articles, two photo pages, an interview and some activities. Since there were a load of people who received their diplomas (including yours truly) on the 4th of November, we expect to place more Bachelor’s assignments in the coming issues. Nevertheless, I would like to exploit this oppurtunity to remind you that we are still looking for new members. So, if you would like to learn about writing articles, Dutch and English and operating InDesign we would like to invite you to sign up via the board. Of course, the coming month is all about the holidays and while I personally dislike them, there is also the momentous first lustre of Astatine, which kind of makes it fun for me also. In any case, enjoy the typically Dutch tradition of “Sinterklaas“, which is an opportunity to learn the meaning of “gezelligheid“, which can’t be translated into English. Try not to overeat on “kruidnoten“ and “speculaas“ and savour your chocolate letters, while you curl up in front of the fireplace and enjoy this edition. Pim Muilwijk, editor in chief

Inhoudelijk 10 22 24 30 34

Bachelor’s assignment: Helicopter StuFi op de schop? Bachelor’s assignment: droplets Black holes Injection moulding

Astatine Actieveledendiner5 Quinquennium: Small going Big! 8 Interview: Herman Hemmes 16 Astatine survey 19 MAC Workshop 21 After Kick-In 33

Overig 2 Colofon 4 AT graduation day 6 From the Chairman 7 From the AT boardroom 23 Column 38 Puzzle

AT graduation day Pim Muilwijk There we were, sitting in the same lecture hall where it all started for most of us; thinking back to all the hours we spend on listening to lectures, studying and generally having a great time. The hall was filled to the brim with all the people that were there to cheer for us: the family and friends of the 21 people who managed to get their Bachelor’s diploma. Luckily we had chairs with our names on them. How did this day start, progress and, more importantly, end? Well, technically, AT started out in “het Paviljoen“, but that was so long ago that only a handful of students were there to tell us about it. For most students HT900 was the space were they spent most of their first year(s). Sitting there and realising that it all eventually amounted to getting those three letters behind your name was, of course, a very rewarding experience. I was so caught up in this stream of nostalgic thought that I nearly missed Jaap calling out my name and had to hurry to make up for it. Stumbling to my feet I found myself signing the first of a stack of documents which stated “Getuigschrift academisch onderwijs“, the last series of Dutch diplomas. I was so exhilarated that I totally screwed up my signature, but who cares?

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In the meanwhile, someone from the staff told some stories about me that were greatly exaggerated and obviously not true and after that I had to explain what I did for my Bachelor’s assignment, which I did in the allotted two sentences. If you want to know more, read my article about it (or my report). After a while they had enough of me and shipped me off with a rose and an engraved crystal, only to repeat the same routine for the next student. When everyone got their diploma there was one last speech by Lolke Folkertsma, who appealed to our life as students and wished us the best of luck in our respective futures, different as they might be. Then, it was time for drinks, obviously. The drink was short and to the point. Get a drink, congratulate another student, toast, finish the drink and repeat. After everyone was satisfied with the amount of liquids they had ingested, a large group of us went to Mr. Hu to get some dinner. Eventually everyone had arrived and after a short introduction to wok 101, we could start eating and drinking to our hearts’ desire. All good things must end, however, and after the finishing speech all the brand new BSc’s went either drinking some more or home. Next stop: the MSc diploma...

Actieveledendiner Enzo Meijer Een jaar lang bestuur zijn is hard werken. Maar als bestuurslid kan je nog zo hard werken, een vereniging is pas een vereniging als ze ook actieve leden en commissies heeft. Zoals iedereen weet, werk je nu eenmaal het beste op een volle maag. Zodoende dook het vijfde bestuur de keuken in, om eigenhandig voor al haar actieve leden een vier gangen diner te bereiden. Nu is het zo dat op 13 oktober, als het vereningsjaar er al dik op zit, dit misschien beschouwd wordt als mosterd(soep) na de maaltijd. Maar in ons geval is het een perfecte gelegenheid om alle actieve leden te bedanken voor hun inzet het afgelopen jaar. Na de sluiting van onze eigen geliefde “Tombe” waren we genoodzaakt dit diner te serveren in het de Borrelkelder van Alembic in de Langezijds. Het grote nadeel van deze kelder dat we de afgelopen borrels signaleerden was de zeer omvangrijke grootte. Voor een diner echter ideaal: na wat schuiven, passen en meten en daarna nog een keer schuiven was er uiteindelijk één grote lange tafel gedekt waar alle 47 hongerige studenten aan plaats konden nemen. Eet smakelijk!

leiding over en bracht deze dan ook precies goed op smaak. Hoofdgerecht Ja, nu komt eetlust pas echt goed op gang en wilt een hongerige student natuurlijk meer. Tijd voor het hoofdgerecht. Aanvankelijk wilden we hier, net als de soep, gaan voor een typisch Nederlands gerecht: Hollandse stoofpot. Bereid met onder andere groene kool en sukadelappen. Maar aangezien Hein vorig jaar enkele maanden in Spanje heeft vertoefd en daar de geheimen van de Spaanse keuken heeft opgedaan, volgde daarom een stukje fusioncooking: oerHollandse stoofpot voorzien van een Spaanse tortilla. Onorthodox, dat klopt, maar de moderne chef-kok houdt nu eenmaal van risico’s. Dessert Om het diner van deze avond passend af te sluiten is er gekozen voor een trio van klassieke desserts. Te beginnen met een bolletje huisgemaakte chocolademousse naar ambachtelijk recept, vervaardigd door Felix. En wat gaat er nou goed samen met chocolade? Inderdaad meer chocolade! Daarom wordt deze chocolademousse begeleid door een heerlijk stukje kleffe brownie. Wederom een klassiek recept uitgevoerd door onderbovengetekende. En zoals ik al schreef, een trio van desserts, als afsluiter nog iets verfrissends, een bolletje citroensorbetijs. Digestief

Koud voorgerecht Om de eerste grote honger te stillen en de maag voor te bereiden op al het lekkers dat nog zal volgen is er gekozen om licht te beginnen. Een klassiek stukje meloen geserveerd met wat rucola en drie stokbroodjes met tapenade. Hierbij gaat een eervolle vermelding uit naar Pim Momberg, die deze olijven eigenhandig heeft geplukt en vermalen tot een mediteraanse tapenade. Warm voorgerecht De maag is nu gedeeltelijk gevuld en de smaakpapillen zijn geactiveerd. Een mooi moment voor een warme Zaanse mosterd-prei-soep. Annegreet nam hier de

Alle hongerige magen gevuld en onze laatste daad als vijfde bestuur der S.V.A.T. Astatine vervuld. “Geen afwas?” vraag je je misschien af? Nee geen afwas. Wel koffie en cadeautjes. Het eten was lekker maar helaas toch van tijdelijke aard; enkelen uren en een wc-bezoek later zijn alle vier gangen slechts nog een herinnering. Daarom nog een vijfde gang van het vijfde bestuur van blijvende aard. Voor alle keukenprinsen en prinsessen: de enige originele Asta-spatel! Dit onmisbare stuk keukengerei is voorzien van naam én logo voor alle actieve leden van het afgelopen collegejaar. Mocht jij nog niet in het bezit zijn van dit stukje handwerk, je kunt deze nog steeds afhalen bij het bestuur. Namens het vijfde bestuur wil ik alle actieve leden nogmaals van harte bedanken voor al hun inzet, tijd en enthousiasme van het afgelopen jaar. Wij hebben er in ieder geval van genoten. Op de Hoogste!

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From the Chairman Jelmer Boter This is the first time I may speak to you from this place. Since the longest general meeting in the history of Astatine, the fifth board has made way for the sixth board of S.V.A.T. Astatine. I want to thank the fifth board for their efforts last year and I’m very happy to have the honour to write this article the coming five editions. In June we were officially presented as the sixth candidate board of S.V.A.T. Astatine and the preparations started. The first meetings were attended and we had to write a policy and make a budget for this year. A lot came onto our way, but eventually we managed well. Preparations ended where our tasks as board started on the 22nd of September. Suddenly, we really were the board, but nothing felt different. It took some time before I realized it. The first time was on the stage during the constitution drink. It took a few days before it really felt like I was the new chairman of S.V.A.T. Astatine. Alexander, Wessel, Monique and I are looking forward to a wonderful year together with the whole association. We will do our very best and hope you will do the same. We have a lot of refreshing ideas for this year to bring Astatine further again. This will be a very special year, because we will be celebrating the first lustrum of S.V.A.T. Astatine. On the 23rd of December 2005 the four founders of our association went to the notary and Astatine really became something. Almost five years later we are a flourishing association, so it’s time to celebrate. The lustrum week will be a bit later than the actual Dies Natalis: from the 9th of February until the 16th of February there will be a whole week of activities and parties. The lustrum committee is working hard to make it a big success and recently presented the theme: “Small going big”. The theme refers to both Astatine and the study Advanced Technology growing big. Astatine isn’t celebrating only during the lustrum week, but the theme comes back during the whole year in different activities and the yearbook will be a special lustrum edition. You can already start with making puzzles on the lustrum site (www.astatinelustrum.nl) and become a Lustrum VIP. On this website you can find any information you want about the forthcoming lustrum.

By the time I’m writing this, the first couple of lecture weeks are over and I hope everyone knows how to study again. I2E-I has finished, so the first results of this year are known and we have totally started. While you read this, the exam weeks are over, so hopefully everyone has their first ECTS for this year and new energy to start the second quartile. Astatine has also started again and the first activities are behind us. The Kick-off for this new year was the Kick-In. The KIC and the KITCAT did their ultimate best and succeeded in organizing a ten day event. In the first few weeks of this college year there already were a couple of activities. We already had the Mac climbing workshop, the ATAC bowling and the tennis tournament. And of course the After-Kick-In Program (also know as ”Na-Intro”). I hope to see you all soon at an Astatine activity. I wrote this article in English, but I want to end with a Dutch sentence: “Op de Hoogste!”

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From the AT boardroom Jaap Flokstra, educational director The new academic year 2010/2011 started last August/ September. This was a special start for Advanced Technology because from this moment on the official language would be English. The study material, the lectures and the project meetings are all in English. I heard that some of the students found it a little difficult while other students said that there were no difficulties. A year ago it was feared that the number of students would drop to maybe half the number that started in 2009. The contrary is true. More students enrolled in the programme, and certainly also a number of students from abroad. Furthermore, it turned out that the average mathematics and physics grades of our first-year students (at secondary school) have increased significantly. The start of the new academic year was celebrated with a kick-off meeting with the first-year students. The president of the university, Anne Flierman, welcomed the students and he specifically addressed the international character of the university and stressed that Advanced Technology was really preparing students for an international career. Later this year the AT board will donate a barrel of beer during one of the Astatine ”borrels” so all our students can join us in the celebration of this new start.

The day I am writing this column, November 4, 2010, is again a unique day in the history of AT. We will make 21 students happy during a special bachelor diploma session. Over the last six months 24 students finished their Bachelor’s degree and that is more than the total number we’ve had since the programme started in 2004. Furthermore, we have our first cum laude student, Geert Folkertsma. The AT-board is very proud of all our BSc graduates and we specifically congratulate Geert with this beautiful result, we hope many more will follow. There is still a lot of work to be done on the educational programme. We have a revision of the curriculum going on, we are working on the third year of the AT-programme, there are discussions about minors of 30 EC, also the so-called “toetsbeleid “ (policy for testing) is keeping us busy . Regarding communication and recruitment special attention is now paid to the question how we can reach students from abroad. Eventually we would like at least a third of our students to have a truly international background. As a consequence we will invest in a very attractive webpage, for AT is an ATtractive programme. Dynamic and attractive!

