Structure and Dynamics Studies of Concentrated Micrometer-Sized Colloidal Suspensions
Fan Zhang, Andrew J. Allen, Lyle E. Levine, Jan Ilavsky, Gabrielle G. Long
We present an experimental study of the structural and dynamical properties of concentrated suspensions of a series of different sized polystyrene microspheres dispersed in glycerol for volume fraction concentrations between 10 % and 20 %. The static structure, probed with ultra-small angle X-ray scattering, shows a behavior very similar to that of hard spheres. The equilibrium dynamics is probed with ultra-small angle X-ray scattering X-ray photon correlation spectroscopy, a new technique that overcomes the limits of visible light scattering techniques imposed by multiple scattering and is suitable for studies of optically opaque materials containing micrometer-sized structures. Contrary to a previous study of a similar, but much smaller nano-colloid system (Lurio et al., Phys. Rev. Lett. 84, 785, (2000)), we found that the intensity autocorrelation functions are better described by a stretched exponential function and microspheres in a concentrated suspension move collectively, rather than behaving as free particles in Brownian motion. We also found that the inverse of the effective diffusion coefficients display a peak with respect to the scattering vector that resembles the peaks in the static structure factors, which indicates that a long-lived, low free-energy state exists. The relaxation time is inversely related to scattering vector, a behavior consistent with models that describe the dynamics in terms of random, local structural arrangements in disordered elastic media.