# Tag: popular science

## TEDxUHasselt: Virtual Lab

Saturday March 20th at 19h00, I have the pleasure of speaking at the TEDxUHasselt 2021 event.

We will visit the virtual lab and I’ll dive into the important questions about computational researchers: Who are we?  Why do we like supercomputers…and rubber ducks? And what does the future hold?

Along the way, I’ll touch on the use of computational research for any subject imaginable: from atoms to galaxies, to the spread of diseases as well as opinions.

Update 22/03/2021:
The entire live-stream is still online available at here. (My presentation starts at ~7’30 😉 ).

## COVID-19 in de groep?

België staat de laatste dagen zowat op zijn kop ten gevolge van de huidige corona-crisis. De cijfers schieten als een pijl de hoogte in, en geen van de tot nu toe genomen maatregelen lijken het tij te keren. Er wordt duchtig gediscussieerd over het L-woord (lock-down), en het lijkt onoverkomelijk. In plaats van een pleidooi voor of tegen te houden, laat ik je zelf beslissen. De situatie in België is immer zo ernstig dat zelfs primitieve benaderingen de situatie al redelijk goed benaderen. Voor nauwkeurige modellen ben je nog steeds aan het goede adres bij de epidemiologen. Deze modellen kun je gebruiken om strategieën te bedenken om dit virus in te dijken en de situatie op termijn te verbeteren. Dit zijn echter vaak vrij abstracte gegevens. Wat we als gewone burger willen weten is eigenlijk gewoon hoe veilig het voor ons is. Hoe groot is de kans dat we in een groep mensen – op het werk, op school, in de bus, of in de sportclub – één of meerdere personen hebben die met COVID-19 besmet zijn?

Als we aannemen dat besmettingen gelijkmatig verdeeld zijn over het land, leeftijdcategorieën en sociale groepen (dit is niet helemaal het geval, maar door het snel groeiende besmettingsaantal wordt dit een steeds betere benadering) dan kan je heel eenvoudig de kans op een aantal besmette personen “n” in een groep van “N” personen benaderen met de formule:

$kans= \frac{N!}{n!(N-n)!} p^{n}q^{N-n}$

waarbij p de kans is dat een willekeurig persoon besmet is, zijnde de besmettingsgraad. Voor een besmettingsgraad van 500/100 000 (waar alle provincies nu boven zitten) is p=0.005. q is de kans dat een willekeurig persoon niet besmet is (zijnde q=1-p). De term met de uitroeptekens (dat zijn “faculteiten”, en die stellen een reeks van vermenigvuldigingen voor, bijvoorbeeld: 4!=4x3x2x1) vertelt ons op hoeveel manieren (combinaties) er n personen besmet kunnen zijn in een groep van N personen.

Dit is allemaal leuk om weten, maar waar het om draait is natuurlijk wat dit voor jou betekent. Laat ons starten met de situatie van enkele weken geleden, toen er 500 besmettingen per 100 000 werden geconstateerd, dan ziet dat er voor groepen tot 50 personen als volgt uit:

De zwarte lijn toont hoe groot de kans is dat er geen enkele besmette persoon in de groep aanwezig is, terwijl de gekleurde lijnen de kans geven voor exact 1, 2, 3, 4 en 10 personen. Voor een schoolklas met 20-25 leerlingen valt dit nog mee, er is “maar” 10% kans dat er 1 of meerdere besmette leerlingen/leerkracht in de groep zitten. Merk op dat dit voor alle klassen van een school afzonderlijk geldt. Voor een kleine school met 6 jaarklassen (en maar 120-150 mensen) is de kans dat er niemand besmet is reeds gezakt tot ongeveer 50%.

Verdubbelen we de besmettingsgraad naar 1% (of 1 000 per 100 000) dan krijgen we dit beeld:

De kans op minstens één besmet persoon in onze denkbeeldige klas is gestegen tot 20%, terwijl de kans dat de kleine school besmettingsvrij is gebleven is ingezakt tot een magere 20%.

Gaan we naar de situatie zoals deze nu is dan hebben we te maken met een besmettingsgraad van ongeveer 3% (3 000 per 100 000). Het plaatje dat we dan krijgen is het volgende:

De kans op geen enkele besmetting in een klas van 20-25 leerlingen is gezakt naar 1 op 2! De kleine school, daar hoeven we ons geen illusies over te maken: de kans dat daar geen besmettingen zijn is tot nagenoeg nul gezakt.  In een specifieke klas is het intussen ook realistisch geworden dat er meer dan 1 leerling besmet is. Bij deze besmettingsgraad is er zelfs een flinke kans (>10%) op een besmet persoon in een kaartgroep van 4 personen.

