

Abstract: From silicon computer chips to aluminum alloy airplane wings, crystalline materials are made up of ordered crystal regions, or grains, which are typically separated by disordered grain boundaries. The presence and motion of these grain boundaries determine material properties ranging from mechanical strength to electrical conductivity. We study grain boundary motion in colloidal crystals composed of micron-diameter spheres suspended in a liquid. The particles are small and light enough that they undergo Brownian motion, which jiggles them into thermally fluctuating crystal structures that can be directly observed with a light microscope, offering a direct view of particle-scale mechanisms for grain boundary motion. We focus on single-layer crystals in which a grain completely enclosed within another grain shrinks, while the surrounding grain grows, a process known as grain coarsening. Prevailing continuum theories predict that the rate of shrinking is constant, but our experiments show that such grains shrink in steps, with periods of slow dissolution punctuated by brief events that quickly shrink the grain. The fast events involve cooperative rotation of hexagon-shaped clusters of particles at the edge of the grain. We determine the cause of this previously unreported mechanism and predict where these rotations will occur, based on the crystal orientations of the growing and shrinking grains. Incorporation of this cluster rotation mechanism into models for grain coarsening could help provide more accurate predictions for how self-assembled crystals age.