The Free Energy Landscape of Hard Sphere Clusters

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Entry by Leon Furchtgott, APP 225 Fall 2010.

The Free-Energy Landscape of Clusters of Attractive Hard Spheres. Meng, G., Arkus, N., Brenner, M. P., & Manoharan, V. N. (2010). Science, 327, 560-563


The paper is interested in the behavior of small (10 or fewer colloidal particles) clusters and their relation to bulk behavior. In particular, the paper discusses the thermodynamics of small clusters: what structures are favored by entropy or by the potential energy, how this competition changes as N grows larger. Through careful experimentation the authors succeed in measuring structures and free energies of small equilibrium clusters. They compare these structures to theoretical predictions and draw conclusions regarding highly favored configurations.

Experimental Setup

Fig. 1. A. Experimental system: microwells filled with polyNIPAM microgel particles and PS microbeads. B. Close-up view of two sticky microspheres and surrounding nanoparticles. C. Pair potential. Note that the depletion attraction is very short-range, so the interaction is strictly pairwise additive. D. Micrograph of several microwells, each with different individual clusters. E. Optical micrographs of colloidal clusters in microwells with N = 2, 3, 4, 5 particles.