Effects of the Attractive Potential Range on the Phase Behavior of Small Clusters of Colloidal Particles

We report the findings of a computational study designed to discover the effects of changing the range of a depletion-based attractive potential on the phase behavior of assemblies of 32 colloidal particles. The tunable model potential has been experimentally realized using a system of silica particles and hydrogel depletants in previous work. Using Monte Carlo umbrella sampling with coarse graining in two order parameters, we generate free energy landscapes that indicate coexistence between phases. When the range of the depletion interaction is short, a fluid-like phase is in equilibrium with a relaxed face-centered cubic (FCC) crystalline phase. As the interaction length is increased to about 20 % of the silica particle radius, a relaxed hexagonal close-packed structure is observed in coexistence with the relaxed FCC structure.