Wall–colloid interaction in nematic solvents: external field effects

Abstract

We propose a molecular theory of colloid-wall interactions in nematic media that predicts a new effective force acting on colloidal particles in the presence of an external field. In contrast to the so-called 'image' interaction that is always repulsive at long distances, the force identified here can be attractive or repulsive, depending on the type of anchoring at the wall and colloidal surfaces. The effective force on a colloidal particle decreases with distance s from the wall 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 our findings is presented.

Transport of particles in liquid crystals

“…45 A rough estimate of the order of magnitude of the elastic energy associated with the director distortions around a strongly anchored micron-size particle placed in an otherwise uniform nematic cell, is anisotropic interactions, such as pairing of particles and formation of multiparticle chains and clusters; 33,39,[46][47][48][49][50][51][52][53] see the recent reviews by Tkalec and Muševič, 54 by Araki, Serra and Tanaka, 55 and by Blanc et al 56 Forces of the same nature are also responsible for attraction of colloidal particles to (particle-free) distortions and defects in the director eld in nematics, 47,57,58 smectics [59][60][61] and blue phases, 62 for trapping and ordering of particles at the LC surfaces, [63][64][65][66] and even for symmetry-breaking that enables transport phenomena such as nonlinear electrophoresis in LCs. 45,67 As discussed in the next section, the balance of anchoring and elastic forces also leads to the effect of colloidal levitation in LC environment, thanks to the elastic repulsion from the bounding wall 44,[68][69][70][71] (an interesting version of the magnetic eld-induced levitation in LCs has been presented by Lapointe et al 72,73 ).…”

“…section, the balance of anchoring and elastic forces also leads to the effect of colloidal levitation in LC environment, thanks to the elastic repulsion from the bounding wall 44,[68][69][70][71] (an interesting version of the magnetic field-induced levitation in LCs has been presented by Lapointe et al 72,73 ).…”

Cross-over between central and non-central conservative effective forces in a modulated colloidal fluid

“…1 Due to fundamental importance, the effective forces for different systems have been worked out in details via experiments, theoretical studies, and computer simulations in various systems. [1][2][3][4][5][6][7][8][9] The medium generated forces on the dispersed particles arise from the free energy cost of distributing the dispersing particles around them. In a homogeneous liquid of uniform density, the distribution of the medium particles around the dispersed particles does not depend on the position of the latter.…”