OPOMNIK: Dr. Guilhem Poy: On the pertinence of the thermomechanical model in the Lehmann rotation of cholesteric and nematic droplets

This talk is focused on the Lehmann effect, an out-of-equilibrium effect which couples a temperature gradient with the rotation of the internal texture of liquid crystal droplets in coexistence with the isotropic phase.

First, we characterized the thermomechanical couplings of Leslie, Akopyan and Zel'dovich by measuring the rotation velocity of the molecules in two translationally invariant configurations with different orientations, below the cholesteric/isotropic transition. Then, we characterized the texture of the droplets observed in the Lehmann experiment, both using optical observations and numerical simulations. More important, we showed for the first time that it is possible to observe the Lehmann effect in achiral nematic droplets, providing that the internal texture is chiral. We also used a photobleaching experiment to show that there is no visible flow in the vicinity of the droplet, which implies that the texture rotation is due
to a local rotation of the molecules -- not to a solid rotation of the droplet. Finally, we proposed a theoretical model of the Lehmann effect based on the thermomechanical coupling of Leslie, Akopyan and Zel’dovich. By applying this model to the numerically computed textures, we fitted the measured rotation velocities and found values for the thermomechanical coupling constants much bigger than those measured below the cholesteric/isotropic transition. This shows that this model is incorrect and that the Leslie paradigm must be abandoned.

Kindly invited!