Experimental observations of non-Gaussian behavior and stringlike cooperative dynamics in concentrated quasi-two-dimensional colloidal liquids

We report, from direct observation of particle trajectories as a function o f time, the presence of stringlike cooperative motion in a quasi-two-dimens ional liquid. We have used digital video microscopy to study the equilibriu m dynamics of suspensions of sterically stabilized uncharged poly(methylmet hacrylate) spheres confined in a thin glass cell. Our experiments reveal th e existence, in semidilute and dense liquid states, of a transition in the qualitative dynamical behavior of the system. At short times particles unde rgo unhindered Brownian motion, at intermediate times they undergo uncorrel ated binary collisions, and at long times these one-particle self-diffusive modes are coupled to collective longitudinal acoustic modes of the fluid, the signature of which is local fluctuating domains of enhanced particle mo bility. We study the properties of these domains by examining the density d ependence of the van Hove self-correlation function and its deviation from Gaussian behavior. We observe that periods of non-Gaussian behavior correla te precisely with the timing of events involved in the relaxation of "caged " particles and their nearest neighbors. In contrast with relaxation proces ses in supercooled liquids, the lifetime of dynamical heterogeneities in a dissipative colloidal suspension is found to shift towards shorter time sca les with increasing particle density. During time periods for which a quasi -two-dimensional system follows Gaussian behavior, we observe that, as pred icted by Cichocki and Felderhof [J. Phys. Condens. Matter 6, 7287 (1994)], the time dependence of the evolution of the effective diffusion coefficient from its short time to its long time value has the form (In t)/t. This las t finding is true for all observed particle densities. To our knowledge, th ese results are the first experimental verification of the existence of mic roscopic cooperativity and the predicted temporal evolution of the diffusio n coefficient for Brownian motion in concentrated quasi-two-dimensional liq uids. [S1063-651X(99)06311-4].