At the opening of the academic year at the Music Centre in Enschede there was another success for AT. The socalled “scriptieprijs”, a prize for the best master thesis, was awarded to our AT-alumnus Sven Krabbenborg, who studied Nanotechnology after his AT-bachelor. At present Sven is a PhD student at our university in the group of Jurriaan Huskens.

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Quinquennium: Small going Big! Lustrum committee You might have noticed posters announcing a theme announcement. You may have seen posters explaining fabulous prizes can be won by doing puzzles. You have possibly been bothered by people in waistcoats on these same issues. You have probably seen a mixture of the words lustrum, lustre and something unintelligible with a lot of Q’s and U’s. But most definitely you will get to know about “Small going big”! On 23 December 2005, the study association for Advanced Technology Astatine was founded. Of course, everyone knows that coming (early) Christmas, Astatine has reached a new milestone: its fifth anniversary, also known as lustrum, lustre or quinquennium! Lustrum week Although it would be very nice to throw a big party just before Christmas to celebrate this fact, the Thursday falls in the middle of the winter holidays. Apart from that, one day is of course too short a celebration for a quinquennium. In order to suit everyone’s planning a bit better, there will be a lustrum week from Wednesday 9 February to Wednesday 16 February, 2011. The theme for this week is related to the early years of every association: Small going Big. Activities In this week, there will be a lot of activities: think of a big gala dinner, grand opening and closing festivities and other special activities throughout the week. To find out everything about the programme, do visit the website of the lustrum: www.astatinelustrum.nl

Clothing Of course, special lustrum clothing is sold in preparation for the festivities, so we can all be in our Sunday best when the time comes. Apart from the yearly Astatine jumper, which will be lustrum-themed, there will be more exotic things like boxer shorts and chamber robes… Logo The logo for the lustrum was presented together with the theme and global programme, a couple of weeks ago. The logo consists of elements from the Astatine logo; some rearranged, others enlarged or removed. Note the seven electrons that have grown into flowers – small going big! You might see this logo around Astatine or the university; and whenever you do, you’ll know it has something to do with the quinquennium. Announcements on puzzles, committee members wearing their waistcoats, and maybe scattered around the campus… Website Finally, make sure to regularly check the website astatinelustrum.nl, because it might contain news on activities, subscriptions for them, order forms for merchandise, important puzzle news…

Puzzles On this website, www.astatinelustrum.nl, you can find a new puzzle each week. Completing them and submitting the answer scores points. What can you do with these points? You don’t get fobbed off with a free pint of beer during the opening, but can instead go to the lustrum as a VIP! This means free entrance to activities, free merchandise and maybe more… There are other prizes in other categories, see the website (www.astatinelustrum.nl) for those!

small going big 8

YOU are NEEDED ...to write reports of Astatine’s activities

...to write in-depth articles on science and assignments

...to edit and lay-out the ATtentie So help or join the ATtenCie

Bachelor’s assignment: Helicopter Geert Folkertsma At the end of the Advanced Technology Bachelor’s programme stands the swan song, the “eindbaas” or (for some) the neverending story: the Bachelor’s Assignment. This is the moment to see what you have learned the last three to five years; and to show that you don’t “know a very little bit of a lot of things”, as some outsiders classify AT, but that you can apply your broad base of the first two years and the specialization of the last one to an in-depth research problem.

Picking an Assignment

In the ATtentie, we aim to regularly get an article from a (former) AT student about his or her Bachelor’s Assignment. Aside from usually making an interesting story, this shows you what you can expect of the assignment in general, and of the research group in question specifically.

So, 3 weeks before I wanted to start I visited him again, only to hear that they were actually pretty short on assignments, due to high numbers of students! There were only 2 assignments readily available, but they were already taken or I did not find them interesting.

This time the honour is mine; and apart from telling you about the assignment itself (which was really cool and challenging), I will explain how I got to do this specific research – hoping that it will help you in finding an assignment yourself. The group Most Advanced Technology students do their Bachelor’s Assignment at a research group that is connected to their prospective Master’s. This was not different for me: since the 2008 symposium Artificial Evolution on robotics, I have been drawn to Mechatronics; as a result I chose Control Engineering as the group where to do my assignment. This group does research on all kinds of robotics and other mechatronic systems: multidisciplinary and very interesting for AT students (see http://www.ce.utwente.nl/ for more information).

You can’t start looking too soon, or so I thought, so I went to Job van Amerongen (chairperson of the group) to talk about an assignment somewhere in February 2010. He told me I could do my BA at CE, but I should come back 2 or 3 weeks before I wanted to start: then they would find an assignment on which topic a PhD student or UD is working and needs work done at that time. He assured me there would be plenty of assignments available.

Luckily, one day before I had wanted to start, another member of the group came back from a foreign trip and told me she had four assignments available, all of which sounded interesting. From silent despair of not having a BA to do, I went to the luxury of picking one! Writing the description The assignment I chose was “controller for unmanned aerial vehicle”, which is nowhere near a solid task description and needed some work defining it. After another meeting with Raffaella (my tutor, for whom I did the assignment), I knew what to do. I would get to play with a miniature helicopter. That is to say, I had to buy one and make a controller so it could fly autonomously; or at least hover in place. How cool is that! Starting up This assignment was really the first thing the group did in practical research for Unmanned Aerial Vehicles (UAVs), so there was not much work to build on. On the one hand this gives a lot of freedom, but on the other hand you really have to find out everything yourself. This includes choosing the helicopter, but also the sensors to use, what kind of computer (microcontroller) to use for the controller: everything. After a week of searching, comparing, reading datasheets and defining requirements, I knew what to buy. Then trouble started.

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At the university, everything a researcher wants, has to be procured through “Afdeling Inkoop” (the

UAV: Helicopter procurement department), which has a process time of something between two weeks and three months. Especially if something has to be bought outside of the Netherlands at a company the UT has not done business with before (which was, not surprisingly, the case for the R/C airplane shop), all kinds of formalities and paperwork have to be processed.

This assignment’s goal is to start on experimental research on UAVs, by designing a controller for a miniature helicopter. This can be split into four objectives:

After two months my assignment would be almost over, so an order time of 2-3 months on the helicopter was, ah, not feasible.

3. Implementation: of the controller in the real heli.

Luckily, the group’s technician found a way to bypass this delay, using his own resources and resourcefulness. Thanks again!

Helicopter

1. Modelling: make a dynamic model of the heli. 2. Controller design: using the model and simulations.

4. Verification: running experiments; does it work? Before making a model of a helicopter, we must know: what is a helicopter? How does it fly?

Doing it Of course it still takes some time to receive goods from the UK, but I could do some theoretical work in the meantime: reading up on literature about UAVs, starting on the modelling, writing firmware… but of course I was really waiting for the chopper to arrive. Then from the moment it came, till the moment I finally called it a day and went on vacation for a couple of weeks before the holidays were over, I filled my days modelling, measuring, writing, testing and crashing. Rewriting the report for the last time just after the start of the new term, I finally concluded the assignment with a presentation shortly after: the last bit of my Advanced Technology Bachelor’s was in the pocket.

Figure 1: Helicopter and its most important parts. Referring to Figure 1: the main rotor generates thrust, blowing the helicopter into the air. This also generates a reaction torque, which is compensated by the tail rotor – or the heli would spin out of control. The swash plate can alter the angle of the main rotor blades to tilt the helicopter forwards/backwards, or left/right.

Research and Report To give an idea of what the assignment was about, I continue with a kind of summary of my report. If you want to read it in full, or are wondering about the references I did not include here, you can visit the Control Engineering website to download my full report: http://www.ce.utwente.nl/rtweb/publications/ MSc2010/pdf-files/025CE2010_Folkertsma.pdf Introduction There is, as all articles start, an increasing interest in Unmanned Aerial Vehicles. Most people think of the drones the US Army uses (think of the Predator), but there are a lot of civilian applications thinkable too. Think of the helicopter the Dutch police use for sniffing weed plantations in corn fields, or the remote inspection of power lines or high buildings exteriors. The University of Twente focuses on these civilian applications by participating in the European research project AIRobots, whose goal is: “to develop a new generation of aerial service robots capable to support human beings […] and interact safely with the environment.”

Figure 2: Coordinates and important forces/torques. Figure 2 shows the coordinate system used: positions x (left/right), y (forward/backward) and z (altitude); and angles φ (pitch, around x), θ (roll, around y) and ψ (yaw, around z). Also the important forces are shown: the thrust L, weight Fz, rotor and tail rotor torque Mr and Mt; and the cylic torque τ generated with the swash plate as explained above. Honeybee The helicopter used in this assignment is the Honeybee Fixed Pitch v2 (Figure 3). It has all the parts explained above, and to make a dynamic model of it, it has to be characterised: forces, weights and intertias must be measured.

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UAV: Helicopter

Figure 3: The Honeybee helicopter used in this project. Characterisation The weight and the moment of inertia of the helicopter can be easily measured with a set of scales and some basic formulas. The force/torque generation is a little bit harder: how to measure the torque generated by the main rotor at full throttle? The thing with flying robots is, they fly… By strapping the helicopter to an electronic weighing scale, the thrust could be measured (by the weight reduction). For the yaw torque, the helicopter was strapped to a frame shown in Figure 4, constraining all movements except rotation around z. Dynamic model After determining all the parameters for force generation, inertias and the swash plate, these can be applied in a dynamic model. This can be done in any simulation program and I will not bore you with the specifics, but the general model structure is interesting. Figure 5 shows the 6 degrees of freedom (the three translational axes with their associated angle in the same colour) and the four actuators: a motor for both the tail and main rotor, and two servos for the swash plate. You can clearly see the coupling between pitch/ roll and the lateral position: if the helicopter tilts forward, it automatically also moves forward. This also makes control rather difficult, but that is something for the next paragraph!

Figure 4: The setup used to measure the yaw torque. Control Generally, the problem of the control of a mechatronic system boils down to providing the proper actuator input, to get a desired output. In the case of the helicopter, the inputs are the four channels (the grey boxes in Figure 5), and the outputs are the 6 coordinates (the coloured blocks in the same figure). The problem is that this is an underactuated system: there are more degrees of freedom (6) than actuators (4). This is already apparent from the dynamic model: it is visible in the coupling between the roll/pitch and lateral position. This problem is solved using “cascaded control loops”, which means that the control outputs for pitch and y are combined: if the helicopter needs to move forward (y controller output), you just tell it to pitch forward (pitch controller). The same is done for roll and x.

Figure 5: The general form of the dynamic model.

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UAV: Helicopter

Figure 6: The control strategy. Feed forward (FF) and feedback (PID) are both used; and the underactuation problem is solved by combining control outputs into one actuator input (cascaded control).

Control (providing the right input) can be done in two ways: feed forward and feedback. Feed forward is providing an input based on your knowledge of the system and the desired output; for instance: poke up the hearth when it gets colder outside, to maintain a nice temperature. Feedback also uses the measured output and compares it to the desired output; in the same example this would be a thermostate. The complicated dynamic system that the helicopter is, requires a combination of both. For instance: you know you have to spin up the rotor if you want to lift off (feed forward), but to maintain a specific height, you also need to measure the actual altitude. All these things combined (cascaded control, feed forward, feedback) give a control strategy shown in Figure 6.

