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Effect of Collective Molecular Reorientations on Brownian Motion of Colloids in Nematic Liquid Crystal

Science  13 Dec 2013:
Vol. 342, Issue 6164, pp. 1351-1354
DOI: 10.1126/science.1240591

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Confusing Colloids in Liquid Crystals

In a simple fluid, particle diffusion such as the motion of colloidal particles shows a change in the mean squared displacement that is proportional with time. Within a nematic liquid crystal, diffusion of the molecules may show anisotropic behavior. Turiv et al. (p. 1351; see the Perspective by Abbott) asked what happens to colloidal particles in a nematic liquid crystal. At short times, anomalous diffusion was observed with motion both slower and faster than the long-term behavior, indicative of a complex coupling between the diffusive motion of the colloidal particles and the motion of the liquid crystal molecules.

Abstract

In the simplest realization of Brownian motion, a colloidal sphere moves randomly in an isotropic fluid; its mean squared displacement (MSD) grows linearly with time τ. Brownian motion in an orientationally ordered fluid—a nematic—is anisotropic, with the MSD being larger along the axis of molecular orientation, called the director. We found that at short time scales, the anisotropic diffusion in a nematic becomes anomalous, with the MSD growing slower or faster than τ; these states are respectively termed subdiffusion and superdiffusion. The anomalous diffusion occurs at time scales that correspond to the relaxation times of director deformations around the sphere. Once the nematic melts, the diffusion becomes normal and isotropic. Our experiment shows that the deformations and fluctuations of long-range orientational order profoundly influence diffusive regimes.

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