Dynamical processes of interstitial diffusion in a two-dimensional colloidal crystal*

We report the first study of the dynamical processes of interstitials in a 2D colloidal crystal. The diffusion constants of both mono- and di-interstitials are measured, and found to be significantly larger than those of vacancies. Di-interstitials are clearly slower than mono-interstitials. We found that, by plotting the accumulative positions of 5- and 7-fold disclinations relative to the center-of-mass position of the defect, a 6-fold symmetric pattern emerges for mono-interstitials. This is indicative of an equilibrium behavior that satisfies local detailed balance that the lattice remains elastic and can be thermally excited between lattice configurations reversibly. However, for di-interstitials the 6-fold symmetry is not observed in the same time window, the local lattice distortions are too severe to recover quickly. This observation suggests a possible route to creating local melting of a lattice (similarly one can create local melting by creating di-vacancies). This work opens up a new avenue for microscopic studies of the dynamics of melting in colloidal model systems.