Thermomechanical Effects in Liquid Crystals

This chapter describes two crossed effects observed in nematic and cholesteric liquid crystals (LC) subjected to a temperature gradient G. The first effect couples the rotation of the molecules to the temperature gradient (thermomechanical [TM] effect), while the second one couples the flows to the temperature gradient (thermohydrodynamical effect). The chapter describes the Éber and Janossy experiment that revealed for the first time the existence of a TM effect in a cholesteric LC. It summarizes the main theoretical results obtained by Sarman and coworkers by molecular dynamics simulations in the case of the TM effect. The dynamic experiments are more spectacular because they show more directly the non-equilibrium TM effect. The chapter describes the experiments in which translationally invariant configurations (TIC) are set into continuous rotation by application of a temperature gradient. The comparative experiments with different types of TICs have nonetheless proven their existence and have allowed us to estimate their order of magnitude.