Activated surface diffusion in a simple colloid system.

We investigate, from single-particle tracking of jumps, the cluster configurations that allow hopping over a geometric activation barrier in surface diffusion. Spherical colloidal particles, their dimers, and their isomeric planar trimers are compared on hexagonal surface lattices commensurate with the elemental size of one particle. The experiments reveal that translational and rotational mobility depend on the shape of these clusters, not only on their mass, because the jump process favors a restricted family of cluster configurations. The resulting strong decoupling between rotation and translation demonstrates the limitations of a naïve Arrhenius picture, even for a simple gravitational potential.