Direct Imaging of Vibrations in Colloidal Crystals: In Equilibrium and in a Steady Drift

Crystals of colloids, micron-size particles in a solvent, typically contain high concentrations of structural defects, limiting their applicability in self-assembly of metamaterials. Defects and grain boundaries play an important role for most properties of these crystals. Most previous research of colloidal crystals, by experiment and theory, focused on spatially averaged vibrational spectra: the differences in local environment between the bulk crystal particles and those at a grain boundary were typically neglected. We employ direct confocal microscopy and recent more accurate particle tracking algorithms to study the potential wells of individual particles in thermally vibrating quasi-two-dimensional colloidal crystals. We demonstrate that the energy landscape probed by a particle sensitively depends on its local environment. Furthermore, we emphasize the commonly neglected role of slight out-of-equilibrium drift of colloidal crystals, demonstrating that particle vibrations depend significantly on the drift velocity, so that the drifting crystals are softer, allowing an effective "drift temperature" to be defined.