Two-dimensional Binary Colloidal Self-Assembly in and Out of Equilibrium

Self-assembly of colloidal particles gives rise to the spontaneous formation of ordered structures both under equilibrium and out-of-equilibrium conditions. We study the self-assembly of a binary mixture of two different particle sizes and find features distinct to the assembly conditions. In equilibrium self-assembly driven by short-ranged depletion interactions, we find that the frustrated assembly kinetics result in amorphous structures even when there is a marginal mismatch between the two particle sizes (i.e., size ratio of 0.88). Our experiments show that the crystalline order of the assembly decreases with an increasing fraction of impurity particles, reaching up to 18% reduction in a 1:1 binary mixture compared to a monodisperse sample. On the other hand, the out-of-equilibrium assembly is driven by light-induced convection flow and forms crystalline monolayers within the timescale of a few minutes. In a dilute suspension, the higher velocity of the smaller particles helps them nucleate into small crystalline grains rapidly in the sparse presence of the larger particles. In a concentrated sample with a crowded initial condition, the two particle types form the monolayer together and hence compromise the crystalline order. While the assembly kinetics play a major role under both equilibrium and non-equilibrium conditions, we find that the flow-driven non-equilibrium assembly offers better control over the assembly structure and dynamics.