Using Depletion to Tune Colloid Shape for Assembly

Depletion interactions arise from entropic forces and their ability to induce aggregation and even ordering of colloidal particles through self-assembly is well established, especially for spherical colloids. We vary the size and concentration of penetrable hard sphere depletants in a system of nonspherical colloids, and show how depletion effectively changes the shape of the colloids and thereby selects different crystal structures. We provide a simple explanation of this effective shape change using cuboctahedra, and explain the stability of the distinct crystals formed via depletion using free energy calculations that consider separately the contribution of the cuboctahedra and depletant entropy. We show that, counterintuitively, the colloid contribution to the free energy stabilizes the simple cubic phase. We corroborate our results by analyzing how the depletant concentration and size affect the emergent directional entropic forces and hence effective particle shape. We propose the use of depletants as a means of easily changing the effective shape of self-assembling anisotropic colloids.