Structures and Assembly Pathways of Colloidal Clusters

We experimentally study colloidal clusters of 6 to 100 spherical particles bound together by short-range attractions. These clusters are a model system for understanding colloidal self-assembly and dynamics, since the positions and motion of all particles can be observed. For 10 particles and fewer, the ground states are degenerate, and, as shown in previous work [1], the probabilities of observing specific clusters depend primarily on their rotational entropy, which is determined by symmetry. Thus, less symmetric structures are more frequently observed. However, for larger clusters the ground states appear to be subsets of close-packed lattices, which tend to have higher symmetry. To understand how this transition occurs as a function of the number of particles, we assemble and then anneal an ensemble of clusters. We fit a generative model to confocal microscopy data and then characterize the number of apparent ground states, their symmetries, and their probabilities as a function of the number of particles. We also study how these structures form by forming a chain of colloidal particles and watching it fold into its minimal-energy configuration.

1. G. Meng, et. al. Science 327 563 (2010).