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58


nanotimes News in Brief


Crystals // Flowing Structures in Soft Crystals


Vienna has discovered intriguing structures formed by tiny particles floating in liquids. Under mecha- nical strain, particle clusters in liquids can spontane- ously form strings and dramatically alter the proper- ties of the liquid.


A


If small particles accumulate, they can form clusters. Within a cluster, the particles may overlap and min- gle, similar to a densely packed shoal of eels, gliding past each other. Remarkably, these clusters are not situated at random positions, but they spontaneously form a regular structure – a “cluster crystal”. The distance between two neighboring clusters is con- stant. “Increasing the density of particles adds more and more particles to each cluster – but the distance between them stays the same”, says Arash Nikou- bashman, PhD-student at TU Vienna. He made the calculations together with Professor Gerhard Kahl (Institute for Theoretical Physics, TU Vienna) and Professor Christos Likos (University of Vienna).


“Previous results had already led us to believe that these particles could exhibit strange behavior under certain external conditions”, the physicists explain. And their hopes were not unfounded: in compu- ter simulations they managed to calculate how the


team of researchers at Vienna University of Technology (TU Vienna) and the University of


crystal-like structure behaves under mechanical strain that causes shears stress – which means that surfaces within the liquid are shifted relative to each other. At first, the crystal structure starts to melt, the connec- tions between the clusters are broken. From these molten particle clusters, a new regular order starts to emerge spontaneously. Long, straight strings of parti- cle are formed, neatly aligned in parallel.


While these strings are created, the liquid gets thin- ner, its viscosity decreases. This is due to the strings being able to slide relative to one another. If the material is subject to even more strain, the strings break up too, a “molten” unstructured ensemble of particle clusters remains, and the viscosity of the liquid increases again. More and more particles are washed away from their original positions and inhibit the flow. This behavior is the same for all kinds of cluster crystals. With a simple theoretical model, the critical strain, at which the ordered structure vanishes completely, can be predicted very accurately. Under shear strain, crystals made of soft, penetrable particles can exhibit new kinds of self-organization. Geometric structures emerge, governed by the kind of forces acting between the particles. This research in the field of “soft matter” in the micro- and nano- meter regime is not only interesting from a theoreti- cal point of view. These materials play an important


11-08 :: August 2011


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