Velocity oscillations and stop-go cycles: The trajectory of an object settling in a cornstarch suspension

Abstract

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.

Dramatic effect of fluid chemistry on cornstarch suspensions: Linking particle interactions to macroscopic rheology

“…1b-d show the time evolution of z,ż andz for Φ m EtOH =10% and H fall =15 cm. As previously observed for a CS suspension using pure water [7,25], after a rapid slowing down due to the impact (grey vertical line),ż oscillates around a terminal velocity. For each oscillation we define its mean velocity,v i and mean accelerationā i (insets of Fig.…”

Forces encountered by a sphere during impact into sand

“…Thus, by probing a granular system with an impactor, it is possible to extract information about the influence of air on the compressibility of the bed, locally and for the bed as a whole. When there is less air present to counteract the compaction caused by the falling ball, the drag on the ball not only increases, but, surprisingly, also departs more significantly from the drag law (3). This means that there is a compacting-decompacting mechanism present that has not yet been seen in dry granular materials.…”

“…The compressibility of the sand bed is observed through the oscillatory behavior of the acceleration curve for various ambient pressures; it shows the influence of interstitial air on the compaction of the sand as a function of time. Recently there has been growing interest in measuring the movement of objects in complex fluids such as granular systems [1,2], cornstarch suspensions [3], or foams [4]. The ultimate goal is to measure and understand the forces that act on an intruder in such fluids.…”

The role of shear in the transition from continuous shear thickening to discontinuous shear thickening

“…Intuitively, the velocity of the plate exhibits stop-go oscillations. 17 The normal force appears when the suspension is jammed. These behaviors can be explained as follows.…”