Effective pair interactions between colloidal particles at a nematic - isotropic interface

– The Landau-de Gennes free energy is used to study theoretically the interaction of parallel cylindrical colloidal particles trapped at a nematic-isotropic interface. We find that the effective interaction potential is non-monotonic. The corresponding force-distance curves exhibit jumps and hysteresis upon approach/separation due to the creation/annihilation of topological defects. Minimization results suggest a simple empirical pair potential for the effective colloid-colloid interaction at the interface. We propose that the interface-mediated interaction can play an important role in self-organization and clustering of colloidal particles at such interfaces. Introduction. – Colloidal particles dissolved in a nematic solvent experience long-ranged interactions [1]. These effective interactions are generated by the distortions of the liquid crystal director around the particles and result in their clustering and self-organization [2, 3]. Upon approaching and at the nematic-isotropic transition, the solvent forms nematic and isotropic domains, which leads to an additional interaction of the particles with the interface between the two phases [4, 5]. The resultant morphology of the network formed by the particles in colloid-liquid crystal composites is very sensitive to this interaction [4, 6]. In fact, colloidal particles can be captured by the nematic-isotropic interface and even dragged by a moving interface [4]. This enables one to manipulate tiny particles in suspension, which in a more general context is of technological importance for manufacturing of e-papers and electrophoretic displays [7], separation of bacterial species and living cells [8], trapping of DNA and polymer particles [9], as well as growth of photonic crystals [10]. The structures of self-assembled colloid layers at free surfaces of nematic films can be tuned by their thickness [11]. Recent experiments of dragging colloidal particles by nematic-isotropic interfaces [4] demonstrated that a single colloidal particle is attracted by the interface. This conclusion is in accordance with theoretical results obtained by minimizing the corresponding Landau-de Gennes