Rotational diffusion in a bistable potential

Using depolarized quasielastic light scattering, we have investigated the rotational diffusion of optically anisotropic colloidal particles in a dilute suspension subject to external electric and magnetic fields (E(0), B(0)). The particles were produced by the polymerization of nematic liquid crystal droplets, leading to birefringent colloidal spheres whose frozen orientational order is rigidly coupled to the particle orientation. The torque on the droplet director u(t) originating from the coupling of E(0) and B(0) to the particles anisotropy in the refractive index and in the diamagnetic susceptibility, respectively, leads to a suppression of the orientational fluctuations about the direction of the external field. We observe a strong dependence of the measured relaxation rates on the field strength and on the orientation of the field relative to the scattering plane. We explain our findings by a solution of the Smoluchowski equation describing rotational diffusion in a bistable potential V (u) which has two equivalent minima separated by a potential barrier whose height is proportional to (E(0), B(0))(2).