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SA Rice

Publications and source records attributed to SA Rice.

13 recordsLinked to original sources
PubMed2000

Melting transition in a quasi-two-dimensional colloid suspension: influence of the colloid-colloid interaction

We report the results of a study, using digital video microscopy, of the melting transition in a quasi-two-dimensional suspension of uncharged silica spheres. This system was chosen to further test the dependence of the two-dimensional melting transition on the functional form of the colloid-colloid interaction. Our experimental data show that the solid phase undergoes a first order transition directly to the liquid phase. The system studied yields no evidence of the existence of a hexatic phase interpolating between the solid and liquid phases in the melting process.

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PubMed2000

Nature of the transition from two- to three-dimensional ordering in a confined colloidal suspension

We report the results of extensive molecular dynamics simulations of solid-to-solid transitions in two- to six-layer colloidal suspensions confined between two smooth parallel walls. The studies are designed to elucidate the ordered particle packings that interpolate between the structures of two- and three-dimensional crystals in a confined space. At a fixed density per layer, as the wall separation increases we find a sequence of stable phases, each characterized by uniform amplitude buckling along the normal to the layer planes. The buckling is coupled to an in-plane ordering transition. The buckled phases alternate with phases whose structures contain only parallel planes of particles. The relative densities of the positively and negatively displaced particles in a buckled layer, the in-plane structures, and the behavior with respect to increasing wall separation of the split density distribution that characterizes a buckled layer, clearly identify these layers as intermediates in the reconstructive transformations ntriangle up-->(n+1) square that occur when the character of the constrained space evolves from being two dimensional to being three dimensional (triangle up denotes layers with hexagonal packing symmetry, while square denotes layers with square packing symmetry). The two transitions, ntriangle up-->n-buckled-->(n+1) square, are found to be first order.

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PubMed2000

Hexagonal to square lattice conversion in bilayer systems

We report the results of extensive molecular dynamics simulations of the reconstructive hexagonal to square lattice conversion in bilayer colloid systems. Two types of interparticle potential were used to represent the colloid-colloid interactions in the suspension. One potential, due to Marcus and Rice, is designed to describe the interaction of sterically stabilized colloid particles. This potential has a term that represents the attraction between colloid particles when there is incipient overlap between the stabilizing brushes on their surfaces, a (soft repulsion) term that represents the entropy cost associated with interpenetration of the stabilizing brushes, and a term that represents core-core repulsion. The other potential we used is an almost hard core repulsion with continuous derivatives. Our results clearly show that the character of the reconstructive hexagonal to square lattice conversion in bilayer colloid systems is potential dependent. For a system with colloid-colloid interactions of the Marcus-Rice type, the packing of particles in the square array exhibits a large interlayer lattice spacing, with the particles located at the minima of the attractive well. In this case the hexagonal to square lattice transition is first order. For a system with hard core colloid-colloid interactions there are two degenerate stable intermediate phases, linear and zigzag rhombic, that are separated from the square lattice by strong first order transitions, and from the hexagonal lattice by either weak first or second order transitions.

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PubMed2000

Direct measurements of constrained brownian motion of an isolated sphere between two walls

We report the results of direct measurements, using video microscopy in combination with optical tweezers, of constrained diffusion of an isolated uncharged PMMA sphere in a density-matched fluid confined between two parallel flat walls. Our experimental methodology allows us to study the hindered diffusion of the sphere as an explicit function of its distance from the walls, without interference from sedimentation or from electrostatic interaction between the particle and the walls. The measured diffusion coefficients are used to test the predictions of the wall drag effect predicted by several approximate theoretical analyses. We find a quantitative agreement with the behavior predicted using a hydrodynamic analysis that independently superimposes the wall drag effects arising from each wall. Our results imply, indirectly, that neglect of multiple interactions with the colloid sphere of the perturbations of the pressure and velocity fields induced by each wall leads to an underestimate of the influence of the wall on the drag force experienced by the particle.

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