Wall–colloid interaction in nematic solvents: external field effects

Abstract

We propose a molecular theory of colloid–wall interactions in nematic media that predicts anew effective force acting on colloidal particles in the presence of an external field. In contrastto the so-called ‘image’ interaction that is always repulsive at long distances, the forceidentified here can be attractive or repulsive, depending on the type of anchoring at thewall and colloidal surfaces. The effective force on a colloidal particle decreases with distances from thewall as exp(−s/ξ), where ξ is a magnetic (electric) coherence length. At weak fields the force is proportional to(Σ/ξ)3 for ‘quadrupolar’colloids and to (Σ/ξ)2 for ‘dipoles’, where Σ is the colloidal diameter. A brief discussion of recent experiments in the light of ourfindings is presented.