Figure 7: The response of the simulated helicopter, when at t=5s it is told to go to (1,1); starting from (0,0).

Simulation The whole point of the model was to be able to test the control strategy in a computer simulation; and so to easily tune it to make it work. After all, if the simulated helicopter “crashes” you just run another simulation, but with the real helicopter that’s quite another story… Figure 7 shows results of a simulation where the helicopter started at position (x,y)=(0,0) and after 5s must move to (1,1) metre. The dashed blue line indicates the time after which this new position is reached within 5%. The height and yaw (not shown in this figure) were kept constant at 1m and 0 rad. The simulation was also used to test the influence of parameter variation: what if the measured force differs 20% or even more in flight? The simulation showed that the setpoint was still reached, but the response looks different: it might be slower or faster and overshoot a bit.

Figure 8: The hardware put on the helicopter (avionics).

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UAV: Helicopter Implementation: hardware The goal of the assignment was to have an actual helicopter hover by itself, so after all this measuring and simulation, it is time for some real action. Well, almost, because the designed controllers must be implemented in the helicopter. Figure 8 shows in a block diagram all the parts. The blue blocks are all sensors: a three-axis accelerometer, a single and two-axis gyroscope, an electronic compass, an ultrasonic height sensor and a GPS receiver. The red block is a microcontroller programmed to measure the full position of the helicopter; also called IMU (Inertial Measurement Unit). The other big block is a microcontroller running all the controller code; and finally the radio, multiplexer and servo/ESCs are the input/output of the system.

The computer runs a program that receives the flight data, records it (for taking measurements) and displays it sort-of real time on a graphical interface: very useful during test flights (Figure 9)! Experiments Now everything is in place, real flights can be made and, if everything went well, the helicopter can hover in place on its own! The first test I did was with the helicopter still in the restraining cage of Figure 4, to test only the yaw controller: do the measurements agree with the simulation results? Figure 10 shows that this is indeed the case: both responses look the same!

Implementation: software There are three pieces of software: one on the IMU, one on the controller and one on the computer. The IMU contains an algorithm (Kalmanfilter) to combine all the sensor data into a position. It uses the gyroscopes for fast measurements, and compensates their drift (slow deviation of the actual value) with the compass and accelerometers. The controller runs a 50Hz loop that receives position data from the IMU, calculates the required outputs and sends them to the servos and motors. In the meantime it also handles communication with the radio (the original remote control of the helicopter) and sends flight information through a wireless connection to the computer. Figure 10: Yaw controller experiment. The dashed line shows yaw setpoint; the dark (red) line simulation result; and light (green) the measurements.

Figure 9: The interface of the computer program that displays flight data.

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UAV: Helicopters Pitch and roll

Conclusion

The next test was the yaw, pitch and roll controllers in real flight; while manually setting rotor speed and pitch/roll setpoints for position control with the remote. Figure 11 shows results for this experiment. It might be a bit tiny now, but looking at the full figure in my report, you see agreement here as well!

My conclusion was that unfortunately the ultimate goal of the project was not reached; the helicopter cannot hover in place. However, there was an explanation for this (see previous paragraph), and looking at the four objectives of the assignment, all but one of them was completed.

The final experiment was complete position stabilisation, but unfortunately this did not work. This was due to a number of difficulties, including inaccurate GPS measurement, a malfunctioning ultrasonic height sensor, a fragile helicopter (crash=broken heli, per definition) and the deadline of my vacation…

All in all, I was happy with the results, but if the CE group carries out more practical research into flying vehicles, I have some recommendations: have a safe vehicle, so not one with a giant rotor spinning at very high speeds, chopping off fingers if you put them in (also making the vehicle more robust); use other sensors to do position control; and find a testing facility (because I had to resort to the technical room also containing Carré’s ventilation systems, or the lawn in front of the Horst if I needed GPS).

Figure 11: Pitch and roll measurements in a test flight. Dashed line is setpoint; green and red (light and darker grey for the greyscale version) are response of the simulation and real helicopter.

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Interview: Herman Hemmes Pim Muilwijk & Geert Folkertsma Een van de stafleden die we nog nooit hadden geïnterviewd kennen we vooral als een rustige man, die ook veel bij het practicum doet. Wat opvalt als je zijn kantoor binnenstapt is dat er niet meer (zoals bij zijn voorganger) metershoge stapels papier, tijdschriften en verslagen op elke vierkante meter van zijn bureau en tafel liggen, maar in plaats daarvan vier dozen waterkokers in de hoek. Wat zegt dit over hem? Daar gaan we achter komen!

Nu terug naar waar het allemaal begon: waar ben je geboren en getogen? Ik ben in Amsterdam geboren en heb daar tot mijn zevende gewoond. Daarna zijn we naar Amstelveen verhuisd en ik heb daar mijn lagere school afgemaakt. Toen de middelbare school in Buitenveldert, weer net over de grens in Amsterdam. Ik heb havo gedaan en ben in de tweede blijven zitten, want ik had voor alle talen een dikke onvoldoende. Uiteindelijk heb ik na de havo in 1 jaar vwo gedaan. Je hebt daarna natuurkunde aan de VU gestudeerd. Hoe heb je die keus gemaakt?

Wat wij ons altijd hebben afgevraagd: waar staat de K in H.K. Hemmes voor? Ik heet voluit Herman Koos Hemmes. Herman was de ene opa, Koos de andere. Nouja, hij heette Hiepko, maar dat hebben mijn ouders me niet aangedaan. Waar woon je? In Enschede, de wijk Deppenbroek. Dat is een tijd een beetje achterstallige allochtonenwijk geweest, maar een jaar of vijftien geleden zijn er grote renovaties begonnen. Ik woon nu in een oud-ABP-flat die tien jaar geleden groot onderhoud heeft gehad. In 2002 ben ik er komen wonen, daarvoor woonde ik aan de Boulevard 1945. Hoe kom je naar de UT? En, een nieuwe vraag, waar doe je boodschappen? Ik fiets altijd naar de UT. Als ik terugkom ga ik naar huis, leg ik mijn spullen weg en ga dan lopend boodschappen doen bij de dichtstbijzijnde supermarkt, in mijn geval de Albert Heijn Deppenbroek.

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Ik vond eigenlijk alles leuk, als het bèta was tenminste. Ik ben bij veel voorlichtingsdagen geweest en heb veel richtingen bekeken: wiskunde, natuurkunde, scheikunde, werktuigbouwkunde... Voor een deel was het eigenlijk pro forma: je hebt toch vaak al in je achterhoofd wat je wilt gaan doen en de rest is alleen maar bevestiging daarvan. Delft trok eigenlijk meer, maar Amsterdam was toch wel makkelijk dichtbij. Achteraf had ik werktuigbouw in Delft ook wel leuk gevonden. Hoe zag je studententijd eruit? En: netjes nominaal, of zoals het toen ging 10 jaar studeren? Het eerste jaar heb ik nog thuis gewoond, daarna ben ik in Uilenstede in Amstelveen gaan wonen, in wat vooral bekend stond als de studentengetto. Ik ben lid geweest van s.v. I.A.N. (studentenvereniging Institutio Amicitiae Nostrae), nu het VU-corps. Ik zat in een gemengd dispuut zonder corpstrekjes, want dat hoefde ik niet. Ik heb iets meer dan zes jaar over mijn studie gedaan. Pas in mijn tweede of derde jaar werd het begrip “nominale studieduur” ingevoerd, natuurkunde werd toen op 5,5 jaar geclassificeerd. Er zat nog wel wat “lucht” in het programma, dus later werd dat gewoon 5 jaar. Waarom en waarop ben je na je studie gaan promoveren? Ik ben ook aan de VU gepromoveerd en ben daar een beetje ingerold. Ik ben afgestudeerd in vastestoffysica, een onderzoek over het oplossen van H2 in metaal. Mijn promotie ging over supergeleiding in metaalwaterstofsystemen. Er was een theorie dat de kritieke

Interview: Herman Hemmes temperatuur van Palladium omhoog zou gaan als het verzadigd wordt met waterstof. Mijn onderwerp was het bouwen van een diamantpers om dat te realiseren. Is het gelukt? De bouw van de drukpers is een tijd lang stilgelegd. Dat kwam omdat er onder andere Cu-Be in moest en beryllium, of eigenlijk vooral BeO, is nogal schadelijk. De werkplaatstechnicus was wel netjes ingepakt en er was afzuiging, maar toen de Centrale Veiligheidsdienst metingen deed vonden ze het toch niet goed, dus er moest een speciale ruimte gebouwd worden. Uiteindelijk had ik na twee jaar de drukpers, maar toen waren er al hogetemperatuursupergeleiders ontdekt... Ik heb het onderzoek natuurlijk wel afgemaakt en zelfs nog een halfjaar verlenging gekregen, en de theorie klopte wel. Ben je daarna meteen naar Twente gekomen? Ja, ik ben toen gaan solliceteren, ook in Twente, en hier zochten ze een postdoc voor een project over hogetemperatuursupergeleiding. Dat project werd gefinancierd door de UT, maar in het eerste jaar heb ik meegeschreven aan een projectvoorstel voor een Europees project. Dat haalden we binnen, dus heb ik daar zo’n 4 jaar aan gewerkt. Daarna was het geld van het eerste project nog niet op, dus heb ik dat nog een jaar gedaan. De laatste tijd zit je meer in onderwijs, met practica en als coordinator van AT. Hoe ben je daar terechtgekomen? Na die projecten was er een onderwijsproject “TEO” (Techniek, Elektronica en Ontwerpen) om een nieuwe lijn in de practica op te zetten. Dat ben ik gaan doen, omdat het anders een WW-uitkering was geworden. Ik heb dat een jaar gedaan; intussen had premier Kok een wet aangepast waardoor ik ineens een vaste aanstelling had. Ik ben nog als UD’er naar de TN-groep Meet- en Systeemtechniek gegaan, wat vooral onderwijs was. Onderzoek was namelijk lastig: er was geen hoogleraar en het was niet duidelijk of de leerstoel opgeheven zou worden. Na een jaar of 4, 5 was er een nieuwe hoogleraar, maar zijn onderwerp sprak me niet aan. Na wat stress daardoor ben ik overgestapt naar het INO (Instituut voor NatuurkundeOnderwijs, de onderwijsgroep van TN). Nu ben ik practicumcoördinator voor AT en opleidingscoördinator. Achteraf was het misschien beter geweest om ergens anders heen te gaan, maar dat zit nog steeds wel in mijn achterhoofd. Ik ben nu twee jaar coördinator, maar het past niet 100% bij me. Als ik iets doe, doe ik het het liefst goed: alle aspecten uitgebreid vergelijken en een weloverwogen keuze maken. Als opleidingscoördinator

