For decades colloids have been known to form ordered crystalline phases when they're packed together closely enough—and to exhibit many of the same phase transitions as atomic and molecular systems, but on length and time scales more easily studied. In 2005 Arjun Yodh and colleagues at the University of Pennsylvania opened up a new realm of possibilities for studying colloidal phase transitions: They devised micrometer-sized polymer particles that reversibly shrink when heated, so the colloid's packing density can readily be tuned to drive the phase transition. Yodh's team used the particles to study heterogeneous melting, which begins at a grain boundary or other defect. Now Yilong Han and his colleagues at the Hong Kong University of Science and Technology have used the heat-sensitive particles to study the poorly understood homogeneous melting, in which a superheated perfect crystal melts from the inside out via spontaneous nucleation of small liquid regions. Han and company looked at some 200 melting transitions; snapshots from one of them are shown in the figure. Blue, green, and orange dots represent solid-phase particles with progressively greater deviations from their lattice positions; liquid-phase regions are shown in red. The researchers found that melting began not with the spontaneous formation of crystal defects, as some computer simulations had predicted, but rather with so-called loop rearrangements, in which particles (such as those marked in white in the first panel) swapped places while leaving the crystal structure intact. (Z. Wang et al., Science 338, 87, 2012.)—Johanna Miller
