Real crystals are rarely perfect; they tend to contain the occasional defect, such as the one in the colloidal packing depicted here. The packing is visualized as a Voronoi diagram, with each 2-µm-diameter colloidal particle represented by a polygonal cell. In what’s known as a dislocation, two extra rows of particles, indicated by dashed yellow lines, have been squeezed into the hexagonal lattice, leaving one particle (white) with only five nearest neighbors and another (gray) with seven. (For more on colloidal crystals, see Physics Today, September 2010, page 30, and December 1998, page 24.) A form of topological defect, dislocations are common in both colloidal and atomic crystals. Understanding how they emerge and interact is key to modeling the kinetics of melting and of solid-to-solid phase transitions. But experimentalists have lacked the means to reproducibly control such defects in the lab. Now William Irvine (University of...

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