Brownian ratchets and the photoalignment of liquid crystals

Abstract

Molecular motors play key roles in areas ranging from biological transport to emerging nanotechnology. They produce current as a result of transfer of energy but not of momentum from a source; many molecular motor scenarios are based on the translational Brownian ratchet mechanism. Here we consider the mechanism of photoalignment of liquid crystals both in the bulk and at the surface by a photosensitive alignment layer. We show that the photoalignment is due to an orientational ratchet mechanism, where the azo-dye molecules, functionalized into a polymer alignment layer, when irradiated by polarized light act as the rotors of Brownian motors which reorient the bulk liquid crystal against an elastic restoring torque. Results of this photoalignment experiment can be obtained directly from a remote experiment set up at the Liquid Crystal Institute, via the WWW. In addition to experimental results, we present a detailed Fokker-Planck description of this system. We discuss the implementation and the results of numerical simulations, and compare these with the experimentally observed dynamics.