PerspectiveMaterials Science

Turning Away from High Symmetry

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Science  29 Jan 2010:
Vol. 327, Issue 5965, pp. 535-536
DOI: 10.1126/science.1184457

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A common sight in cold climates is the growth of crystalline ice “feathers” or “ferns” on a cold window pane. These structures are initiated by heterogeneous nucleation—the ice crystals form on the surface of a different substance, in this case, specks of dust. It is much harder to start crystal formation in pure fluids because the seed crystal must form spontaneously in solution (hence pure water can supercool and remain as a liquid for long times below its freezing point). Homogeneous nucleation is largely a mystery; observing small clusters of molecules directly is difficult, and their formation is a rare event. On page 560 of this issue, Meng et al. (1) report an exhaustive experimental study of the equilibrium cluster configurations in a model system consisting of microscopic plastic spheres that were designed to have a short-ranged, reversible attraction that drives them together (2). This work underscores the subtle geometrical difficulties associated with crystal nucleation, and helps us to understand the “rules” by which nature self-assembles small structures and ultimately crystals.