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 onto a large array of holographic optical tweezers. These optical traps constitute an effective substrate potential whose symmetry, separation and depth of modulation all 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 kinetically hindered 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.