Motion of a sphere through an aging system

TL;DR

This study examines how a sphere moves through an aging, yield-stress clay suspension, revealing that aging affects drag force and fluidization behavior, with only a thin fluidized layer around the sphere.

Contribution

It provides new insights into the motion of objects in aging yield-stress fluids, highlighting the effects of aging on drag and fluidization layers.

Findings

Drag increases with speed and aging time.

A minimum speed exists below which steady motion is impossible.

Only a thin layer around the sphere is fluidized during motion.

Abstract

We have investigated the drag on a sphere falling through a clay suspension that has a yield stress and exhibits rheological aging. The drag force increases with both speed and the rest time between preparation of the system and the start of the experiment, but there exists a nonzero minimum speed below which steady motion is not possible. We find that only a very thin layer of material around the sphere is fluidized when it moves, while the rest of suspension is deformed elastically. This is in marked contrast to what is found for yield-stress fluids that do not age.