kun je niet zoveel tijd aan dingen besteden en moet je veel ad hoc regelen. Dat levert soms wel stress op. Even op de zaken vooruitlopend: heb je al een opvolger op het oog? Nee, daar heb ik nog niet over nagedacht. Ik weet ook niet wie het zou moeten doen, en of ze er wel zijn: mensen die het kunnen én willen. Tegenwoordig komen er vaak mensen van buiten voor dit soort posities. Je hebt een tijd TN-practica geregeld en doet dat nu bij AT; bovendien kom je daar ook ST’ers tegen. Zit er duidelijk verschil tussen deze studenten? Er zijn natuurlijk allemaal verschillende studenten, maar er zijn wel “gemiddelde types” bij de drie richtingen. ST-studenten nemen het probleem zoals het komt; ze voeren het script, de practicumomschrijving, uit en doen wat de begeleider zegt. TN-studenten zijn veel meer eigenwijs, ze willen begrijpen wat er gebeurt en duiken meer in de theorie. AT-studenten willen het ook wel begrijpen, maar dan vooral in grote lijnen; soms zijn ze er moeilijk toe te bewegen het probleem of de stof écht goed uit te diepen. Wat vind je van het “scheikundepracticum” bij Lab Practice 1? Wij vonden het niet echt scheikundig. Het is inderdaad niets met pilletjes en poedertjes. We horen bij de evaluaties en van docenten ook wel dat studenten te weinig kunnen in een chemisch lab. Ze zouden wel wat meer basisvaardigheden op dat gebied mogen krijgen. In de voorlichting wordt de scheikunde vaak in een adem met engineering genoemd, terwijl dat in het eerste jaar en in de bachelor toch anders ligt. De engineering-lijn is heel duidelijk. De gamma-lijn is ook duidelijk, maar kleiner; AT is tenslotte een technische studie. Scheikunde komt echter vooral tot uiting in keuzevakken als Basic Chemistry en in het derde jaar. Het zou eigenlijk beter zijn als er een scheikundevak, Basic Chemistry-achtig, in het eerste jaar zit. Had je zelf Advanced Technology willen studeren, als dat toen had bestaan? Het is natuurlijk moeilijk dat achteraf te zeggen. De UT was erg ver weg, maar ik denk het toch wel. Ik wilde eigenlijk alles blijven doen – behalve de talen natuurlijk. Dat is ook het leuke aan AT: je kunt echt alle kanten op. Veel studenten weten nog niet precies wat ze willen worden en tijdens de studie ontdekken ze pas wat echt leuk is. De BaMa-structuur is gunstig voor AT: je kunt dan een specifieke master doen na de bachelor. Andere opleidingen hebben er minder baat bij; daar fietsen studenten 17

Interview: Herman Hemmes gewoon door na de bachelor. Het leuke aan AT is dus dat je alle kanten op kunt. Het huidige plan om het derde jaar Engelstalig te maken is door vakken te clusteren en zo standaard “stromingen” te maken voor studenten. Hoe valt dit te rijmen met alle kanten op kunnen? Het idee is op dit moment inderdaad om een bundeling te maken voor typische vakken van populaire masters. Veel studenten gaan naar de engineering-kant (mechatronica, andere EWI-opleidingen) of CTW; een paar CT en redelijk wat NT. Voor die populaire richtingen zou je dus een stroming kunnen maken, eventueel in de vorm van een Engelstalige minor. Veel studenten hebben nu een vakkenpakket speciaal toegespitst op hun master of studieverleden. Is dat dan niet meer mogelijk? Naast die stroming kun je natuurlijk andere vakken doen, zeker Nederlandstalige studenten. Masters moeten echter ook wat losser worden in hun toelatingseisen: vaak komt het erop neer dat je eigenlijk hun eigen bachelor gedaan moet hebben. Je zou bijvoorbeeld ook een bachelorvak kunnen “inhalen” in de master. Nu we het over vakken hebben: we vragen ook altijd naar een nieuw vak. Stel dat je een vak voor AT zou mogen opzetten, wat zou dat dan zijn? Het zou een groot vak zijn, misschien zelfs wel meer dan een. Alle ingrediënten van de opleiding moeten erin, maar dan in de vorm van een ontwerpproject. Dus inclusief systeemanalyse, ontwerpen en de realisatie. Vooral tussen die laatste twee zit vaak een spanning, dat merken veel studenten bij Instrumentatie/Lab Practice 2 en vooral Project 2 (accelerometers). Dat vak zit eigenlijk te vroeg; dan is alles nog een beetje houtjetouwtje, ook het ontwerp en de systeemanalyse. Sommige studenten hebben er wel een soort intuïtie voor, die kunnen makkelijk iets in elkaar knutselen. Studenten moeten beter kunnen knutselen? Nieuwe studenten kunnen goed met een computer omgaan en met veel geluk hebben ze nog met (technisch) lego gespeeld. Daar krijg je al veel ruimtelijk en technisch inzicht van: hoe werkt een tandwielkast, welke mechanische constructies werken wel en niet in de praktijk; dat heeft bijna niemand meer tegenwoordig. Vroeger had bijna elke technische student een eigen brommer die hij uit elkaar kon halen en weer in elkaar zetten.

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Behalve bij accelerometers wordt het ook niet echt gedaan. We klooien maar wat aan bij de projecten. Vroeger kregen eerstejaars TN-studenten nog een werkplaatscursus: leren zagen, boren, frezen. Nu gaan jullie met Project 2 nog de werkplaats in om wat in elkaar te zetten, maar straks wordt dat misschien op de 3d-printer van Control Engineering gedaan. Het is allemaal goedkoper, mooier en efficiënter, maar de praktische vaardigheden gaan er niet op vooruit. Heb je zelf “knutselhobby’s”? Mijn hobby is vooral hobby’s verzamelen... Ik vind inderdaad knutselen leuk: lego, modelvliegtuigen, bootjes, zelf vliegtuigjes met benzinemotor ontwerpen. Ik ben dan aan het uitzoeken hoe iets gaat en zodra ik dat door heb, stop ik er weer mee, want dan is het eigenlijk niet meer leuk. Zodra ik mijn brommer reversibel uit elkaar kon halen ben ik daarmee gestopt; na een paar werkende modelvliegtuigjes had ik dat ook wel gezien. In mijn Linux-tijd had ik op een gegeven moment veel uitgeprinte handleidingen, die heb ik toen eens geprobeerd in te binden. Dat heb ik nog een paar keer gedaan en uiteindelijk net geen boekpers meer gekocht; toen had ik het wel weer gezien. Ik bind nog wel eens iets in, zoals laatst een serie columns uit de Focus voor een afstudeerder. Nog twee vragen. Ten eerste: heb je een favoriete wetenschapper? In mijn promotietijd heb ik Neil Ascroft (van boeken Ascroft&Marmin) eens horen spreken. Hij gaf college ergens over; het was een geweldig verhaal en je snapte alles. Toen ik thuiskwam en het wilde toepassen bleek dat hij vakkundig om alle problemen heen had geschipperd in zijn verhaal, waardoor ik er eigenlijk achteraf niet zoveel aan had. Didactisch wel goed, of juist niet? Als ik zelf iets uitleg moet ik van tevoren goed nadenken hoe ik het doe en een beetje polijsten, want anders praat ik mijn gedachten achterna en dan kom ik niet uit waar de bedoeling was. Onderwijs is altijd een afweging tussen de studenten iets leren en een mooi verhaal vertellen. Ten slotte: wie is ons volgende slachtoffer? Als volgende lijkt het me leuk om Arie van Houselt te interviewen. Hij is docent van een chemievak, dus misschien het begin van een nieuwe lijn in de interviewreeks?

Astatine survey Geert Folkertsma At least once each year, Astatine conducts a questionnaire of its members. It goes into study-related things such as what Master’s degree programme you will do after Advanced Technology (if any), but also Astatine related questions such as: how often do you go to events, and what should be done to make them more interesting? At the end of last academic year another survey was conducted, the results of which have not yet been published. Because, apart from some interesting information for the board and organising committees (why didn’t you visit more drinks?), there is some interesting information in these survey results. Do you know where your fellow students will end up in a couple of years? Figure 1: Answers to “how do you know about activities and events?” (More than one answer was possible.) Response 89 people filled out the survey, which was sent to 240 members. Because of the nature of the questionnaire (a lot of questions about Astatine), this 37% probably includes a lot of active Astatine members (committees, events), so it is not a perfect representation of the Advanced Technology students, especially where questions regarding activities (how often do you go to lunch lectures?) are involved. However, assuming little correlation between study direction (physics, mechanical engineering, et cetera) and activism, the statistics on Master’s degree programmes is quite accurate.

Activities The graphs for the activities are not so interesting. They do show that you think yourselves very busy (more than half does not show up on activities because they don’t have time for it), but not to worry: 70-80% of the respondents think all the activity types (excursions, colloquiums, drinks, et cetera) frequency is precisely on the spot. Apparently, you don’t care if activities are organised nobody has time to visit – except for the 2-5 people that show up for 75-100% of all the activities.

Promotion Before people join activities, they have to know about them. There are several ways in which Astatine seeks to promote its activities, the most effective of which is (see figure 1) still word-of-mouth advertisement. If one person signs up for an event, he gets his (or she her) mates to join as well. Is a promotional talk at the start of a lecture such a bad way to hear about activities (only slightly more than 20%)? Well no, but this is only used for special activities such as the symposium or the lustrum. The skewed response group shows in one of the other questions related to promotion: according to this survey, two thirds of the Astatine members read their newsletter and find it useful (figure 2)!

Figure 2: Do you read the newsletter? Master’s degree programme Then the most interesting question of all: apart from the 25 people who didn’t know yet what they would do after their Bachelor’s degree (also first-years filled out the survey), the next page shows you where Advanced Technology students generally spend another two years! 19

Master survey

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MAC Workshop Ellen Norde Het zal voor het gros van de AT’ers gelden: bachelordiploma halen, masterdiploma halen en vervolgens de arbeidsmarkt op. Je mag ervan uitgaan dat je na het afronden van je master over voldoende technische kennis beschikt, maar geldt dit ook voor tal van andere vaardigheden? Met dit als uitgangspunt organiseerde de MaC (Mastercommissie) op woensdagmiddag 6 oktober een workshop ‘Presentatietechnieken en commerciële vaardigheden’ verzorgd door UniPartners Twente. Na ontvangst met koffie en thee, beet Peter Riezebos het spits af met een voordracht over diverse aspecten van communicatie. Om ervoor te zorgen dat we wat betreft het begrip “communicatie” op één lijn zaten, werd eerst een eenvoudige versie van het zenderontvanger model (figuur 1) besproken. Het moge duidelijk zijn dat een boodschap onder invloed van ruis niet of gewijzigd aankomt bij de ontvanger.

Figuur 1: het zender-ontvanger-model. Interessanter is echter om na te gaan hoe de betreffende boodschap door de ontvanger wordt geïnterpreteerd. Bij verbale signalen is dit goed in te schatten, maar in de meeste gevallen betreft dit slechts 20% van de totale boodschap. De rest bestaat uit non-verbale signalen en daarom is het belangrijk om je bewust te zijn van je non-verbale uitstraling. Deze conclusie werd extra duidelijk geïllustreerd door een praktijkvoorbeeld. Twee workshopbegeleiders, Joost en Jan Willem, werden naar voren gehaald met voor de deelnemers de taak om eigenschappen toe te kennen aan de nonverbale signalen die door de twee werden uitgezonden. Om maar even een voorbeeld te stellen: wie denkt nou niet van een zelfverzekerd type met een blouse, halflang haar en een luxueus horloge dat hij ijdel is, VVD stemt en bedrijfskunde studeert? Hoewel er door verschillende deelnemers wel degelijk grotendeels dezelfde eigenschappen werden toegekend aan Joost of Jan Willem, bleek toch dat dit nauwelijks overeen kwam met de werkelijkheid.

