Surface-sensitive particle selection by driving particles in a nematic solvent

Electrophoresis and sedimentation (or ultracentrifugation) are powerful means for separating particles, proteins, and DNA, exploiting the difference in particle charge, mass, and size. Surface properties of colloids and proteins are closely related to their physical, chemical, and biological functions. Thus, the selection of particles in terms of their surface properties is highly desirable. The possibility of replacing a simple liquid like water by a complex liquid may provide a novel route to particle separation. Here we report a new principle of surface-sensitive particle selection by using nematic liquid crystal as a solvent. When we immerse a particle in nematic liquid crystal, topological defects are formed around a particle if there is a strong enough coupling between the particle surface and liquid crystal orientation (so-called surface anchoring effects). Then these defects strongly influence the motion of particles. Here we study this problem by using a novel numerical simulation method which incorporates elastic and nematohydrodynamic couplings properly. We find that the surface anchoring properties change both direction and speed of motion of a particle driven in an oriented nematic liquid crystal. This principle may be used for separating particles in terms of their surface properties.