Chemical Leslie effect in a chiral smectic-C^{★} film: Nonsingular target patterns.

Abstract

We analyze experimentally and theoretically the winding and unwinding of the c[over ⃗] director in a chiral smectic-C^{★} film crossed by an ethanol flow. This leads to a target pattern under crossed polarizers when the +1 defect imposed by the boundary conditions is pinned on the edge of the film. We show that the target is deformed at the center of the film when it is subjected to a flow of ethanol because of the presence of two recirculation vortices of chemohydrodynamical origin. This deformation and the two vortices disappear during the unwinding of the target when the ethanol flow is stopped. This unambiguously shows that the target deformation is due to the vortices and not to the elastic anisotropy. These two points are confirmed theoretically. An estimate of the two so-called chemomechanical and chemohydrodynamical Leslie coefficients is also derived from this study.