Drainage induced convection rolls in foams

Forced drainage describes the procedure in which liquid is added to a column of foam at a constant flow rate. For sufficiently high flow rates this results in a convective bubble motion, the symmetry of which depends on the experimental details. We review recent experiments, in particular for the case of convection in a foam column which is tilted away from the vertical. The experimentally-determined dependency of the onset of convection with the tilt angle is well described by a combination of standard drainage theory and simple foam rheology. We also present experimental data for foam convection in a Hele-Shaw cell. The measured foam density and velocity profiles provide an excellent basis upon which to develop a theory of the phenomenon.