Iedereen zal je eerst beoordelen op basis van de non-verbale boodschappen die je uitzendt, ook je toekomstige werkgever. Kortom, wees je daarvan bewust en werk aan het beeld dat je bij een sollicitatie of functioneringsgesprek wilt uitdragen. Het tweede gedeelte van de workshop behandelde presentatietechnieken voor een korte en een langere presentatie. Bij de korte, ook wel elevator pitch genoemd, is het van belang dat je een duidelijk statement hebt en dat je ervoor zorgt dat je in het geheugen van je toehoorders blijft hangen. Voor de AT’ers die project Startrix nog niet hebben gevolgd: je stapt met je gesprekspartner in een lift en imponeert hem met een kort en bondig, maar overtuigend verhaal in de tijd die de lift nodig heeft om jullie van de begane grond naar de 20e verdieping te brengen. In de praktijk bleek dat een goede pitch voorbereiding nodig heeft. De deelnemers hadden als opdracht zich voor een jury te presenteren als “Beste student van Nederland”. Na twee pitches werd duidelijk dat de toehoorders niet altijd het belangrijkste deel van het verhaal konden reproduceren en dat ook hier een zenuwachtige of arrogante lichaamshouding je pitch negatief beïnvloedt. Bij een langere presentatie is daarnaast de opbouw van je verhaal van belang. De alom bekende driedeling: inleiding, middenstuk en conclusie blijkt nog steeds effectief. Of het nou gaat om een informerend of overtuigend verhaal. Al dan niet ingestudeerde gebaren ter verduidelijking van je verhaal werken ook goed, zolang ze maar niet te zenuwachtig of te onnatuurlijk overkomen. Als laatste nog een gouden tip voor de vervelende kwestie “waar laat je je handen?” Sla je handen ineen en druk met de duim van je ene hand op de muis van je andere hand. Dit geeft je een gevoel van aanwezigheid en zorgt ervoor dat je zonder enige zweem van zenuwachtigheid en in een actieve houding je presentatie kan houden. Na ruim twee uur, een portie informatie en weetjes verder, kwam er een einde aan deze workshop. Dat ook UniPartners over commerciële vaardigheden beschikt, blijkt maar weer als we getooid met een pen en telefoonsok richting huis gaan.

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StuFi op de schop? Pim Muilwijk Door de economische malaise waarin Nederland zich verkeert ziet het huidige kabinet zichzelf genoodzaakt om flink te bezuinigen. In totaal moet er 18 miljard over de hele linie gekort worden. Eerder al werd besloten om te snijden in de kinderopvang en om de btw voor kunst te verhogen. Is het nu de beurt aan de studenten? We schrijven 16 oktober 2007: de minister van onderwijs, Ronald Plasterk, overweegt op advies van commissie Rinnooy Kan om de studiefinanciering af te schaffen en studenten hun studie volledig met een lening te laten betalen. Enkel als de lening na de studie niet terugbetaald kan worden zal deze omgezet worden in een gift. Het bericht komt als een donderslag bij heldere hemel en treft ongeruste studenten. Heeft hun laatste uur geslagen? Opluchting alom wanneer blijkt dat het ging om een mogelijkheid die volgens Plasterk “geen serieuze optie” belichaamt. Geen sociaal leenstelsel, wel afslanken Krap twee jaar later besluit het kabinet nogmaals om de studiefinanciering niet in te ruilen voor een sociaal leenstelsel. Echter wordt er wel bezuinigd op de basisbeurs, welke met een jaar wordt ingekort, waardoor studenten nog maar drie jaar studiefinanciering krijgen. Straf de “langstudeerders” Dit brengt ons op het punt waar we nu zijn. Als het aan het kabinet ligt zal de basisbeurs voor de masterfase komen te vervallen en verruild worden voor een sociaal leenstelsel. De aanvullende beurs lijkt vooralsnog wel gehandhaafd te blijven. De structurele bezuiniging ten gevolge van deze maatregel bedraagt 0,11 miljard euro. Verder wordt het collegegeld voor studenten die langer dan een jaar uitlopen verhoogd met € 3.000 en moeten deze studenten betalen voor hun OV-jaarkaart. Overigens zal deze maatregel met terugwerkende kracht gelden voor studenten die momenteel staan ingeschreven. Alsof dat niet genoeg is, worden instellingen per “langstudeerder” ook nog eens € 3.000 gekort. Hiermee hoopt de overheid nog eens 0,40 miljard structureel te bezuinigen. Als laatste wordt de terugbetalingstermijn voor studieleningen met vijf jaar verlengd naar 20 jaar. Dit zou goed moeten zijn voor een bezuining van 0,01 miljard euro: in totaal net iets meer dan een half miljard euro. Dit klinkt als een hoop geld, maar is in werkelijkheid nog geen 3% van de geplande bezuinigingen. Toch kunnen de gevolgen van deze “kleine” maatregel grote 22 gevolgen hebben voor de gewone student.

Straf de universiteiten En dat niet alleen voor de student, de universiteit levert per langstudeerder dus € 3.000 euro in, waardoor er een financieel tekort ontstaat dat kan oplopen tot meer dan vijftien miljoen euro en dat op een begroting van 280 miljoen (in het geval van de UT). Het gevolg is dat er ook door de universiteiten bezuinigd moet worden en we over een paar jaar te boek staan als “Onderwijsfabriek Twente”. Geld is echter niet het enige probleem. Wat te denken van de instroom bij de bètastudies? Eerstejaars kijken wel uit om de als zwaar te boek staande technische studies te gaan doen wanneer ze bij studievertraging een boete boven het hoofd hangt. Dit gaat lijnrecht in tegen het beleid dat de 3TU de laatste jaren gevoerd heeft. Technologie is volgens de UNESCO de belangrijkste stimulerende factor voor toekomstige economische ontwikkeling en de technische sector kampt nu al met een structureel tekort aan gekwalificeerd personeel. Kortom, een dergelijk kortzichtig plan ondermijnt de kenniseconomie. Bovendien zijn het de hogeropgeleiden die later de meeste belasting mogen betalen en zo de schatkist spekken. Straf de actievelingen Niet alleen eerstejaars zullen bij zichzelf te rade gaan, ook hogerejaars studenten zullen wel een tweede keer nadenken voordat ze actief worden bij bijvoorbeeld een studievereniging, zeker nu de harde knip en het bindend studieadvies ook ingevoerd worden. Dit wordt verergerd doordat de overheid geen onderscheid zal gaan maken in de reden van studievertraging (welke overigens ook door ziekte kan worden opgelopen). Deze studenten krijgen zo geen kans om klaargestoomd te worden als de bestuurder van de toekomst. Een bezorgde studievereniging omschrijft dit als het kweken van “magnetronstudenten”: snel klaar, maar smakeloos. Leve de hypocriete babyboomers Kortom, dit plan van ondermeer Maxime Verhagen, die zelf 11 jaar studeerde voor het luttele bedrag van 4.000 euro (zijn boete zou € 37.000 bedragen), laat weer goed zien hoe wereldvreemd het kabinet is. In 1998 verweet Pim Fortuyn in zijn boek “babyboomers” zijn generatiegenoten dat ze onverantwoordelijk omgingen met de maatschappelijke instituties waarover zij het beheer kregen. Deze keer is het de beurt aan deze Pim om jullie op te roepen hier wat aan te doen: http://www.minimaalnominaal.nl/

Column Geert Folkertsma Ik ben naar de kapper geweest. Ik houd niet zo van naar de kapper gaan, bovendien heb ik liever te lang dan te kort haar. Ik ga dan ook niet zo vaak. Als ik ga, doe ik dat in mijn geboortedorp, want daar kent de kapper mij en hoef ik niet uit te leggen wat ik wil.

De vorige hond heeft me in mijn gezicht gebeten toen ik 2 was en naar hem toekroop om te aaien. Ik hield er een litteken en een houten speelgoedvliegtuig aan over. Die hond is sinds een paar jaar dood, dus daar hebben we het niet meer over. Het vliegtuig is op een paasvuur beland.

“Hallo, Geert.”

“Je haar is een beetje droog. Wat voor shampoo gebruik je?”

“Dag Jan.”

“Gewoon van Andrélon of de Kruidvat.”

Ik neem plaats aan de wacht- en leestafel, want gemiddeld is hij een kwartier na de afgesproken tijd pas klaar met de vorige klant. Tussen de Glamour en Cosmo (of hoe heten die bladen) zoek ik naar een Donald Duck om de tijd te doden.

“Dat is niet goed voor je haar. Je kunt beter één met conditioner gebruiken. In die goedkope zitten alleen maar zouten.”

“Het is alweer een tijdje geleden hè?”

Toen ik klein was ging ik hier ook heen, totdat mijn moeder een tondeuse kocht. Vanaf de middelbare school mocht ik weer naar de kapper, voor een “hip” kapsel. Ik staarde tien minuten naar een boek met foto’s van mooie mannenhoofden met sportief haar, en wees uiteindelijk één aan. Een halfuur later kwam ik thuis, zonder sportief kapsel, maar met een scheiding. Ik had zo hard mogelijk gefietst in een poging hem eruit te laten waaien, maar dat was mislukt. Het huilen stond me nader dan het lachen.

“Ja, er mag flink wat af.” Ik ben niet ijdel; het kan me niet zoveel schelen hoe mijn haar zit. Ooit werd ik door een huisgenoot van een vriend uitgemaakt voor “derderangs paupertuig”, alhoewel dat geloof ik meer ingegeven werd door de legerjas die ik vaak draag. “Waar studeer je nu?” “Enschede.” “Bij de Technische HT.” De eerste paar keer legde ik uit dat het tegenwoordig de UT heet, maar het blijft niet hangen. “Ja.” Even later: “Hé, je hebt grijs haar.” Hier komt hij ook elke keer opnieuw achter. Ik mompel instemmend, waarna hij een grap maakt over zorgen, of hard studeren. De eerste paar keer riep hij zijn vrouw (ook kapper) erbij om het te laten zien, maar ik geloof dat zij het intussen wel weet. Mijn kapper is niet intelligent. Dat hoeft ook niet, als hij maar knipt, maar het maakt hem niet tot een interessante gesprekspartner. Na de standaardvragen (Hoe is het met je broertje? Waar studeert je zus ook alweer? Zit je nog bij de muziek?) valt het meestal stil. Of we hebben het over zijn hond. Dat is een dankbaar onderwerp, want daar weet hij veel vanaf. Het is tenslotte zijn hond.

Wat kan mij die shampoo schelen? “Ik ben maar een arme student.”

“Je moet eigenlijk wat vaker komen, dan blijft het model er beter in zitten.” Hij zou het moeten toejuichen dat ik nog naar hem kom! Bovendien vind ik eens in de paar maanden wel genoeg. “Ja, dat zou beter zijn hè?” Na een halfuurtje is het klaar, en alhoewel hij de scheiding nog af en toe probeert, is het resultaat prima. Wel verdenk ik hem ervan het telkens iets langer te laten, wellicht in de hoop dat ik eerder terugkom. Bij het afrekenen probeert hij me soms een conditioner, mousse of iets dergelijks aan te smeren. Vast met de beste bedoelingen, maar ik bedank. Laatst kreeg ik een proefflesje mee, maar ik weet niet meer waar dat is. In het klantensysteem wordt bijgehouden wie ik ben en wanneer ik ben geweest. Bij het afrekenen hoor ik altijd wanneer de laatste keer was. “De vorige keer is al 17 weken geleden.” Ik heb een nieuw record.

