The mean-square displacement [DELTA-r2(tau)] of particles in concentrated suspensions is measured at times sufficiently short to observe the transient nature of hydrodynamic interactions. For all volume fractions-phi, the velocity autocorrelation function decays as a power law R(tau) is similar to tau–3/2. A remarkable scaling with phi is observed for the time-dependent self-diffusion coefficient D(s)(tau) = [DELTA-r2(tau)]/6-tau: If D(s)(tau) is scaled by its asymptotic value and if time is scaled by a viscous time inversely proportional to the shear viscosity of the suspension, all the data fall onto a single master curve.

We report the formation of a solid gel network from purely liquid emulsion droplets. The gel remains rigid at droplet volume fractions as low as 10(-3). The gelation process is identified as diffusion-limited cluster aggregation. We find a surprising order in the gel structure. This ordering is induced by the aggregation kinetics, which result in an unexpected spatial correlation between the growing clusters, ensuring that the network is formed from clusters of nearly equal size and spacing.

The coalescence of monodisperse silicone oil-in-water emulsions stabilized with sodium dodecyl sulfate has been studied. We report the existence of a sharp destabilization threshold, controlled by surfactant chemical potential, osmotic pressure, and droplet diameter, at which the rate of coalescence increases dramatically. We present evidence that the stability of the emulsions can be characterized by two microscopic parameters: a minimum stable value of the surfactant chemical potential and a maximum value of the pressure exerted upon a droplet-droplet interface.