Hexagonal vortices during grain coarsening in colloidal crystals

In polycrystalline materials, a grain boundary loop arises when one crystal grain is embedded within a surrounding crystal. The enclosed grain shrinks to minimize free energy, and continuum theory predicts a linear rate of dissolution. However, both our experimental studies and brownian dynamics simulations of grain dissolution reveal a combination of two distinct behaviors: a fast process and a slow process. While the slower dissolution behavior matches with existing models for grain boundary motion, the faster process involves coordinated rotations of groups of particles, acting like mini-grains or "granules." These granule rotations can be understood by comparing the hexagonal patterns of particle displacement vorticity to the Moiré pattern obtained by overlaying the two misaligned crystal lattices. We estimate the free energy cost of grain dissolution by directly computing the free space available to each particle, and use this to interrogate the differences between granule rotation and typical grain boundary motion. Other studies have found hexagonal vorticity in magnetically-driven crystals, or during grain splitting, but here we find granule rotation is a generalized mechanism of grain boundary motion for undriven grain coarsening.