Foam Drainage Rheology

Stephan Koehler
Physics, Emory University

       Although foam is composed of two inviscid substances, water and
air, it has solid-like properties. For sufficiently large shear stresses, a
foam flows; however, the apparent viscosity of the foam is several orders of
magnitude greater than that of water. The dissipation mechanisms of
continuously sheared foams are poorly understood, and also difficult to
measure. I will discuss a new technique for obtaining the rheology of wet
foams that do not coarsen on the time-scale of the experiments. These
measured shear stresses show a surprising correlation with the liquid volume
fraction of the foam, which naturally leads to a simple two-parameter model.
This model can be justified by considerations of the shear which is focused
in the thin films separating the bubbles.