N 95-14573 dynamics of hard sphere colloidal dispersions

One of the most fundamental systems in nature is that of hard spheres, particles that do no interpenetrate but otherwise do not overlap. Hard spheres display many of the thermodynamic, hdyrodynamic, and transport properties of both molecular and colloidal systems, including transitions from disordered fluid to crystalline solid or disordered glasslike states and a host of nonideal static and dynamic phenomena. Physical realization of this most basic of interacting systems is possible with the inert gases only at extreme temperatures or pressures, but several wellstudied colloidal particles provide very faithful approximations and are amenable to study at convenient length and time scales through dynamic and static light scattering. The synthesis of monodisperese spheres, stabilization by repulsions of range short relative to the radius, and index matching in organic solvents to minimize van der Waals attraction snd multiple scattering provides convincing hard sphere behavior over moderate time scales [ 1,2]. However, experiments with equilibration times of of days to weeks, such as crystallization, suffer from sedimentation due to the density difference between the solvent and particles [3]. The specific shortcomings of existing studies pertain primarily to the disorder-order transition, a subtle entropy driven process that leads to coexistence between a disordered phase with liquid-like order at a volume fraction 4~0.494 and a face-centered-cubic solid at 4~0.545. Experiments confirm fairly well the phase boundaries and suggest a glass transition at $=0.56-58, but detect a crystal structure more closely described by random stacking of hexagonally packed layers [4]. In addition, small angle light scattering studies of crystal growth do not yield the form factors expected for the linear growth of uncorrelated crystallites, but a considerably more complex form [5]. Either or both of these could arise from subtle effects of gravitational settling, which produces settling velocities comparable to diffusion velocities early in the process of nucleation and growth of crystallites. Furthermore, the equilibrium state under gravity consists of an almost closepacked, inhomogeneous crystal or glass, far from the homogeneous sample desired. So our objective is to perform on homogeneous, fully equilibrated dispersions the full set of experiments characterizing the transition from fluid to solid and the properties of the crystalline and glassy solids. These include measurements quantifying the nucleation and growth of crystallites, the structure of the initial fluid and the fully crystalline solid, the Brownian motion of particles within the crystal, and the elastiGity of the crystal and the glass. Experiments are being built and tested for the ideal microgravity environment. Here we describe the ground based effort, which exploits a fluidized bed to create a homogeneous, steady state dispersion for the studies. The difference between the microgravity environment and the fluidized bed is gauged by the Peclet number Pe, which measures the rate of convectiodsedimentation relative to Brownian motion. We have designed our experiment to accomplish three types of measurements on hard sphere suspensions in a fluidized bed the static scattering intensity as a function of angle to determine the structure factor, the temporal autocorrelation function at all scattering angles to probe the dynamics, and the amplitude of the response to an oscillatory forcing to deduce the low frequency viscoelasticity. Thus the scattering instrument and the colloidal dispersion were chosen such that the important features of each physical property lie within the detectable range for each measurement.