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.

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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.

The addition of enough non-adsorbing polymer to a hard-sphere suspension causes the particles to aggregate to form a space-Ðlling gel. The integrity of the gel persists for a Ðnite period of time, and then the space-Ðlling structure collapses suddenly to form a denser sediment. This phenomenon of " delayed sedimentation Ï is ubiquitous in many weakly-Ñocculated suspensions. In this work, we observe the processes occurring in the bulk of a colloidÈpolymer gel using dark-Ðeld imaging, and probe the arrangement and dynamics of the particles in the system using two-colour dynamic light scattering. The e †ect of shear is also studied. A number of physical mechanisms relevant to a comprehensive explanation of delayed sedimentation are proposed and discussed.

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