Condensed Matter Theory

Abstract:

We examine the possibilities to use the intrinsic 3D defect networks in blue phases I and II as arrays of trapping sites for colloidal particles. Our approach based on the phenomenological Landau-de Gennes description and topological theory has proven to be extremely useful in dealing with nematic colloids. A perturbed orientational order leads to effective anisotropic long range inter-particle coupling and consequently to numerous organizations of colloidal particles not present in simple liquids. Recent developments that led to the blue phases with extended stability range make them more attractive for use. In these phases the competition between nematic ordering and intrinsic tendency to form double twisted deformations yields complex director patterns and disclination networks. The spatially deformed order that mediates the attraction of particles to the network sets the ground for a possible self-assembling of 3D superstructures with extended stability ranges. Here we first describe the trapping mechanism on the case of a single discilination line and then use the results to demonstrate the trapping in the blue phase II. Effects of particle sizes ranging from submicron to 50 nanometers are examined. The assembling in blue phases is expected to form photonic crystals that can be easily manipulated via affecting the liquid crystal matrix and/or colloidal particles. © 2010 SPIE.

Abstract:

In patchy particle systems where there is competition between the self-assembly of finite clusters and liquid-vapour phase separation, reentrant phase behaviour is observed, with the system passing from a monomeric vapour phase to a region of liquid-vapour phase coexistence and then to a vapour phase of clusters as the temperature is decreased at constant density. Here, we present a classical statistical mechanical approach to the determination of the complete phase diagram of such a system. We model the system as a van der Waals fluid, but one where the monomers can assemble into monodisperse clusters that have no attractive interactions with any of the other species. The resulting phase diagrams show a clear region of reentrance. However, for the most physically reasonable parameter values of the model, this behaviour is restricted to a certain range of density, with phase separation still persisting at high densities.

Abstract:

We investigate the wetting properties of surfaces patterned with fine elastic hairs, with an emphasis on identifying superhydrophobic states on hydrophilic hairs. We formulate a 2D model of a large drop in contact with a row of equispaced elastic hairs and, by minimizing the free energy of the model, identify the stable and metastable states. In particular, we concentrate on partially suspended states, where the hairs bend to support the drop--singlet states, where all hairs bend in the same direction, and doublet states, where neighboring hairs bend in opposite directions--and find the limits of stability of these configurations in terms of the material contact angle, hair flexibility, and system geometry. The drop can remain suspended in a singlet state at hydrophilic contact angles, but doublets exist only when the hairs are hydrophobic. The system is more likely to evolve into a singlet state if the hairs are inclined at the root. We discuss how, under limited circumstances, the results can be modified to describe an array of hairs in three dimensions. We find that now both singlets and doublets can exhibit superhydrophobic behavior on hydrophilic hairs. We discuss the limitations of our approach and the directions for future work.