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 be tween two smooth parallel walls. The studies are designed to elucidate the ordered particle packings that interpolate between the structures of two- a nd three-dimensional crystals in a confined space. At a fixed density per l ayer, as the wall separation increases we and a sequence of stable phases, each characterized by uniform amplitude buckling along the normal to the la yer planes. The buckling is coupled to an in-plane ordering transition. The buckled phases alternate with phases whose structures contain only paralle l planes of particles. The relative densities of the positively and negativ ely displaced particles in a buckled layer, the in-plane structures, and th e behavior with respect to increasing wall separation of the split density distribution that characterizes a budded layer, clearly identify these laye rs as intermediates in the reconstructive transformations n Delta -> (n + 1 ) square that occur when the character of the constrained space evolves fro m being two dimensional to being three dimensional (Delta denotes layers wi th hexagonal packing symmetry, while square denotes layers with square pack ing symmetry). The two transitions, n Delta -> n - buckled -> (n + 1)square , are found to be first order. PACS number(s): 64.70.Dv.