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

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