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 bottom. Concentrated particulate suspensions consist of a homogeneous fluid containing particles, larger than 1 μm. They can be found everywhere, and their flow is important in nature, industry, and even health care [1]. In spite of their significance, many aspects of the flow of these dense suspensions remain poorly understood. In order to study these materials, people have used methods inspired by classical rheology, and typically characterized them in terms of a constitutive relation of stress versus shear rate [2][3][4][5][6]. A general result is that, when increasing the shear rate, dense suspensions first tend to become less viscous (shear thinning) and subsequently shear thicken.Probably the most conspicuous example of a dense suspension is formed by a high concentration of cornstarch in water. Recent rheological experiments in cornstarch have revealed the existence of a mesoscopic length scale [6,7], a shear thinning regime that terminates in a sudden shear thickening [8], a dynamic jamming point [4], and fracturing [9]. Merkt et al. [10] observed in a vertically shaken, thin layer of cornstarch suspension that, among other exotic phenomena, stable oscillating holes can be formed at certain frequencies and amplitudes [10,11], which were subsequently described using a phenomenological model based on a hysteretic constitutive equation [12]. At present, however, we are still far from a detailed understanding of dense suspensions.In this Rapid Communication we subject a cornstarch suspension to a basic experiment, in which we observe and describe the settling of a spherical object in a deep bath of suspension. This yields two interesting observations. In the bulk, we find that the object velocity is oscillating in addition to going toward a terminal value. Near the bottom we observe a second phenomenon: The object comes to a full stop before the bottom, but then accelerates again, and this stop-go cycle can repeat up to seven times. We will show that both bulk and bottom behavior are conceptually different from that observed in a wide range of other fluids. We propose a jamming model for the stop-go cycles near the bottom that specifically includes the liquid-grain interactions.Experiment. Our experimental setup is shown in Fig. 1(a). It consists of a 12 × 12 × 30 cm 3 glass container containing a mixture of cornstarch and liquid. For the liquid we use either demineralized water or an aqueous solution of CsCl with...

We present results for objects settling in a cornstarch suspension. Two surprising phenomena can be found in concentrated suspensions. First, the settling object does not attain a terminal velocity but exhibits oscillations around a terminal velocity when traveling through the bulk of the liquid. Second, close to the bottom, the object comes to a full stop but then reaccelerates before coming to another stop. This cycle can be repeated up to 6 or 7 times before the object reaches the bottom to come to a final stop. For the bulk, we show that shear-thickening models are insufficient to account for the observed oscillations and that the history of the suspension needs to be taken into account. A hysteretic model, that goes beyond the traditional viscoelastic ones, describes the experiments quite well but still misses some details. The behavior at the bottom can be modeled with a minimal jamming model.

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 ͑i͒ decreasing the depth of the sand bed and ͑ii͒ 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.

We study the dynamics of holes created in vertically vibrated dense suspensions and viscous Newtonian liquids. We find that all holes oscillate with the driving frequency, with a phase shift of π/2. In Newtonian liquids holes always close, while in suspensions holes may grow in time. We present a lubrication model for the closure of holes which is in good agreement with the experiments in Newtonian liquids. The growth rate of growing holes in suspensions is found to scale with the particle diameter over the suspending liquid viscosity. Comparing closing holes in Newtonian liquids to growing holes in dense suspensions we find a sinusoidal, linear response in the first, and a highly non-linear one in the latter. Moreover, the symmetry of the oscillation is broken and is shown to provide an explanation for the observation that holes in dense suspensions can grow.

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