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Nonlinear nonequilibrium dynamics in a nematic liquid crystal.


Stieger,  Tillmann
Group Non-equilibrium soft matter, Department of Dynamics of Complex Fluids, Max Planck Institute for Dynamics and Self-Organization, Max Planck Society;

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Stieger, T. (2016). Nonlinear nonequilibrium dynamics in a nematic liquid crystal. PhD Thesis, Technische Universität, Berlin.

Liquid crystals are elongated molecules with a rich and surprising phase behavior. Nonequilibrium conditions open a myriad possibilities of manipulating matter, and reach collective states not accessible under equilibrium conditions. We perform nonequilibrium molecular dynamics simulations of a nematic liquid crystal flowing around a colloidal particle. Because of a mismatch between the nematic far field alignment and the local orientation of the liquid-crystal molecules at the surface of the colloid, defect topologies arise if the host is in thermodynamic equilibrium. We study the flow-induced modications of these topological defects. We find that Saturn ring defects are convected downstream along the flow direction, which is in agreement with experimental observations [1]. As Poiseuille flow is initiated, the Saturn ring is deformed. The degree of deformation is analyzed quantitatively in terms of characteristic geometric parameters fitted to suitable projections of the Saturn ring. Our results suggest that smaller Saturn rings are shifted downstream while approximately maintaining their circular shape, whereas larger ones exhibit an elastic deformation in addition. Additionally, we show that flow distorts Boojum defects into an asymmetrically larger downstream lobe. For a Janus colloid, exhibiting a Boojum defect and a Saturn ring defect, we find that the Boojum defect facing the upstream direction is destroyed and the Saturn ring is convected downstream. Furthermore, we study a similar system of a nematic liquid crystal flowing around a cylindrical pillar. We report flow-induced cavitation in an anisotropic fluid. Cavitation domains nucleate due to a sudden drop in pressure upon flow past the cylindrical obstacle. The inception and growth of cavitation domains ensue in the laminar flow regime. We study the physical principles governing the cavitation phenomena in nematic liquid crystals, and identify a critical value of the Reynolds number for cavitation inception that scales inversely with the characteristic order parameter of the nematic liquid crystal. Strikingly, the critical Reynolds number can be as low as about 50% of the cavitation threshold in the isotropic liquid crystal. These findings suggest that long range ordering, and its tunability, can be potentially applied as a novel control parameter to modulate cavitation inception in anisotropic fluids. Additionally, we find very good agreement with earlier micro fluidic experiments [2] at smaller flow speeds before cavitation initiates. Our simulations are able to reproduce the structural changes within the micro fluidic channel at different flow speeds.