#### Deze laatste grafiek is de grafiek van het heden. Dit is de grafiek die je leslokaal, je busrit, je sportvereniging, je kantooromgeving of je werkploeg beschrijft. De keuze is aan ons allemaal om hier onze conclusies uit te trekken. Wachten we op iemand anders om ons te zeggen wat te doen? Of nemen we ons leven en dat van onze familie en vrienden zelf in handen?

Voor wie zelf wil spelen, om bijvoorbeeld de besmettingsgraad van jouw gemeente in te vullen, kan dit met dit excel werkblad, of het online rekenblad (ziet er iets minder mooi uit). Je hoeft enkel de besmettingsgraad aan te passen. De rest gaat vanzelf. Het enige wat je dan nog moet doen, is zelf je conclusies trekken.

## Start to science-communicate

Today and tomorrow, there is a 2-day summer school on science communication held at the University of Antwerp: Let’s Talk Science! During this summer school there are a large number of workshops to participate in, and lectures to attend, dealing with all aspects of science communication.

Wetenschapsbattle Trophy: Hat made by the children for the contestants of the wetenschapsbattle. Mine has diamonds and computers. 🙂

I was invited to represent Hasselt University (and science communication done by its members) during the plenary panel session starting the summer school. The goal of this plenary session was to share our experiences and thoughts on science communication. The contributions varied from hands-on examples to more abstract presentations of what to keep in mind, including useful tips. The central aim of my presentation was directed at identifying the boundary between science communication and scientific communication. Or more precisely, showing that this border may be more artificial than we are aware of. By showing that everyone’s unique in his/her expertise and discipline, I provided the link between conference presentations and presentations for the general public. I traveled through my history of science communication, starting in the middle: with the Science Battle. An event, I wrote about before, where you are asked to explain your work in 15 minutes to an audience of 6-to 12-year-olds. Then I worked my way back via my blog and contributions to “Ik heb been vraag” (such as: if you drop a penny from the Eiffel tower, will this kill someone on the ground?) to the early beginning of my research: simulating STM images. In the latter case, although I was talking to experts in their field (experimental growth and characterization), their total lack of experience in modelling and quantum mechanical simulations transformed my colleagues into “general public”. This is an important aspect to realize, not only for science communication, but also for scientific communication. As a consequence this also means that most of the tips and tricks applicable to science communication are also applicable to scientific communication.

For example: tell a coherent story. As noted by one of my favorite authors – Terry Pratchett – the human species might have better been called “Pan Narrans”, the storytelling ape. We tell stories and we remember by stories. This is also a means to make your scien(ce/tific) communication more powerful. I told the story of my passion during science explained and my lecture for de Universiteit van Vlaanderen.

A final point I touched is the question of “Why?”. Why should you do science communication? Some may note that is our duty as scientists, since we are payed with taxpayer money. But personally I believe this is not a good incentive. Science communication should originate from your own passion. It should be because you want to, instead of because you have to. If you want to, it is much easier to show you passion, show your interest, and also take the time to do it.

This brought me back to my central theme: Science communication can be simple and small. E.g. projecting simulated STM images on the wall’s of the medieval castle in Ghent (Gravensteen) during a previous edition of the Ghent Light Festival.

Simulated STM of nanowires projected on the Gravensteen (Ghent) during the 2012 Light Festival). Courtesy of Glenn Pollefeyt

## Universiteit Van Vlaanderen

A bit over 1 month ago, I told you about my adventure at the film studio of “de Universiteit Van Vlaanderen“. Today is the day the movie is officially released. You can find it at the website of de Universiteit Van Vlaanderen: Video. The video is in Dutch as this is a science-communication platform aimed at the local population, presenting the expertise available at our local universities.

In addition to this video, I was asked by Knack magazine to write a piece on the topic presented. As computational research is my central business I wrote a piece on the subject introducing the general public to the topic. The piece can be read here (in Dutch).

And of course, before I forget, this weekend there was also the half-yearly daylight saving exercise with our clocks.[and in Dutch]

## Newsflash: Materials of the Future

This summer, I had the pleasure of being interviewed by Kim Verhaeghe, a journalist of the EOS magazine, on the topic of “materials of the future“. Materials which are currently being investigated in the lab and which in the near or distant future may have an enormous impact on our lives. While brushing up on my materials (since materials with length scales of importance beyond 1 nm are generally outside my world of accessibility), I discovered that to cover this field you would need at least an entire book just to list the “materials of the future”. Many materials deserve to be called materials of the future, because of their potential. Also depending on your background other materials may get your primary attention.

In the resulting article, Kim Verhaeghe succeeded in presenting a nice selection, and I am very happy I could contribute to the story. Introducing “the computational materials scientist” making use of supercomputers such as BrENIAC, but also new materials such as Metal-Organic Frameworks (MOF) and shedding some light on “old” materials such as diamond, graphene and carbon nanotubes.

