(379a) Structure Formation During Vibration of Suspensions of Hard Spheres

Complex fluids, when subjected to vertical vibration, have been shown to exhibit a variety of interfacial morphologies considerably different from those observed in Newtonian fluids. Currently there is no accepted model or theory to explain many of the observed phenomena. Here we investigate the surface instabilities in thin layers of vertically vibrated, highly-filled dispersions of glass microspheres in aqueous solutions of varying density. In contrast to what has been reported previously, we find that the behavior is similar to that of a Newtonian fluid when the fluid and particle densities are approximately equal.  Large fluid-particle density differences and low particle volume fractions reveal standing waves around the outer edge of the dispersion.  At high particle volume fraction, spontaneous craters form and grow into protrusions that quickly collapse.  Under some conditions, the suspension gathers itself into a single, spherical ball floating on a thin fluid layer.  Fluid density, volume fraction and particle size impact the presence and topology of the observed phenomena.