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.
Pub Type: Journals
microstructure charaterization, small-angle X-ray scattering, X-ray photon correlation spectroscopy, colloidal dispersions, particle dynamics