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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.

Cite as: https://hdl.handle.net/11858/00-001M-0000-002C-3BAF-5
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.