Rotational diffusion in a bistable potential

Open Access
Publisher Website
Google Scholar
Abstract
Using depolarized quasielastic light scattering, we have investigated the ro- tational diffusion of optically anisotropic colloidal particles in a dilute suspension subject to external electric and magnetic fields (E0, B0). The particles were produced by the polymer- ization 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 E0 and B0 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 (E0 ,B 0)2. Colloidal particles in a solvent do not only experience random forces due to the collisions with the surrounding molecules, but are also imparted random torques that lead to fluctuating particle orientations described by a unit vector u(t) also called particle director. If the colloids possess optical anisotropy, e.g., due to intrinsic birefringence or shape birefringence, these orientational fluctuations can be probed by depolarized quasielastic light scattering (1). For isolated, freely rotating spherical particles in the limit of high solvent viscosity η, the rotational