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The conformation and diffusion of a single DNA molecule confined between two parallel plates are examined using both single molecule experiments and Brownian dynamics simulations accounting for hydrodynamic interactions. The degree of chain stretching and the diffusivity are characterized as a function of the chain confinement and the channel geometry. Good agreement is found between the simulations, experiments, and scaling theory predictions.

This Letter investigates the defect structures that arise between two colloidal spheres immersed in a nematic liquid crystal. Molecular simulations and a dynamic field theory are employed to arrive at molecular-level and mesoscopic descriptions of the systems of interest. At large separations, each sphere is surrounded by a Saturn ring defect. However, at short separations both theory and simulation predict that a third disclination ring appears in between the spheres, in a plane normal to the Saturn rings. This feature gives rise to an effective binding of the particles. The structures predicted by field theory and molecular simulations are consistent with each other.

Insertion of spherical particles into a uniform nematic liquid crystal gives rise to the formation of topological defects. In the present work, we investigate how a spherical particle accompanied by its topological defects interacts with neighboring disclination lines. We perform two- and three-dimensional dynamic simulations to analyze the effect of a particle on the annihilation process of two disclination lines. The dynamics of the liquid crystal is described by a time-dependent evolution equation on the symmetric traceless order parameter that includes some of the salient features of liquid crystalline materials: excluded volume effects, or equivalently, short-range order elasticity and long-range order elasticity. At the surface of the particle, the liquid crystal is assumed to exhibit strong homeotropic anchoring. The particle is located between two disclination lines of topological charges +1/2 and -1/2. Two-dimensional simulations indicate that the topological defects bound to the particle mediate an interaction between the two disclination lines which increases the attraction between them. This result is confirmed by three-dimensional simulations that provide a complete description of the director field and of the order parameter around the particle. These simulations indicate that a spherical particle between two disclination lines can be surrounded by a Saturn ring, and suggest that the dynamic behavior of disclination lines could be used to report the structure of a defect around the particle.

The effects of confinement on the director field configurations are studied for a spherical particle immersed in a nematic liquid crystal. The liquid crystal is confined in a cylindrical geometry and the particle is located on the axis of symmetry. A finite element method is used to minimize the Frank free energy for various sizes of the system. The liquid crystal is assumed to possess strong anchoring at all the surfaces in the system. Two structures are examined for strong homeotropic anchoring at the surface of the particle: configuration with a Saturn ring disclination line and configuration with a satellite point defect (hedgehog defect). It is shown that the equilibrium locations of the Saturn ring and of the hedgehog point defect change with confinement. It is also found that confinement induces an increase in the elastic free energy that differs substantially with the type of topological defect under consideration. In particular, the evaluation of the total free energy that includes an approximate contribution for the core defect shows that, for micrometer-sized particles in confined systems, the Saturn ring configuration appears to be more stable than the hedgehog defect. This result is in contrast to the bulk situation, where the hedgehog is more stable than the Saturn ring, and it helps explain recent experimental observations of Saturn ring defects around confined micrometer-sized solid particles.

The low-temperature phase coexistence of size-asymmetric primitive model electrolyte solutions has been investigated by means of Monte Carlo simulations. Binodal curves and critical parameters are reported as a function of size ratio lambda = sigma(+)/sigma(-) in the range 0.05 to 1. Both the critical temperature and the critical density are found to decrease as lambda decreases. These trends are in conflict with available theoretical predictions. For highly asymmetric systems, the depression of the critical parameters is accompanied by the formation of large chainlike and ringlike structures, thereby giving rise to considerable finite-size effects.

Properties such as the glass transition temperature ( T(g)) and the diffusion coefficient of ultrathin polymeric films are shown to depend on the dimensions of the system. In this work, a hard-sphere molecular dynamics methodology has been applied to simulate such systems. We investigate the influence that substrates have on the behavior of thin polymer films; we report evidence suggesting that, depending on the strength of substrate-polymer interactions, the glass transition temperature for a thin film can be significantly lower or higher than that of the bulk.