Lehmann effect in chiral liquid crystals and Langmuir monolayers: an experimental survey

In 1900 Otto Lehmann observed that the internal texture of cholesteric droplets, when submitted to a thermal gradient, was constantly rotating. This phenomenon was explained phenomenologically in 1968 from symmetry arguments by F.M. Leslie in the framework of the nematodynamics. Six years later, Pierre-Gilles de Gennes noted in a premonitory way, in his seminal book The Physics of Liquid Crystals, that the heat current responsible for the Lehmann effect could also be an electric or a diffusion current, suggesting the existence of an electric or chemical Lehmann effect. This led to numerous experiments, sometimes wrongly interpreted, and to the recent discovery of the chemical Lehmann effect in Langmuir monolayer and ferroelectric smectic films. These experiments are reviewed and discussed in this paper.