Devices are getting ever smaller. That’s why the computing power of your mobile phone now surpasses that of a bulky desktop computer from a few years ago. But strange things happen when you enter the world of sub-micrometre materials.
‘They tend to behave very differently from the bigger stuff,’ says Erik van der Giessen, Professor of Micromechanics at the University of Groningen. He will be awarded the Koiter Medal by the American Society for Mechanical Engineers on 14 November for his work on the deformation of small objects.
From the comfort of his office, Van der Giessen uses a teaspoon to illustrate his work. ‘Look, if I hold it on one side and then pull down the other side a bit, it springs back. But if I pull it down further, it will bend permanently. The spoon is now deformed and won’t return to its original shape.’ Which is no big surprise, of course, but why does this happen?
‘Basically, because the material the spoon is made of isn’t perfect. There are defects in the crystal structure of the metal, and it’s these faults that allow the spoon to bend. If they weren’t there, the spoon would break like a piece of chalk.’ Van der Giessen has worked on mathematical models to describe the behaviour of metal sheets. ‘This is important for the automobile industry, for example. If you want to bend a sheet of metal into a car bonnet or the like, you need to know how much force must be applied and what strains the bending process induces in the material.’
For cars or airplanes you can simply test a spoon-sized piece of the material and extrapolate this to an entire car or Airbus. ‘But when you scale down to below one micrometre, things are totally different,’ says Van der Giessen. Engineers ran into this problem in the 1990s as devices became ever smaller. ‘It turns out that the smaller the object, the harder it is to permanently deform it. But the theories at the time weren’t able to describe this.’
Van der Giessen devised a method to simulate the mechanical behaviour of very small devices. ‘There are a lot of atoms even in a sub-micrometre device.’ It was impossible to simulate all 1011 (that’s 100,000,000,000) atoms in a cubic micrometre. ‘But a colleague in the United States and I found a way to only model the defects in the materials crystal structure, which meant we could leave out most of the atoms.’
He hasn’t seen the jury reports, but this work is most likely what won him the Koiter medal. In the past few years, however, Van der Giessen has worked on a number of problems in materials science, so it may have been for something else. ‘Like the behaviour of plastics and synthetic rubber on an atomic scale.’ He has also ventured into something totally different from spoons, cars and airplanes: the mechanics of biomolecules, the building blocks of living cells.
This interest was sparked by something Van der Giessen read in the popular science magazine Scientific American. ‘It was about the structural elements of cells, the cytoskeleton. The article argued that these were what are known as tensegrity structures.’ Tensegrity (a contraction of ‘tensional integrity’) structures are made of rigid bars held together by cables. The tension of the cables and the stiffness of the bars combine to make a stable structure that can be both very tall and very light.
‘My first reaction was that this couldn’t be true,’ says Van der Giessen. ‘In a tensegrity structure, if you cut one cable the whole thing comes down. The components of biological structures are constantly changing, with molecules being broken down and rebuilt. To me, that ruled out tensegrity.’
From his work on synthetic (and natural) rubber, Van der Giessen knew that those polymers are made up of very long molecules. Stretching occurs by unfolding, in much the same way as when you pull a randomly coiled bicycle chain. ‘The biopolymers which make up the cytoskeleton are straighter but wrinkled, which means they can stretch by straightening out.’ Once again with a colleague, he developed a simulation programme that described the behaviour of biopolymers.
‘It turns out that the biopolymers in the cytoskeleton are connected in a foam-like network, which adds strength to the material.’ These foam-like structures can be used in artificial materials as well. ‘NASA is working on these kinds of materials, because they actually become stronger when they are deformed.’
These days, Van der Giessen’s main interest is not in rockets, but in biomedical applications. ‘For example, when we make artificial vesicles for drug delivery, how can we make them both flexible enough to pass through capillaries and strong enough to keep the drugs inside until they can be released?’ And there are even more speculative applications: ‘Can we use mechanistic properties of tumour cells to distinguish them from normal, healthy cells? This could open up all sorts of possibilities.’
The Koiter Medal is named after the Dutch engineer Warner T. Koiter (1914-1997). He studied at Delft University of Technology and spent almost his entire career there working on solid mechanics theories used in the engineering of aircraft and rockets. Van der Giessen, who also studied and worked in Delft before moving to Groningen, explains why Koiter is so important. ‘A rocket is a cylinder, and the wall has to be as thin and light as possible. But when you put pressure on a cylinder, as happens during the launch of a rocket, at some stage the wall may buckle. Koiter developed a mathematical model to predict when this would happen.’ He did this during World War II, partly whilst in hiding from the Germans.
After the War, Koiter played an important part in the development of Dutch aeronautical science and engineering. When he left Delft for the US in the 1960s, his colleagues petitioned the Minister of Education to intervene. As a result, he was given special privileges, reducing his administrative burden so he could focus on research, which enticed him to return. ‘Koiter really was a world leader in solid mechanics.’
When the American Society for Mechanical Engineers came with the plan to establish a Koiter Award, Van der Giessen was asked to lobby Delft Technical University for funding. The University agreed and the first Koiter Medal was awarded to Koiter himself in 1997. Van der Giessen, who helped secure funding for the award, is now set to receive the sixteenth medal himself.
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