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Bachelor’s assignment: droplets Pim Muilwijk Finishing AT includes doing the Bachelor’s assignment; as was, is and will be the case for most AT students. Most of the time this means doing some research in an area you think is interesting, writing a report about it with some unpronounceable, long and often meaningless title and a conclusion which basically says: “At first I didn’t know how to start and now I know how the experiment should’ve been performed”. Depressed already? Don’t be, you’ll probably learn a lot from it, as I did. If you paid ATtention while reading the table of contents of this ATtentie, you will have noticed that this is not the only Bachelor’s assignment in this issue. Geert also wrote a very nice article about his assignment. While my assignment didn’t include playing around with toys and nearly chopping of the heads of innocent bystanders, it can at least fill some of the pages of this periodical. Physics of Complex Fluids When I was looking for a Bachelor’s assignment I had only two prerequisites: the assignment had to be interesting and it should have nothing to do with my master (Nanotechnology). The reasoning behind this was that I would encounter enough nano-related subjects in my master’s assignment and I wanted to see if I could cope with a field which I wasn’t very good at. Therefore I ended up at the Physics of Complex Fluids group, which does research in the properties of liquids on scales ranging from a few nanometers to many micrometers. So, in the end I couldn’t totally escape the nanoworld. You can read more about this group at: http://www. utwente.nl/tnw/pcf/ So, I made an appointment with Frieder Mugele, the professor of the group, and discussed some possible assignments with him. I ended up choosing for the assignment which featured droplets on fibres and was immediately introduced to the three PhD students who were doing research in that area: Jolet and Riëlle de Ruiter (yes, they’re twins) and Burak Eral, who seemed very happy to have a student to perform all the tedious measurements for them. Of course this was not the case and they helped me a lot with everything I needed to know and do. They explained to me what they wanted to accomplish and I set to work. However, the Bachelor exam committee was not content with this and demanded a description, which was 24 promptly delivered by my supervisors.

Morphological transitions of a droplet on a fibrous network

Now, at least I had a title which described the work I was doing; observing the changes in the shape of a drop on two crossed fibres. I was the first to research this particular behaviour, since no scientist had ever been bothered by it. Later on I would learn that a German group was performing some numerical calculations, which will also be mentioned further on in this article, but I could scoop the practical work. This also meant that I had to think of an experimental setup and everything that goes with it. Luckily, my supervisors already had some experience producing fibres and methods for measurements on single fibres, so I only needed to expand on these methods and perform the experiments. This was, of course, easier said than done and I ended up spending the first few months on preparations. There was, for example, a certain procedure to make fibres with a Teflon layer (which has very beneficial properties), which involved cleaning the fibres with different chemicals for about an hour, dipping them in a Teflon solution and then baking them in an oven, a process which I referred to as “baking cookies”. This whole procedure would take up the better part of a day and could, so it turned out, suddenly stop working. Whenever I would do a measurement on these fibres they would break down and I would have to start all over again. After about a month it became clear that the workshop which provided the stainless steel fibres had run out of steel wire and had started using a different type of wire which wasn’t compatible with our procedure. Therefore, we had to come up with a new one, which took us another few weeks. In the end we solved it, but it is these little things that can severely upset your work. This little inconvenience cost me a lot of time, which should’ve been used for actual measurements, so I had to make up for it in the summer vacation. And so I found myself sitting in the laboratory on a quiet Saturday when I turned 22, thinking of the sacrifices I was making for science. It turned out that this particular Saturday was very well spend and that the data I obtained would be enough to write half my report about, so no regrets there. Of course I also spend a lot of time on writing the report, which has been summarized in the following pages. The full version, which includes all the omitted references, can be acquired by emailing the author: [email protected].

Bachelor’s assignment Introduction While the world of wetting takes place on a relatively small scale it has a big impact on our daily lives. Often unknowingly or unaware we use specially coated appliances and detergents that make our lives easier. Even in nature we can find beautiful designs, such as the Lotus flower, with its super hydrophobic leaves, which are applied today for creating self-cleaning surfaces. My research will go into the behaviour of droplets on crossed fibres, which is relevant for the textile industry, because most textiles consist of crossed fibres. It is also relevant for cleaning up oil spills, such as the big one British Petroleum dropped in the Gulf of Mexico, because they often use sponge-like materials in the cleaning process. In fact, the day after my Bachelor’s presentation, a delegation of BP visited the PCF group, where my research was also mentioned.

With σsv, σsl and σlv for the surface tensions of the solid-vapour interface, the solid-liquid interface and the liquid-vapour interface respectively and θY for Young’s equilibrium contact angle. When a voltage is applied over the droplet a diverging electric field is created at the contact line (figure 2). This causes charges to accumulate at the contact line and the drop is pushed down, decreasing the contact angle. This is also the reason that we tend to use fluoropolymers like Teflon or other materials which display a large contact angle, because electrowetting can only reduce the contact angle. In the case of Teflon, the nonpolarity makes it hydrophobic and since fluorine has a very high electronegativity it reduces the polarizability, mitigating the London dispersion forces, making it lipophobic as well. The result is that liquids have a hard time wetting a Teflon coated material and will show a large contact angle.

We already know how droplets on a single fibre behave as a function of contact angle and volume (relative to the fibre radius), but there has been no research into the behaviour of crossed fibres. Therefore, the behaviour of a drop on two fibres with an angle with respect to each other will be investigated. Electrowetting Maybe you have already heard of wetting or electrowetting, but in case you haven’t I will explain both phenomena briefly. When a liquid and a solid surface are brought together, intermolecular interactions arise. The force balance (shown in figure 1) between these forces determines the amount of contact that is maintained between the solid surface and the liquid. The ability of the liquid to maintain contact with the surface is referred to as wetting.

Figure 2: A droplet at 0 V (dotted) and at an applied voltage (solid). Also note the diverging electric field. [1] This behaviour is captured in the electrowetting equation, which is basically Young’s equation with an added capacitance to compensate for the electrostatic forces:

With θ the observed contact angle, ε0 and εd the dielectric constants of vacuum and the dielectric respectively, d the thickness of the dielectric layer and U the applied voltage. The added term is sometimes called the electrowetting number η and measures the relative strength of the electrostatic and surface tension forces.

Figure 1: The force balance of a droplet on a surface. When the system is in thermodynamic equilibrium all forces in the horizontal direction must also be in equilibrium. Setting the sum of all forces equal to zero and rearranging leads to Young’s equation, which relates the interfacial energies to Young’s equilibrium angle:

Now, the cosine of the contact angle can be plotted against the electrowetting number, which is called an electrowetting curve. From this curve we can obtain the thickness of the dielectric layer and Young´s equilibrium contact angle. Also, when we know these constants we can linearly extrapolate to obtain contact angles for a given voltage. At high voltages the linear behaviour will break down, which is called contact angle saturation, but in my experiments the voltages stayed well below the critical voltage after which contact angle saturation occurs.

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Bachelor’s assignment

Figure 4: A series of images of a drop of silicone oil on a single fibre with applied voltage. Behaviour on single fibres Experimental setup When a drop of oil is placed on a conducting fibre which is coated with a dielectric material, its contact angle can be increased by applying a voltage. This is in agreement with the electrowetting equation, since as the liquid and vapour phases are reversed, the electrowetting number changes sign. At zero voltage, the contact angle is small and the drop will adopt the axisymmetrical barrel morphology. Upon increasing the voltage and thus the contact angle, the shape of the droplet changes to one resembling a clamshell (leading to the name asymmetric clamshell morphology). In principle, the drop can adhere to any side of the fibre, but in practice it is found that, due to minor imperfections on the fibre, there will be some preferred orientation. When the voltage is subsequently reduced, a transition back to the barrel morphology takes place. The two possible morphologies are sketched in figure 3.

The experiment was set up around the existing Dataphysics OCA30 system, which was build for contact angle determination and features a stage with 3 degrees of freedom, which is perfect for mounting a cuvette. The situation around the cuvette is sketched in figure 5.

Figure 3: Droplet morphologies on a fibre. [2]

Figure 5: A sketch of the experimental setup. I fixed one fibre to the cuvette and suspended the other from an electrical clamp which I could rotate to change the angle between the fibres. The electrical clamp also served as electrical connection. I also fixed an electrode to the cuvette which ran to electrical ground. Then, I filled the cuvette with Milli-Q water, which is purified and deionized water, and placed a droplet of silicone oil on the junction. To determine the contact angle, the OCA camera and an additional PCO PixelFly camera were used to record a side-view and a head-on view for each fibre. It should be noted that the head-on and side-view for the suspended fibre are semi head-on and side-views for fibre angles other than 90°, because of the rotation of the fibre. The actual contact angle determination is done using MATLAB scripts, which scan the pictures of the OCA and PixelFly using a threshold value to determine the drop profile after which the contact angle is obtained from a linear fit at the contact line.

Since the two fibres setup is actually a combination of two single fibres I expected to see transitions similar to those on single fibres. Therefore I first wanted to observe and confirm the transitions on a single fibre, which are shown in figure 4. At 0 V the drop is neatly centred on the fibre. At 110.7 V the drop is already off-axis and at 113.5 V the barrel-to-clamshell transition takes place. After the transition the voltage is decreased and at 39.5 V the drop almost engulfs the fibre again. At 22.6 V there is only a tiny deformation left and at 19.8 V the clamshell-to-barrel transition takes place. Now, the applications become more clear, since a droplet in clamshell morphology adheres less to a fibre than a droplet in barrel morphology and is therefore easier to detach.

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Bachelor’s assignment

Figure 6: Two fibres with an angle of 90 degrees with to each other. Behaviour on crossed fibres When the voltage is increased the barrel-to-clamshell Since there is not nearly enough space to showcase all transition will take place on both fibres simultaneously my results I will just show and explain one picture and and when the voltage is reduced the clamshell-tosummarize the remaining data. If you are interested in barrel transition will also take place simultaneously. Therefore, I called this the “Simultaneous transitions” the other results you will have to read the report. region, because both transitions took place at the same Figure 6 shows the dependence on the contact angle voltage. Decreasing the junction angle below 45° shows for a drop on two fibres with an angle of 90° with respect some very different behaviour. When the junction angle to each other. The odd rows represent the side-views of is about 40° the drop initially engulfs both fibres and the first fibre and the semi head-on views of the second when the voltage is increased the barrel-to-clamshell fibre, while the even rows represent the semi side-views transition takes place on the first fibre, followed by of the second fibre and the head-on views of the first the second fibre at higher voltage, however, when the fibre. Transitions are marked with a red asterisk and the voltage is decreased the clamshell-to-barrel transition volume of the drop is estimated at about 30–35 micro- only takes place on the first fibre and the droplet will litre. We can see that upon increasing the voltage the descend between the fibres, positioning itself between barrel-to-clamshell transition takes place on the first the fibres rather than engulfing them. When the drop fibre, followed by the second fibre at higher voltage. is back at zero voltage it also forms a smaller lobe on When the voltage is decreased, the clamshell-to-barrel the opposite side of the big lobe. Therefore, I called this transition takes place on the second fibre, followed the “Starting of lobing” region, because it started out as by the first fibre at lower voltage. Therefore, I called an engulfing drop and ended up as two lobes; a bigger this the “Separate transitions” region, because of the and a smaller one. When the junction angle is reduced visible separate transitions. When the junction angle is to about 30°, the drop is divided in a big lobe at one decreased to 70° the behaviour will be similar, but there side of the junction and a smaller lobe at the opposite are some minor differences. Again, upon increasing side of the junction. Upon increasing the voltage the the voltage the barrel-to-clamshell transition will take barrel-to-clamshell transition will take place on the first place on the first fibre, followed by the second fibre at fibre, but it is barely noticeable. Further increasing the higher voltage. When the voltage is reduced however, voltage leads to the drop rising out of the junction in a the clamshell-to-barrel transition will take place on continuous fashion. Upon decreasing the voltage, the both fibres simultaneously. Therefore, I called this the lobe will descend between the fibres again and “semi-simultaneous transitions” region, because one the clamshell-to-barrel transition will take transition took place at the same voltage. This trend place on the first fibre. Therefore, I called 27 continues when the junction angle is reduced to 50°.

