Composite colloidal assembly by critical Casimir forces.

We investigate the phase behavior of mixtures of two populations of colloidal particles dispersed in a binary solvent system near its critical composition. The surfaces of particles are chemically modified to elicit a specific solvent affinity for one of the solvents. In this way, fluid-mediated interactions, which involve the critical Casimir effect, become particle population specific. As a result, the colloidal mixture shows a complex crystallization behavior reminiscent of the crystallization of atomic alloys. We show that the exquisite temperature dependence and reversibility of the critical Casimir interaction allow sampling of the entire phase diagram of the binary system and can even be used to anneal the crystalline microstructure, analogous to temperature cycling in atomic alloy phases.