Relaxation mechanisms of photoinduced periodic microstructures in ferrofluid layers

We consider theoretically and numerically a periodic concentration grating induced in a layer of ferrofluid in the presence of the external magnetic field by nonuniform optical heating through photoabsorption. The stationary profiles of the periodic microstructures are governed by the equilibrium of the diffusive, thermodiffusive, and magnetic fluxes. The anisotropy of the diffusion coefficient and the magnetically driven microconvection contribute to the relaxation of these structures. The temperature-concentration coupling is shown to increase the initial effective diffusive relaxation rate by up to 50%. Microconvection dominates in the relaxation process even at small values of the control parameter and rapidly destroys the periodic part of the concentration grating. We describe this process in the weakly nonlinear regime by an approximate Galerkin model.