Evolution of a colloidal critical state in an optical pinning potential landscape

As a step toward isolating the influence of a modulated substrate potential on dynamics and phase transitions in two dimensions, we have studied the behavior of a monolayer of colloidal spheres driven by hydrodynamic forces into a large array of holographic optical tweezers. These optical traps constitute a substrate potential whose symmetry, separation, and depth of modulation can be varied independently. We describe a particular set of experiments, in which a colloidal monolayer invades the optical pinning potential much as magnetic flux lines invade type II superconductors, including cooperative avalanches, streaming motion, and a symmetry-altering depinning transition. The jammed intermediate state in this process resembles the critical state long associated with flux entering zero-field-cooled superconductors. By tracking the particles' motions, we are able to determine the microscopic processes responsible for the evolution of the colloidal critical state and compare these with recent simulations of flux-line dynamics.