Rotational diffusion in a bistable potential

Abstract

Abstract. -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 (E0, B0). 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 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, B0) 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 part of the field auto-correlation function g 2 of the scattered depolarized electric field E VH (t) decays exponentially with time t; the corresponding decay rate is proportional to the rotational diffusion constant D r = k B T /(8πηR 3 ), where k B T is the thermal energy and R is the particle radius.This simple situation changes completely when the rotational symmetry is broken by an external field (such as an electric or magnetic field) that couples to permanent or induced (electric or magnetic) dipoles localized on the particle. The orientational distribution then develops a peak around the direction of the external field, and, consequently, the amplitudes

Differential dynamic microscopy microrheology of soft materials: A tracking-free determination of the frequency-dependent loss and storage moduli

“…The particles were prepared and purified according to the protocols described in detail in Refs. [39,41]. The final mass fraction of the dispersion was approximately 4%.…”

“…The tracer used in this study consists of spherical colloidal particles, with a polymerized nematic liquid-crystal core, whose birefringence makes the particles optically anisotropic, with a uniaxial symmetry [18,19,39,40]. The particles were prepared and purified according to the protocols described in detail in Refs.…”

Study of translational and rotational dynamics of birefringent colloidal particles by depolarized light scattering in the far- and near-field regimes

“…The particles used in this study are spheres made of polymerized liquid crystal in nematic phase, which makes them birefringent. Optically anisotropic particles have been used in electro-optic switching studies, 15 to investigate rotational motion in concentrated systems, 16 or as tracers in rotational microrheology experiments. 17,18 Here, depolarized dynamic light scattering in the far-and near-field regimes is used to measure the short-time translational and rotational diffusion coefficients of such systems in an extended scattering vector range.…”

Highly birefringent colloidal particles for tracer studies

“…Rotational motion can be observed in depolarized quasi-elastic light scattering experiments on optically anisotropic particles [4,5], using time-resolved phosphorescence anisotropy measurements on specially labelled particles [6], or nuclear magnetic resonance [7]. Measured autocorrelation functions of the scattered electric field measured by depolarized QELS now contain information not only on the particle displacements but also on their…”