Cooperative motion and granule rotation during rapid grain shrinking in colloidal crystals

In 2D polycrystals, fully enclosed grain boundary loops tend to dissolve over time. Our colloid experiments show that grain boundary loops shrink via two processes: steps of rapid dissolution and plateaus of slow dissolution. This contradicts continuum theory, which predicts a constant rate of dissolution. We find that the periods of rapid dissolution involve the cooperative motion of particles together in rotating, hexagonal ‘granules.’ During such granule rotation, particle displacements follow a string-like pattern as previously observed in glassy dynamics at grain boundaries, but here we find that motion is directed along the outlines of the underlying Moiré pattern. Furthermore, we employ Brownian Dynamics simulations of grain boundary loops to study the effect of crystal area fraction, finding that tighter crystals shrink more quickly, via cooperative hexagonal granule rotation. Collectively these results point to the importance of cooperative granule rotation in enabling rapid grain dissolution.