Résumé
Symmetry arguments show that a flow of matter through a chiral medium can induce a collective molecular rotation. We report the study of such a linear coupling between a gas flow and molecular motion in thick free standing films of SmC* liquid crystal. This effect could be used to develop liquid crystalline micromotors potentially useful for microfluidics and micromechanics. Typical evolving target patterns have been recently observed by Tabe et al. [1] in a Langmuir monolayer of chiral liquid crystal molecules deposited on glycerol. We designed a more robust system composed of a free standing film of chiral molecules, without any liquid substrate. The flow of gas through the film is tuned by controlling the gas chemical potential. Direct pattern observation needs no Brewster angle microscope and is done under optical polarizing microscope. The whole set-up allows a quantitative study of the flow/rotation coupling phenomenon. The system shows a very rich spatio-temporal behavior. Here we focus on patterns obtained in a film suspended on a circular frame. In this geometry, a steady-state is attained where a spiral pattern is formed (see fig.1). We obtain the set of equations which govern the dynamics of the film under the gas flow. In a particular case, the equations involve only the c-director dynamics [2]. In the steady-state regime, they reduce to the competition of the elastic torque and an external torque induced by the flow. The observed static spiral patterns confirm the existence of such an external torque.