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Colloquium: Topological defects in a living nematic

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Igor Aronson, The Pennsylvania State University
When
09 November 2017 from 4:00 PM to 5:00 PM
Where
117 Osmond Laboratory
Contact Name
Vincent Crespi
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A living nematic is a realization of active matter that combines a nematic liquid crystal with swimming bacteria. The material exhibits remarkable tendency towards spatio-temporal self-organization manifested in the formation  of  dynamic textures of self-propelled half-integer topological defects (disclinations). The well-established and validated model of nematic liquid crystals coupled to  the bacterial dynamics is used to describe intricate properties of such a living nematic. The model yielded a testable prediction on the accumulation of bacteria in the cores of 1/2 topological defects and depletion of bacteria in the cores of -1/2 defects. We also studied such living nematic near normal inclusions,  or tactoids, naturally realized in liquid crystals close to the isotropic-nematic (I-N) phase transition. On the basis of computational analysis, we have established that the tactoid's I-N interface spontaneously acquires negative topological charge which is proportional to the tactoid's size and depends on the concentration of bacteria.  The observed negative charging is attributed to the drastic difference in the mobilities of 1/2 and -1/2 topological defects in active systems. The effect is  described  in the framework of a kinetic theory for point-like weakly-interacting defects with different mobilities. Our dedicated experiments fully confirmed both theoretical predictions. The results hint at new strategies for control over active matter.

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