Nonmonotonic settling of a sphere in a cornstarch suspension\ud

Cornstarch suspensions exhibit remarkable behavior. Here, we present two unexpected observations for a sphere settling in such a suspension: In the bulk of the liquid the velocity of the sphere oscillates around a terminal value, without damping. Near the bottom the sphere comes to a full stop, but then accelerates again toward a second stop. This stop-go cycle is repeated several times before the object reaches the bottom. We show that common shear thickening or linear viscoelastic models cannot account for the observed phenomena, and propose a minimal jamming model to describe the behavior at the botto

Dense suspensions: force response and jamming

The response of dense suspensions to an external force was studied using two different experiments.\ud \ud In the first experiment, objects were settled in a deep bath of a dense cornstarch suspension. This is the only suspension to result in two unexpected phenomena: Velocity oscillations in the bulk, and stop-go-cycles near the bottom. Both are believed to be caused by a change in the particle configuration within the suspension.\ud \ud In the second experiment, vertical vibrations were exerted to a thin layer of dense suspensions. Depending on suspension and shaking parameters, a lot of different unexpected phenomena were found: Stable and growing holes and fingerlike protrusions, to name a few.\ud All have in common that hydrostatic pressure is overcome, which would normally cause a hole to collapse in any Newtonian liquid, even when vibrated. \ud A convection roll was found on the edge of the holes in the suspensions, which might be explained by a break in symmetry in the oscillations of the hole edge, which in turn can cause a change in particle configuration in the suspension.\ud \ud A cornstarch suspension was found to be a remarkable one. Cornstarch was found to be the only suspension to show non-monotonic settling, and shows the richest phenomenology when shaken.\ud The unique particle properties of the cornstarch, edgy particles with a relatively flat size distribution in the range of 5-20 micrometer, might be the cause of this behavior. No other, easily available, particles were found to have these properties.\u

Effect of finite container size on granular jet formation

When an object is dropped into a bed of fine, loosely packed sand, a surprisingly energetic jet shoots out of the bed. In this work we study the effect that boundaries have on the granular jet formation. We did this by (i) decreasing the depth of the sand bed and (ii) reducing the container diameter to only a few ball diameters. These confinements change the behavior of the ball inside the bed, the void collapse, and the resulting jet height and shape. We map the parameter space of impact with Froude number, ambient pressure, and container dimensions as parameters. From these results we propose an explanation for the thick-thin structure of the jet reported by several groups ( J. R. Royer et al. Nat. Phys. 1 164 (2005), G. Caballero et al. Phys. Rev. Lett. 99 018001 (2007), and J. O. Marston et al. Phys. Fluids 20 023301 (2008))