| Liquid Crystal (LC) films composed of chiral compounds are known to exhibit a unidirectional molecular rotation under transmembrane thermal, ionic and mass flows, which is called Lehmann rotation. We have investigated the molecular precession in ultrathin smectic C* films driven by transmembrane gas flow, focusing on the dependence of the rotational direction on the flow materials and film thickness. When the film was monolayer, the rotational direction was not dependent on the transferred materials but determined by the molecular chirality and the flow direction. While in the thick films with several layers or more, the precession direction sometimes is reversed depending on the film thickness. It is considered that there are two origins in Lehmann effect; one is the microscopic twist in each chiral molecule and the other is the macroscopic torque due to the helical structure of smectic C* films. |