Sphere settling in an aging yield stress fluid: link between the induced flows and the rheological behavior

The flow pattern induced by the settling of a non-Brownian sphere in a fluid depends on the rheological properties of that fluid. For instance, at small Reynolds numbers, the pattern presents a fore–aft symmetry in a Newtonian fluid, whereas, in some viscoelastic polymer solutions, it can exhibit a negative wake, i.e., an upward flow in the sphere’s wake. This study is an experimental work on the settling of a sphere in a suspension of a synthetic colloidal clay, laponite. The fluid is a yield stress fluid that ages, i.e., whose rheological properties evolve over time. We show that the settling velocity of a given sphere, as well as the induced flow pattern, are strongly modified as the fluid ages. In particular, the flow pattern asymmetry increases with the age of the fluid, and a negative wake eventually forms. We relate those modifications to rheological measurements and suggest, in line with works dealing with polymer solutions, that it is the increase in the fluid viscoelasticity that is responsible for the formation of a negative wake. The flow field measurements are also compared with flow-induced birefringence measurements, and we show that very slow relaxation processes are involved in the sphere settling.