Hexagonal to square lattice conversion in bilayer systems

We report the results of extensive molecular dynamics simulations of the re constructive hexagonal to square lattice conversion in bilayer colloid syst ems. Two types of interparticle potential were used to represent the colloi d-colloid interactions in the suspension. One potential, due to Marcus and Rice, is designed to describe the interaction of sterically stabilized coll oid particles. This potential has a term that represents the attraction bet ween colloid particles when there is incipient overlap between the stabiliz ing brushes on their surfaces, a (soft repulsion) term that represents the entropy cost associated with interpenetration of the stabilizing brushes, a nd a term that represents core-core repulsion. The other potential we used is an almost hard core repulsion with continuous derivatives. Our results c learly show that the character of the reconstructive hexagonal to square la ttice conversion in bilayer colloid systems is potential dependent. For a s ystem with colloid-colloid interactions of the Marcus-Rice type, the packin g of particles in the square array exhibits a large interlayer lattice spac ing, with the particles located at the minima of the attractive well. In th is case the hexagonal to square lattice transition is first order. For a sy stem with hard core colloid-colloid interactions there are two degenerate s table intermediate phases, linear and zigzag rhombic, that are separated fr om the square lattice by strong first order transitions, and from the hexag onal lattice by either weak first or second order transitions.