## Dangerous travel physics

Tossing coins into a fountain brings luck, tossing them of a building causes death and destruction?

We have probably all done it at one point when traveling: thrown a coin into a wishing well or a fountain. There are numerous wishing wells with legends describing how the deity living in the well will bring good fortune in return for this gift. The myths and legends often originate from Celtic, German or Nordic traditions.

In case of the Trevi fountain, there is the belief that if you throw a coin over your left shoulder using your right hand, you will return to Rome…someday. As this fountain and legend are iconic parts of our western movie history, many, many coins get tossed into it (more than 1 Million € worth each year, which is collected an donated to charity).

In addition to these holiday legends, there also exist more recent “coin-myths”: Death by falling penny. These myths are always linked to tall buildings, and claim that a penny dropped from the top of such a building could kill someone if they hit him.

## Traveling with Newton

In both kinds of coin legends, the trajectory of the coin can be predicted quite well using Newton’s Laws. Their speed is low compared to the speed of light, and the coins are sufficiently large to keep the world of quantum mechanics hidden from sight.

The second Law of Newton states that the speed of an object changes if there is a force acting on it. Here on earth, gravity is a major player (especially for Physics exercises). In case of a coin tossed into a fountain, gravity will cause the coin to follow a roughly parabolic path before disappearing into the water. The speed at which the coin will hit the water will be comparable to the speed with which it was thrown…at least if there isn’t to much of a difference in height between the surface of the water and the hand of the one throwing the coin.

But, what if this difference is large? Such as in case of the penny being dropped from a tall building. In such a case, the initial velocity is zero, and the penny is accelerated toward the ground by gravity. Using the equations of motion for a uniform accelerated system, we can calculate easily the speed at which the coin hits the ground:

x = x0 + v0*t + ½ * g * t²

v=v0+g*t

If we drop a penny from the 3rd floor of the Eiffel Tower (x0=276.13m, x=0m, v0=0 m/s, g=-9.81m/s²) then the first equation teaches us that after 7.5 seconds, the penny will hit the ground with a final speed (second equation) of -73.6 m/s (or -265 km/h)*. With such a velocity, the penny definitely will leave an impression. More interestingly, we will get the exact same result for a pea (cooked or frozen), a bowling ball, a piano or an anvil…but also a feather. At this point, your intuition must be screaming at you that you are missing something important.

## All models are wrong…but they can be very useful

The power of models in physics, originates from keeping only the most important and relevant aspects. Such approximations provide a simplified picture and allow us to understand the driving forces behind nature itself. However, in this context, models in physics are approximations of reality, and thus by definition wrong, in the sense that they do not provide an “exact” representation of reality. This is also true for Newton’s Laws, and our application above. With these simple rules, it is possible to describe the motion of the planets as well as a coin tossed into the Trevi fountain.

So what’s the difference between the coin tossed into a fountain and planetary motion on the one hand, and our assorted objects being dropped from the Eiffel Tower on the other hand?

##### Friction as it presents itself in aerodynamic drag!

Aerodynamic drag gives rise to a force in the direction opposite to the movement, and it is defined as:

FD= ½ *Rho*v²*CD*A

This force depends on the density Rho of the medium (hence water gives a larger drag than air), the velocity and surface area A in the direction of movement of the object, and CD the drag coefficient, which depends on the shape of the object.

If we take a look at the planets and the coin tosses, we notice that, due to the absence of air between the planets, no aerodynamic drag needs to be considered for planetary motion. In case of a coin being tossed into the Trevi fountain, there is aerodynamic drag, however, the speeds are very low as well as the distance traversed. As such the effect of aerodynamic drag will be rather small, if not negligible. In case of objects being dropped from a tall building, the aerodynamic drag will not be negligible, and it will be the factors CD and A which will make sure the anvil arrives at the ground level before the feather.

Because this force also depends on the velocity, you can no longer make direct use of the first two equations to calculate the time of impact and velocity at each point of the path. You will need a numerical approach for this (which is also the reason this is not (regularly) taught in introductory physics classes at high school). However, using excel, you can get a long way in creating a numerical solution for this problem.[Excel example]

As we know the density of air is about 1.2kg/m³, CD for a thin cylinder (think coin) is 1.17, the radius of a penny is 9.5 mm and its mass is 2.5g, then we can find the terminal velocity of the penny to be 11.1 m/s (40 km/h). The penny will land on the ground after about 25.6 seconds. This is quite a bit slower than what we found before, and also quite a bit more safe. The penny will reach its terminal velocity after having fallen about 60 m, which means that dropping a penny from taller buildings (the Atomium [102 m], the Eiffel Tower [276.13 m, 3rd floor, 324 m top], the Empire State Building [381 m] or even the Burj Khalifa [829.8 m]) will have no impact on the velocity it will have when hitting the ground: 40km/h.

