Microscopic vs. macroscopic origin of the Lehmann effect in cholesteric liquid crystals

In a recent letter (EPL 97, 36006 (2012)), we have shown that the Leslie thermomechanical coupling cannot alone explain the Lehmann effect (namely the rotation of cholesteric droplets when they are subjected to a temperature gradient). This result was obtained by measuring in a compensated cholesteric mixture the "Lehmann coefficient" as a function of temperature both below and at the transition to the isotropic liquid. In this article, we detail these experiments and present new ones performed with other compensated mixtures and a diluted cholesteric mixture. The new results confirm the macroscopic origin of the Lehmann effect, in contrast to the Leslie thermomechanical effect that is clearly of microscopic origin.