Bachelor’s assignment this the “Lobing behaviour” region, because the droplet starts out as two lobes. Decreasing the junction angle to 10° presents radical different behaviour than observed before, because the drop tends to form a liquid bridge between the fibres rather than engulfing them. Instead of one big and one small lobe, two equally large lobes are formed on either side of the junction. Increasing the voltage leads to the lobes rising out of the junction in a continuous fashion, whereas decreasing the voltage again leads to the lobes descending between the fibres in a continuous fashion. Therefore, I called this the “Dual lobes” region, because of the two equally large lobes. Finally, when we reach a junction angle of 0°, that is, parallel fibres, we observe the droplet forming a liquid bridge between the fibres again, however, since the droplet cannot rise out of the junction this time (its movement is impaired by the fibres) it bulges out to the side upon increasing the voltage. Decreasing the voltage does the exact opposite. Therefore, I called this the “Bulging” region, because the droplet is observed to bulge.

Discussion

When I calculated the contact angles of the drop at the barrel-to-clamshell and clamshell-to-barrel transitions I noticed something interesting; the behaviour of the drop is independent of the junction angle for large junction angles (> 45°), as shown in figure 7. For angles smaller than 45 degrees the barrel-to-clamshell and clamshell-to-barrel transitions are either barely noticeable or vanish entirely. Instead, there will be lobes which rise and descend continuously upon increasing and decreasing the voltage.

Another point for discussion was the “Starting of lobbing region”. This was the region that featured a drop engulfing both fibres at first and then after the voltage ramp ended up as a lobe, rather than as an engulfing drop again. The reasoning was that when the voltage ramp is applied again the drop will behave as if it is in the “Lobing behaviour” region. Therefore, it is probably best to think of the “Starting of lobing” region as an unstable form of the “Lobing region”.

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The big question that arose right after the measurement data was processed was whether the separate transitions were a result of the differences in the properties of the fibres, or of the symmetry breaking nature of the transitions. A case for the first explanation can be made by looking at the properties of the fibres of which the junction was composed. Therefore we made electrowetting curves of the single fibres and determined the thickness of the dielectric layer. It turned out that the dielectric layer of one fibre was 5 times as thick as the other fibre and as a result the transitions took place at a higher voltage on the fibre with the thickest dielectric layer. When we compare this data to the crossed fibre experiment we found that the transitions also took place first on the fibre with the thinnest dielectric layer, thus confirming the influence of the properties of the fibre. Then again, it could well be that, even when the properties of both fibres become nearly identical, there are still separate transitions because of the symmetry breaking nature of the transitions themselves.

Figure 7: The contact angles of the transitions are independend of the junction angle for α > 45°.

Bachelor’s assignment

Figure 8: Numerical calculation of two fibres at 30 degrees. The big and small lobe correspond with experiments. [3] There were also some encouraging results. A German group performed numerical calculations on crossed fibres and it turned out they had looked at two fibres with an angle of 30° with respect to each other (figure 8). This corresponded nicely with the behaviour I observed for two fibres with an angle of 27° with respect to each other. Recommendations As I stated at the beginning of this article, you start out not knowing what to do and you end up describing how the experiment should have been performed. Even though I got good results, I can recommend using the electrowetting curves of the single fibres from which the junction will be composed for extrapolating the contact angle and determining the properties of the fibre, because not only is the contact angle of the droplet at the transition independent of the junction angle for large junction angles (>45°), it is also the same value as for the transition on a single fibre. Furthermore, it should be informative to look at fibres with matching properties, to determine the cause of the different transitions. One aspect, which was not considered, in my experiments, was the drop volume, which could have an effect on the behaviour. Therefore, the experiment should also be performed with different drop volumes.

Junction angles below 45° display interesting behaviour that is not observed on single fibres, such as lobes and liquid bridges. Therefore, it would be interesting to repeat the experiments for more junction angles. Finally, since the aforementioned German group is already performing numerical calculations of drops on crossed fibres, they could as well do some calculations with the parameters I have used in my experiments. This would be an easy and quick method of checking how well the theory agrees with the observed behaviour. One last recommendation for everyone who still has to begin or is still working on their assignment: do not forget to enjoy the opportunities you are given! [1] F. Mugele, “Fundamental challenges in electrowetting: from equilibrium shapes to contact angle saturation and drop dynamics,” Soft Matter, vol. 5, no. 18, pp. 3365 - 3528, 2009. [2] H. B. Eral, J. de Ruiter, R. de Ruiter, J. M. Oh, C. Semprebon, M. Brinkman, and F. Mugele, “Drops on functional fibres: from barrel to clamshells and back,” To be published, 2010. [3] C. Semprebon, O. Claussen, and M. Brinkmann, “Notes on two fibers,” To be published, 2010.

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Black holes Jeroen van den Berg Our universe is filled with strange objects and phenomena. One of the most curious objects we can find is the black hole. Contrary to what the name might suggest, a black hole is not a hole at all. It is a very compact and massive object, so massive that even light cannot escape its gravitational field when in close proximity. Therefore black holes appear completely black (as in the absence of colour). Thus we cannot see a black hole directly, however due to its strong gravitational field its presence can be detected and it produces some strange phenomena. Although research has been done on black holes and their properties in recent times, there are still some unanswered questions. To understand what a black hole really is one first has to get a bit acquainted with the concept of spacetime, introduced by Albert Einstein with his theories of special and general relativy. Spacetime is a mathematical concept of, as I would describe it, the “frame” in which our universe exists. This frame consists of four dimensions, three spatial dimensions and one time dimension. According to Einstein’s theory of general relativity, gravity can be described as the curvature of this spacetime, the slope of this curvature can be seen as the strength of the gravitational field. Thus masses, which produce a gravitational field, curve spacetime in a way as represented in figure 1.

Figure 1: A two-dimensional analogy of the curvature of spacetime by a planet. Notice the white lines do not represent a curvature of space itself, but rather a coordinate system imposed by the curved spacetime.

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Einstein developed his theory of general relativity as a refinement of Newton’s theory of gravity, since Newton’s theory could not accurately describe the motion of planets and the behaviour of galaxies. Furthermore, there is no time dependency in Newton’s treatment of gravity, meaning its effects exist everywhere instantly. This is inconsistant with Einstein’s theory of special relativity, which states that all forces, even gravitational forces, cannot be transmitted faster than the speed of light. The spacetime curvature threatment of gravity avoids this limitation because gravity is no longer seen as a force propagating between bodies. The birth of black holes Einstein developed his general theory of relativity (GR) to better describe gravity (this theory won’t be treated in this article since it is far to complex), the result was a set of ten differential equations that describe how mass and energy curve spacetime. These equations are known as the Einstein Field Equations. The object of study in GR is the so-called metric tensor, which describes the local geometry of spacetime:

Here , ds2 is a line element squared and represents the curvature of spacetime; dxα and dxβ are infinitesimal coordinate displacements of local coordinates xα and xβ; were x is a 4-dimentional tensor, 3 spatial + time, with index α or β running from 0 to 3. The curvature is described as a coordinate transformation from xα to xβ, which calls for a factor gαβ, a 4x4 matrix, with values of [-1 1 1 1] across the diagonal (lost me already? Don’t worry, I don’t understand it either). Karl Schwarzschild came with a solution for this metric in the case of a stationary spherical body of mass or a point mass. The result was in spherical coordinates:

G is the gravitational constant, c the speed of light, m the mass of the object and r the radius of the object. As can be seen, something funny happens when r becomes 2Gm/c2 . The radial dependence becomes infinite and the time dependence becomes zero. This is called a mathematical singularity, in this case a gravitational singularity. In spacetime it can be represented as in figure 2. This gravitational singularity is called a black hole and the radius at which the singularity occurs is called the Schwarzschild radius rs.

Black holes

Figure 2: A two-dimensional representation of a gravitational singularity in spacetime, i.e. a black hole. Schwarzschild was puzzled by what this mathematical singularity would mean; eventually he regarded it as meaningless, using a simplified model of a star and showing that it would require an infinite pressure gradient to compress the star to its Schwarzschild radius. Many scientists also showed that it is possible to rewrite his solution with a coordinate transformation that would eliminate the singularity. Einstein himself, however, was not so sure about the non-singularity of the solution and could not stand the idea. He spent several years trying to prove that the singularity did not exist. Eventually he published a paper in which he describes a system consisting of many rotating masses with his own theory of relativity. He argued if such a system could collapse under its own gravity and form a stable star with a radius equal to the Schwarzschild radius, that it could not be stable because then the masses had to be rotating faster than the speed of light when reaching the Schwartschild radius: therefore black holes could not physically exist. Although Einstein’s calculations are correct, his point is irrelevant: it does not matter if the star becomes unstable at its Schwarzschild radius, because then it collapses anyway.

is called the Chandrasekhar limit. With the discovery of the neutron Oppenheimer reasoned that a proton and an electron could form a neutron and thus more massive stars could collapse even further to form neutron stars, in which gravity is opposed by neutron degenerate pressure. Again there is a certain limit to the mass of a neutron star for it to remain stable. He used Einstein’s Field Equations and the thermodynamic relationship between pressure and density to calculate this mass limit, the so-called Tolman–Oppenheimer–Volkoff limit (TOV limit), which is 1.5 to 3 solar masses (a solar mass is the mass of our sun). After this limit it might be possible for the star to become something like a quark star depending on quark degenerate pressure, though this has never been confirmed theoretically or experimentally. Oppenheimer became curious as to what would happen when a star overcame all limits and collapsed past its Schwarzschild radius. After tedious calculations a student of his showed that as the star is collapsing it would seem to an observer from a distance that time is going slower on the star, until at the Schwarzschild radius time has stopped. Thus the star is frozen in this state. It also becomes less and less visible because due to the strong curvature of spacetime the wavelength of the emitted light gets larger and larger, or is redshifted, that is why we can’t see these frozen stars. An observer standing on the surface of the star would not notice anything from the time delay.

