Lehmann rotation of cholesteric droplets driven by Marangoni convection

Soft Matter
P OswaldA Dequidt

Abstract

We show experimentally and theoretically that the Lehmann effect recently observed by Yoshioka and Araoka (Nat. Commun., 2018, 9, 432) in emulsified cholesteric liquid crystal droplets under temperature gradients is due to Marangoni flows rather than to the thermomechanical or chemomechanical couplings often invoked to explain the phenomenon. Using colloidal tracers we visualize convection rolls surrounding stationary cholesteric droplets in vertical temperature gradients, while a shift in the position of internal point defects reveals the corresponding inner convection in nematic droplets thermomigrating in a horizontal temperature gradient. We attribute these phenomena to the temperature dependence of the surface tension at the interface between these partially-miscible liquids, and justify their absence in the usual case of purely lyophobic emulsions. We perform a theoretical analysis to help validate this hypothesis, demonstrating the strong dependence of the precession velocity on the configuration of the cholesteric director field.

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Citations

Mar 28, 2020·Journal of Physics. Condensed Matter : an Institute of Physics Journal·Jun Yoshioka, Koji Fukao
Nov 22, 2019·Nature Communications·Federico LanciaNathalie Katsonis
Feb 13, 2020·The European Physical Journal. E, Soft Matter·P Oswald
Jun 20, 2020·The Journal of Physical Chemistry. B·Shinji BonoYuka Tabe

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