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We calculate the long-range pair potential between spherical colloid particles suspended in a uniform liquid crystal. At weak director anchoring on the particle surface the director distortions decay as ␦nϳr Ϫ3 sin2 away from its center. This leads to an anisotropic interaction potential of the form Uϳd Ϫ5 with d the distance between two colloid particles. This interaction also depends on the particle position with respect to the nematic director, giving strong repulsion for particles along the director axis and for particles located in the perpendicular plane, and attraction at oblique angles. We also analyze the system behavior in an external field, when the director decays exponentially away from the particle and, hence, interaction forces are short range. ͓S1063-651X͑97͒02603-2͔PACS number͑s͒: 61.30. Gd, 82.70.Dd, 64.70.Md Colloid suspensions are characteristically mesoscopic systems with structure and time scales such that typical shear rates can bring them out of equilibrium and into some exotic states. Even without a flow the structure and properties of colloids pose a number of theoretical and experimental challenges ͓1,2͔. Of much interest are various novel interactions, for instance, hydrodynamic and polymeric solvent-mediated forces. Many obvious practical applications of colloid systems add to the fundamental interest of this class of objects.Colloid suspensions in a liquid crystal matrix are qualitatively different from their isotropic analogues due to the long-range deformation field n͑r͒ created by particles in the liquid crystal ͓3͔. Perhaps one of the most important applications of liquid crystal colloids is the moulding processing of filled liquid crystalline polymers and the suspension of abrasive particles in lyotropic mesophases. Recently the phase equilibrium of a nematic colloid has been examined experimentally ͓4͔. In that work the authors also explore the theoretical model, accounting for the steric interaction between particles and the concept of ''distortion'' of nematic order by individual particles.Obviously, the effect of a suspended particle on the orientational order in its surrounding will depend on the strength and type of director anchoring on its surface. Colloids with extremely weak anchoring will disturb the static director field very little, although in the flow one must expect a significant effect due to the Leslie-Ericksen coupling ͓5͔. Particles with very strong anchoring create a topological mismatch with the otherwise uniform director field and develop singularities. The resulting director texture can have a quadrupolar symmetry, such as that of the nematic matrix itself. In this case one obtains a pair of polar boojums for planar anchoring, or a (Ϫ1/2) disclination ring for radial ͑homeotropic͒ anchoring. The equilibrium quadrupolar director field around a single spherical particle has been analyzed theoretically ͓3͔, where it has been shown that in all cases the director distortions decay as r Ϫ3 away from the particle. It is possible that large particles with an a...

Mixing model colloidal particles with a thermotropic nematic liquid crystal results in a soft solid with significant storage modulus (G' approximately 10(3)-10(5) Pa). The soft solid comprises a network of particle aggregates, formed by the exclusion of particles from emergent nematic domains as the mixture is cooled below the isotropic-nematic transition. The unusually high storage modulus of the colloid-liquid-crystal composites may be due to the local frustration of nematic order within the particle aggregates. The birefringent soft solid is potentially important as a switchable electro-optical material that can be readily handled and processed.

We consider the elastic and orientational response of a uniform nematic elastomer subjected to an extension perpendicular to its director. By allowing a possibility of local shear in the material, we show that the effect of “soft elasticity” leads to a new regime of director reorientation, through a highly non-uniform stripe domain state (in contrast to earlier predictions and observations of a discontinuous uniform director jump). The molecular theory developed here gives predictions on two levels: of the general texture of the stripe state plus the interval of strains in which it occurs, and of topological properties of the director rotation that are very general and depend only on chain anisotropy of elastomer but not on details of the specific material. On the other hand, parameters like the threshold strain for the domain formation depend on the chemical composition and on the model used to describe its effect. We discuss and explain experimental observations of stripe domains both in the perpendicular geometry and when the stretching direction is at an oblique angle to the director, leading to asymmetric stripes and different topology

Abstract. We study the phase ordering colloids suspended in a thermotropic nematic liquid crystal below the clearing point Tni and the resulting aggregated structure. Small (150 nm) PMMA particles are dispersed in a classical liquid crystal matrix, 5CB or MBBA. With the help of confocal microscopy we show that small colloid particles densely aggregate on thin interfaces surrounding large volumes of clean nematic liquid, thus forming an open cellular structure, with the characteristic size of 10 − 100 µm inversely proportional to the colloid concentration. A simple theoretical model, based on the Landau mean-field treatment, is developed to describe the continuous phase separation and the mechanism of cellular structure formation.