Colloids on free-standing smectic films.

We study the structure of a free-standing smectic-A film around a micron-size polystyrene colloid adsorbed onto the film. We find that a colloid or a cluster of colloids is surrounded by an optically distinct and radially decorated meniscus ending with a sharp edge. The observed strong and finite-range attraction between the adsorbed colloids is driven by the fusion of menisci. We interpret the structure of the smectic meniscus in terms of a model dominated by the surface free energy and we argue that the characteristic appearance of the meniscus is due to layer undulations.

Melting of colloidal molecular crystals on triangular lattices.

The phase behavior of a two-dimensional colloidal system subject to a commensurate triangular potential is investigated. We consider the integer number of colloids in each potential minimum as rigid composite objects with effective discrete degrees of freedom. It is shown that there is a rich variety of phases including "herringbone" and "Japanese 6 in 1" phases. The ensuing phase diagram and phase transitions are analyzed analytically within variational mean-field theory and supplemented by Monte Carlo simulations. Consequences for experiments are discussed.

Structures and transitions in thin hybrid nematic films: a Monte Carlo study.

We confirm by Monte Carlo simulations of a Lebwohl-Lasher lattice spin model the existence of a biaxially ordered nonbent structure in a liquid-crystalline cell subject to opposing boundary conditions. We report on the observation of the bending transition from the biaxial to the bent-director structure when the temperature of the system is lowered. The structural transition is monitored both by the change of the order parameters and by heat capacity. We discuss the thickness dependence of the transition temperature by means of wetting-induced phenomena and elastic deformations. We propose the correspondence to the phenomenological description, which agrees well without any fitting parameters.

Van der Waals interaction mediated by an optically uniaxial layer.

We study the van der Waals interaction between macroscopic bodies separated by a thin anisotropic film with a uniaxial permittivity tensor. We describe the effect of anisotropy of the media on the magnitude and sign of the interaction. The resulting differences in the van der Waals interaction are especially important for the stability of strongly confined liquid crystals, and nanostructures characterized by highly uniaxial macroscopic molecular arrangement, such as in self-assemblies of long organic molecules forming films, membranes, colloids, etc. We introduce an improved expression for the Hamaker constant which takes into account the uniaxial symmetry of a medium. In special cases neglecting the optical anisotropy even leads to an incorrect sign of the interaction.

Equilibrium structures and pretransitional fluctuations in a very thin hybrid nematic film.

The stability of different structures of a nematic liquid crystal in a planar hybrid film is examined within the framework of a Gaussian description of order fluctuations. In a very thin film the director field is not bent smoothly but exhibits a steplike change if the anchorings at the confining substrates are strong (G approximately > or =10(-3) J/m(2)) and comparable in magnitude. A (dis)continuous structural transition to the bent-director state which occurs with increasing film thickness or decreasing temperature is governed by the lowest bending director fluctuation mode. Its relaxation rate exhibits a critical slowdown when the film thickness approaches the "supercooling" limit or transition point, respectively. The (dis)continuity of the structural transition depends on the temperature and film thickness. The upper limit for the corresponding tricritical point is determined. The lowest order parameter mode, which corresponds to fluctuations of the position of the central exchange region, is characterized by a nearly critical behavior of the relaxation rate. The spectra of the two noncritical biaxial fluctuation modes are degenerate, whereas the fluctuation profiles are just mirror images with respect to the middle plane of the film.