## I have a Question: about thermal expansion

“I have a question”(ik heb een vraag). This is the name of a Belgian (Flemisch) website aimed at bringing Flemisch scientists and the general public together through scientific or science related questions. The basic idea is rather simple. Someone has a scientific question and poses it on this website, and a scientist will provide an answer. It is an excellent opportunity for the latter to hone his/her own science communication skills (and do some outreach) and for the former to get an good answer to his/her question.

All questions and answers are collected in a searchable database, which currently contains about fifteen thousand questions answered by a (growing) group of nearly one thousand scientists. This is rather impressive for a region of about 6.5 Million people. I recently joined the group of scientists providing answers.

An interesting materials-related question was posed by Denis (my translation of his question and context):

#### What is the relation between the density of a material and its thermal expansion?

I was wondering if there exists a relation between the density of a material and the thermal expansion (at the same temperature)? In general, gasses expand more than solids, so can I extend this to the following: Materials with a small density will expand more because the particles are separated more and thus experience a small cohesive force. If this statement is true, then this would imply that a volume of alcohol should expand more than the same volume of air, which I think is puzzling. Can you explain this to me?

#### Answer (a bit more expanded than the Dutch one):

Unfortunately there exists no simple relation between the density of a material and its thermal expansion coefficient.

Let us first correct something in the example given: the density of alcohol (or ethanol) is 46.07 g/mol (methanol would be 32.04 g/mol) which is significantly more than the density of air which is 28.96 g/mol. So following the suggested assumption, air should expand more. If we look at liquids, it is better to compare ethanol (0.789 g/cm3) to compare water (1 g/cm3) as liquid air (0.87 g/cm3) needs to be cooled below  -196 °C (77K). The thermal expansion coefficients of wtare and ethanol are 207×10-6/°C and 750×10-6/°C, respectively. So in this case, we see that alcohol will expand more than water (at 20°C). Supporting Denis’ statement.

Unfortunately, these are just two simple materials at a very specific temperature for which this statement is true. In reality, there are many interesting aspects complicating life. A few things to keep in mind are:

• A gas (in contrast to a liquid or solid) has no own boundary. So if you do not put it in any type of a container, then it will just keep expanding. The change in volume observed when a gas is heated is due to an increase in pressure (the higher kinetic energy of the gas molecules makes them bounce harder of the walls of your container, which can make a piston move or a balloon grow). In a liquid or a solid on the other hand, the expansion is rather a stretching of the material itself.
• Furthermore, the density does not play a role at all, in case of the expansion of an ideal gas, since p*V=n*R*T. From this it follows that 1 mole of H2 gas, at 20°C and a pressure of 1 atmosphere, has the exact same volume as 1 mole of O2 gas, at 20°C and a pressure of 1 atmosphere, even though the latter has a density which is 16 times higher.
• There are quite a lot of materials which show a negative thermal expansion in a certain temperature region (i.e. they shrink when you increase the temperature). One well-known example is water. The density of liquid water at 0 °C is lower than that of water at 4 °C. This is the reason why there remains some liquid water at the bottom of a pond when it is frozen over.
• There are also materials which show “breathing” behavior (this are reversible volume changes in solids which made the originators of the term think of human breathing: inhaling expands our lungs and chest, while exhaling contracts it again.) One specific class of these materials are breathing Metal-Organic Frameworks (MOFs). Some of these look like wine-racks (see figure here) which can open and close due to temperature variations. These volume variations can be 50% or more! 😯

The way a material expands due to temperature variations is a rather complex combination of different aspects. It depends on how thermal vibrations (or phonons) propagate through the material, but also on the possible presence of phase-transitions. In some materials there are even phase-transitions between solid phases with a different crystal structure. These, just like solid/liquid phase transitions can lead to very sudden jumps in volume during heating or cooling. These different crystal phases can also have very different physical properties. During the middle-ages, tin pest was a large source of worries for organ-builders. At a temperature below 13°C β-tin is more stable α-tin, which is what was used in organ pipes. However, the high activation energy prevents the phase-transformation from α-tin to β-tin to happen too readily. At temperatures of -30 °C and lower this barrier is more easily overcome.This phase-transition gives rise to a volume reduction of 27%. In addition, β-tin is also a brittle material, which easily disintegrates. During the middle ages this lead to the rapid deterioration and collapse of organ-pipes in church organs during strong winters. It is also said to have caused the buttons of the clothing of Napoleon’s troops to disintegrate during his Russian campaign. As a result, the troops’ clothing fell apart during the cold Russian winter, letting many of them freeze to death.