The effective interaction of micron-size particles immersed in a nematic liquid crystal at temperatures above the nematic-isotropic phase transition is examined. Using the Landau-de Gennes theory we describe the surface-induced ordering of the liquid crystal. Modeling a spherical particle by a sequence of conical surfaces enables us to draw an analogy with the partial ordering in a thin nematic film and to obtain an analytical expression for the two-particle interaction. Special attention is paid to the dependence of the effective interaction on temperature and on the interfacial coupling parameters, which are chosen in agreement with recent experiments. We show that the interaction as a function of the interparticle distance exhibits a potential barrier of the order of kT. The height of the barrier decreases exponentially with increasing temperature while it grows linearly with the interfacial coupling parameters.

The birefringence induced by partially ordered polymer networks dispersed in an isotropic phase of liquid crystals is studied. Polymer networks were formed by the polymerization of 1-4 % mixtures of prepolymer in the nematic phase of liquid crystals. The partial nematic ͑paranematic͒ ordering is analyzed in terms of the Landau-de Gennes approach using a simple model of an array of thin fibrils, which can, on a large scale, form bundles of polymer-rich material. The comparison of the theory and birefringence data clearly shows that the main building bloc of the network is a few nanometers thick fibril. The average thickness of fibrils varies slightly with the polymer concentration. The information obtained on large scale ͑micrometer͒ structures is not conclusive and must be complemented by other methods. The network on the average retains 30-50 % of the order parameter of the environment where it was formed. ͓S1063-651X͑97͒12202-4͔

We have observed capillary condensation in nanometer-thin films of nematic liquid crystals using force spectroscopy. The liquid crystal was confined to a submicron gap between a flat substrate and a microsphere, attached to the cantilever of an atomic force microscope. A long-ranged and strongly attractive force due to capillary condensation of a nematic phase was observed at temperatures close to the bulk isotropic-nematic phase transition. The critical point, terminating the first-order confined isotropic-nematic transition line, was determined for the first time.

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