Formation of a black hole During the same period that Einstein tried to disprove Schwarzschild’s singularity, J. Robert Oppenheimer studied the collapse of stars. Stars are formed by gravitational collapse of a cloud of interstellar matter. Because the gas is compressed the temperature rises until it is high enough to begin nuclear fusion, freeing energy. This causes a thermal pressure gradient, which causes expansion, compensating for the gravitational pressure, which causes compression. Eventually the star is burned out and gravitational collapse resumes until gravity is opposed by electron degeneracy pressure, caused by the Pauli Exclusion Principle. The result is a white dwarf. There is a limit to mass that allows a white dwarf to be stable. This limit

Figure 3: The ergosphere of a rotating black hole, caused by frame dragging

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Black holes Physical properties of black holes Black holes are characterized by an event horizon. The event horizon is the boundary at which not even light can escape its gravitational field. For stationary black holes, the event horizon is the Schwarzschild radius. This is a bit different with rotating black holes, which have an ergosphere, see figure 3. Inside this ergosphere it is impossible for light or matter to stand still, though it is not impossible to escape from it. The ergosphere is formed due to a phenomenon called frame-dragging. Frame-dragging is caused by a rotating mass dragging the near spacetime with it. The result is that if you want to stay still inside the ergosphere you will need to move faster than light in the opposite direction. As said before, it is not impossible to escape from the ergosphere; it even allows an object to leave with a higher energy than when it entered it, “stealing” rotational energy of the black hole. Black holes also have a photon sphere. At the photon sphere, a photon travelling tangential to the surface of the black hole will be trapped in a circular orbit. This orbit is dynamically unstable: every small deviation from its path in either direction will lead to an increasing deviation. Thus any photon with an inbound trajectory that passes the photon sphere will spiral towards the event horizon, unable to escape. For stationary black holes the photon sphere is 1.5 times the Schwarzschild radius. Other compact objects like neutron stars may also have a photon sphere, if their size is less than 1.5 times their Schwarzschild radius. Observing black holes Stephen Hawking showed that black holes do infact emit a small amount of thermal radiation. This radiation is not a result of the black hole itself but because of virtual particles. According to the uncertainty principle, σx σp≥h/2, there is always an uncertainty in time and space, allowing virtual particals to miraculously appear on a very short scale and for a very small time with an unknown momentum and energy before disappearing again. These particles always appear in pairs, a particle and its anti-particle. Hawking argued that if a pair of virtual particles would appear at the event horizon of a black hole, than one particle would be sucked into the black hole and the other would become real, being emitted outwards. Because the escaped particle became real with a certain energy, and energy has to remain constant, the other particle must have had a negative energy decreasing the black hole’s mass. Thus it appears to an outside observer that the black hole emitted a particle and lost mass. This allows black holes to evaporate, small black holes evaporate much faster than large black holes. When absorbing material, black holes also 32 form so-called accretion disks, see figure 4.

Figure 4: Accretion disk caused by a black hole accreting matter from a nearby star: notice energy being propelled at the poles. The tendency to form such discs is because of the conservation of angular momentum. Due to friction the angular momentum moves to the outside of the disk allowing the matter to fall further inwards. As a result they release potential energy increasing the temperature of the gas. Sometimes the temperatures get so high that the black hole will emit vast amounts of radiation at its poles. Black holes also cause gravitational lensing. Gravitational lensing is the effect that light beams get deflected because of the strong curvature of spacetime close to the black hole. As a result light gets bend around the black hole, creating strange optical phenomena, see figure 5.

Figure 5: Light being bent around a black hole due to gravitational lensing. There are other strange phenomena caused by black holes and we still do not know what happens inside a black hole, what happens with entropy, information? These are still questions that need to be anwsered.

After Kick-In Jelmer Boter The Kick-In is far behind us, the first five lecture weeks are history, so it’s time for a weekend full of fun: the Astatine After Kick-In Program (formally known as ”Na-Intro”) in de Lutte, almost Germany. This awesome weekend starts on Friday the 1st of October with a 17 km bicycle tour to our residence for this weekend: a farmhouse in ”de Lutte”, between Oldenzaal and the German border. The only things present here is running water and a roof above our heads. A primitive weekend is ahead of us! On our way we made a pitstop at the McDonald’s to have dinner: burgers made of plastic and limp fries. The first evening the NiCat planned a smuggling trip. Together with Wessel, Alexander, Guus and Hein I was dropped near ”het Lutterse Zand”. We had to bring a crate of Grolsch ”beugels” filled with water back to the farm. Each time we were hunted down we lost three bottles and for each bottle we returned to the farm we received a free beer! Our route took us through the forest, through cornfields, over ditches and in trenches. All in total darkness, some of us could not see anything. Despite all efforts and thorns in our flesh we lost three of our bottles, but we were the first group which reached the farm and could drink a well-earned cool beer! After some more beers the Friday was over and we went to our beds to sleep a bit. A new day during the ”Na-Intro” is started with the fantastic song ”Wakker met een wijsje” from ”Kinderen voor Kinderen”. So was this Saturday in the early morning.

It is a happy song, but for some strange reason most people don’t wake up very happily while hearing it. With a day filled with sports, the program for this Saturday was different then previous years. In teams there was a battle in different sports and activities, e.g. football, a crate race, opening a bottle of beer with nothing but a sheet of paper and other beer games. As a member of the board of Astatine I had other plans for this Saturday. Accompanied by Alexander and Wessel I went to Oldenzaal to buy punishments and other nice stuff for the cantus of the evening. The Senate consisted of Alexander as ”Schachtenmeester”, Wessel as ”Abactis”, Monique as ”Procantor” and myself as ”Senior”. With our supervision the participants of the After Kick-In Program drank beer, sang songs, drank more beer and got punished for their misbehaviour. The rest of the evening was filled with talking, smoking a cigar, playing games and a lot more. It was late at night by the time everybody was in their beds for some hours sleep on their air mattress in a cold barn. The next morning was started again with that wonderful song and a lot of coffee. After waking up, grabbing your stuff, cleaning, some hanging around and without ”ransbal” it was time to start the bike trip all the way back to Enschede. Here some took a bath in the Aquadrome to end the weekend. Despite of the fact that there were only a few participants, this years the After Kick-In Program was a success. NiCat, thank you for organising it!

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Injection moulding Jochem Giesbers Many plastic products are made through injection moulding (Dutch: spuitgieten), which is a manufacturing process in the industrial world. This process is often used with thermoplastic and thermosetting materials, but is also used for manufacturing small, complex metal parts (see the section about MIM). This article mainly covers the process for plastics. In order to fully describe the separate components of the injection moulding process, a basic understanding of the process itself is essential. Figure 2 shows a simple, schematic view of the injection moulding machine. The process starts with plastic granules (Dutch: korrels) being poured into the machine. The granules are fed to a heated barrel in which they melt and are mixed by a reciprocating screw. The screw forces the melted plastic into the mould cavity in which the plastic hardens and forms the product. This process is repeated over and over again as the mould cavity has to be emptied after every cycle. Plastic granules There are many different plastics that can be used in injection moulding. Examples are polyvinyl chloride (which includes many compounds), polyethylene, acrylonitrile butadiene styrene and polypropylene. These raw materials are polymerized and refined by specialized companies and then shipped as granules to companies that use injection moulding. These granules usually have the shape of small cylinders (see figure 1) and are shipped in huge cubic meter bags.

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Figure 1: Black polypropylene granules Circa 70 percent of the granules comes from recycling. Without recycling, these plastics would pose a major threat to the environment. In fact, many plastic granules escape the plastic production process and find their way into the ocean. In the United States more than 27 million metric tons of granules are manufactured. Even a small fraction of this is enough to severely pollute the oceans.

Figure 2: Basic injection moulding machine

Injection moulding Melting the granules When the granules have been shipped to the plastics company, they are pumped into a small chamber that holds one batch of granules that fits in the barrel. Once a new cycle starts, the batch falls through the hopper into the barrel and is compressed towards the nozzle. The screw forces the granules to scrape along the barrel. The resulting friction and the heating elements around the barrel cause the granules to melt into a homogeneous fluid material. Once the material’s pressure (depending on the product, but usually between 20 and 200 MPa) and homogeneity have the right value, the nozzle opens. Injecting the plastic As soon as the nozzle opens, the material will enter the mould cavity under the high pressure stated before. The mould itself is pressed together in order to keep it perfectly sealed. This is usually done with a force between 50 and 2000 kN. Once the mould cavity has been filled with hot, liquid material, the cooling process starts. Plastics have a high thermal resistivity, so cooling down is the main part of the injection moulding cycle. During the cooling process, the mould is still pressed together. This measure reduces the effect of crimp. Water or oil is pumped around the outside of the mould, as a cooling mechanism.

When the product has solidified, the mould opens and the product is pushed out and falls onto a conveyor belt (usually it first lands in a pool of water for further cooling). At that moment a new cycle of the injection moulding process will start. Defects Designers of products that are to be produced with injection moulding have to take into account that certain geometrical features can lead to defects. For example, it is very important that the wall thickness in products is consistent and shows no large changes. It will cause the product to shrink (crimp) unevenly during cooling, creating a so-called “sink mark”. This can result in a section of the product intended to be straight becoming curved, which most likely will result in the part being incompatible with other components. During the cooling process, internal stresses will arise. The designer has to take these stresses into account or, in the worst case, cracks will form in the material and the product can be considered a failure. Two component moulding Usually it is interesting to have different material properties in a product, like adding a soft surface to a stiff object. In this case, first the material for the outer surface is injected, then the material for the core and lastly the material for the surface again to close the nozzle with. This type of two component moulding is called “sandwich moulding” or “coinjection”. The main advantage of this method is that it requires just one injection moulding machine. Even though two injection units and a more complex mould are neccesary, the benefit of a single machine saves a company a lot of money. Working in an injection moulding factory

Figure 3: A simplified injection moulding diagram During injection there are two very important physical quantities that limit the process: the pressure and the temperature. The diagram above shows a qualitative representation of these limits. If the plastic granules’ temperature is too low, they do not melt, if the temperature is too high, the plastic decomposes. If the injection pressure is too low, there will be a so-called “short shot”, meaning the mould cavity is not completely filled with liquid plastic. Increase the pressure too much and a “flash” is likely to occur: the mould is forced to slightly separate, causing a burr at the surface of the final product.

Injection moulding machines have a lot of variables and a single error usually shuts down the whole machine. Employees in charge of the machines, simply called moulders, have to be present at all times. Malfunctions can usually be solved with ease, but it is a job that needs to be done or a lot of revenue will be lost. I myself have worked at a company producing PVC pieces that connect pipes. It is quite interesting to see how such a company operates and to be actually part of it, instead of simply visiting it with, for example, your student association. So if you ever get the chance to work temporarily in such a production company, take on that job!

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Puzzle

The last pages of every ATtentie are dedicated to a puzzle with which our readers can enjoy winning a gift voucher for a movie theater.

This issue, I thought of a brand new game, which I happened to chance upon by reading a curriculum vitae of an AT teacher.

At the moment there is only one person who submitted the colouring picture. So, if you’d like to have a go at scoring that voucher with a high probability of winning, please submit your artistic outbursts to attencie@ astatine.utwente.nl.

On the following page you will find a, not so high resolution, image of this well-known AT teacher. Our question of course is now:

The correct solution for the previous problem, which was already mentioned in the previous issue of the ATtentie, was of course: “zandzeepsodemineraalwatersteenstralen“. The winnier, by lot, is Guido Boerboom. As you can see he is very happy with his price.

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Can you tell us who this handsome gentleman is? This time the teacher in question can play along, because we found the picture. We’d like to encourage other teachers to send their pictures as well to keep this game alive. If you send us a picture of yourself as a student we will reward you with a free Astatine mug!

Puzzle

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Stel je voor: de magie en kracht van de elementen, beheersbaar gemaakt tot nut van de mens. Het is dichterbij dan je denkt. Want morgen is vandaag en dat vraagt om nieuwe toepassingen. Bijvoorbeeld door commerciële technologie aan te wenden voor geneeskundige doelen. Onze gascentrifugetechnologie is op verschillende manieren inzetbaar. Bijvoorbeeld om efficiënt uranium te verrijken waardoor verrijkingsfabrieken snel en veilig brandstof kunnen leveren voor de productie van kernenergie. Minder bekend is dat op dezelfde manier ook stabiele isotopen worden geproduceerd. Hiermee kan de medische wetenschap kanker onderzoeken en bestrijden. Alles draait om de behoeften van de moderne mens. Talenten met een passie voor complexe technologie die ons leven veraangenaamt kijken op thefuture has arrived.nl. Nu, niet morgen.