Physics Colloquium - Friday, April 14th, 2006, 4:00 P.M.

E300 Math/Science Center; Refreshments at 3:30 P.M. in Room E200

John R. de Bruyn
University of Western Ontario

Drag in Yield-Stress Fluids

In a Newtonian fluid, the drag force on a slowly moving sphere is given by Stokes' Law and is proportional to the sphere's speed. In complex fluids, the situation is not so simple and in general no straightforward analog of Stokes' Law is known. I will discuss the results of experimental measurements of the drag force in yield stress fluids. These measurements do not simply tell us how the drag force depends on speed - they also provide information about the structure of these materials and the dynamical processes that take place when they flow. I will also present microscopic measurements of the viscoelastic moduli of a yield-stress fluid, and discuss the relationship between the microscopic properties and the bulk behaviour.