Spatially controlled reversible colloidal self-assembly.

We studied the localized self-assembly of colloidal crystals on a topographically patterned substrate. A competition between particle and pattern interactions provided the ability to reversibly assemble quasi-two-dimensional colloidal crystals on a periodic landscape. The assembly process was visualized and controlled in real-space and real-time using video microscopy. Independent measurements and computer simulations were used to quantify all interactions controlling self-assembly. Steady-state studies characterized spatially inhomogeneous, coexisting fluid and crystal microstructures at various stages of assembly. Microstructures arise from a balance of local sedimentation equilibria within potential energy features and a tunable pairwise depletion attraction between colloids. Transient colloidal crystal self-assembly occurred via a quasiequilibrium process as characterized by continuously evolving spatial profiles of local density, bond orientational order, and self-diffusivities.