Univ Lyon, ENS de Lyon, Univ Claude Bernard, CNRS, Laboratoire de Physique, F-69342 Lyon, France.
Soft Matter. 2019 Mar 20;15(12):2591-2604. doi: 10.1039/c8sm02574f.
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.
我们通过实验和理论证明，最近由 Yoshioka 和 Araoka 在温度梯度下的乳化胆甾相液晶液滴中观察到的 Lehmann 效应是由 Marangoni 流引起的，而不是通常用来解释该现象的热机械或化学生物力学耦合。我们使用胶体示踪剂可视化了在垂直温度梯度中围绕静止胆甾相液滴的对流卷，而内部点缺陷位置的移动则揭示了在水平温度梯度中热迁移的向列相液滴中的相应内部对流。我们将这些现象归因于这些部分混溶液体界面处表面张力对温度的依赖性，并解释了它们在通常情况下纯疏液乳状液中不存在的原因。我们进行了理论分析以帮助验证这一假设，证明了进动速度对胆甾相螺旋结构的强烈依赖性。
