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ePaper created 2013-08-02, 08:40:53 | version 1.25.20
2|2013 Research in Jülich 13 become more hectic the higher the tem- perature,” explains Gompper. The vibrat- ing molecules hit the surface of a shell like billiard balls, for example when they swim through water. This sustained fire has no effect on larger shells, such as ping-pong balls or bathyspheres. In mi- croscopically small shells, however, ther- mal fluctuations change the surface. Gompper illustrates this fact by scrunching up a piece of paper and then smoothing it out. The paper now resem- bles a hilly landscape with randomly dis- tributed ridges and valleys. “This is roughly what you can imagine the sur- face of a microcapsule to look like when it has been deformed by thermal fluctua- tions.” Paper can also be used to demon- strate that the consequences are not merely of an aesthetic nature. Gompper holds a smooth piece of paper at its short edge and swings it like a pendu- lum. The paper bends, it’s elastic. Then he repeats the same movement with the scrunched piece of paper. It remains rig- id. “Smooth and crumpled surfaces have different properties. One of them is stiff- ness. We have studied the influence of this property on the stability of shells.” In order to measure how stable a shell is, pressure tests are usually car- ried out. These are comparable to press- ing a ping-pong ball with your thumb re- peatedly until it is permanently dented. The necessary force can be calculated using a formula developed by the fa- mous mathematician, aerospace engi- neer, and physicist Theodore von Kármán more than 50 years ago. Gomp- per and Vliegenthart have now tested whether this formula also applies when the shells have a diameter of less than 10 micrometres. VIRTUAL EXPERIMENTS For this purpose, the physicists mod- elled a virtual shell on a computer, simu- lated the effects of thermal fluctuation, and performed virtual pressure tests. Whereas the colleagues overseas are working on a theory that numerically de- scribes the effects of the deformed sur- face, scientists at Jülich are primarily using simulation methods and computer models. “We already simulated a plane with an elastic and crumpled surface seven years ago. This was our point of departure for developing a virtual shell with this chaotic surface,” says Vliegenthart. Modelling, he says, was the hardest part of the work: Vliegenthart constructed a shell con- sisting of 100,000 grid points, each of them connected to its neighbours on the surface via springs. He then performed virtual experi- ments: as in a real pressure test, he applied pressure to points or larger ar- eas on the shell until it collapsed. This experiment was repeated under different conditions to measure the critical pres- sure. The result: depending on the size, material, and shell thickness, up to 50% less force is required than predicted by Kármán’s equation – a finding that will be taken into consideration by scien- tists involved in the development of nanoferries. :: Christoph Mann RESEARCH AT THE CENTRE | Simulation Thermal fluctuations mean that atoms are always on the move, and collide with nearby surfaces in the process. This has no effect on larg- er shells, such as ping-pong balls, but the surface of tiny shells such as microcapsules is crumpled when continuously bombarded with atoms (image 1). As a conse- quence, the microcapsules are less resistant to pressure and collapse more easily (image 2). A dent in the sphere Image 2 Image 1 Imagagaggaggee 22222222I Institute