Phoretic dynamics of colloids in a phase separating critical liquid mixture

Abstract

Different phoretic effects have been widely exploited for designing self-propelled colloidal systems, but the underlying wave vector dependent dynamics has been little investigated. In this work, the out-of-equilibrium dynamics of colloids in near-critical liquid mixtures undergoing spinodal phase separation was probed by x-ray photon correlation spectroscopy (XPCS). The emergent dynamics of charge stabilized silica and silica-nickel Janus particles upon a temperature jump into the two-phase region of the solvent mixture displayed similar features at the initial stage. In both systems, the phoretic dynamics is dominated by velocity fluctuations induced by the solvent concentration fluctuations and the hydrodynamic flow during the coarsening process. Furthermore, relaxation rates of the diffusive part of the dynamics manifested an anomalous wave vector dependence akin to the superdiffusive behavior with an effective diffusion coefficient significantly larger than the Brownian limit. For smaller temperature jumps, velocity fluctuations exhibited a broader distribution with silica and Janus colloids showing qualitatively different behavior. The velocity fluctuations decayed with time and the dynamics reverted to diffusive behavior upon completion of the phase separation. Presented results illustrate the ability to probe faster collective dynamics pertinent to active colloids using multispeckle XPCS.