FLUID-FLOW-INDUCED PATTERN-FORMATION IN LIQUID-CRYSTALS IN A ROTATINGMAGNETIC-FIELD

We present a numerical study of pattern formation in the asynchronous regime of a homeotropically aligned nematic liquid crystal under the i nfluence of an in-plane continuously rotating magnetic field. Experime ntally, this system has been shown to produce several pattern-forming states. Here, we test the hypothesis that these dynamic patterns exist due to a coupling between spatial gradients in the nematic director a nd gradients in the fluid flow. We derive the equations of motion coup ling the director and hydrodynamic flow from the Leslie-Erickson equat ions and numerically integrate them in the asynchronous regime. We sho w that there are two pathways by which a sample that is initially in a homogeneous state can evolve into a dynamic pattern. The first path i s through a large external disturbance which nucleates pattern formati on and the second is through the amplification of thermal fluctuations , both of which are experimentally observed. We find that the fluid-fl ow coupling is essential in order to reproduce the experimental behavi or. Intuitively, the coupling of the director with the flow creates a lower effective viscosity than in the absence of the flow.