Aqueous foams. Aqueous foams are systems out of equilibrium. If you produce foam with uniform bubble size and liquid content in an enclosed container, the foam will evolve to a non-uniform profile in bubble size and liquid fraction. Essentially, the top of the foam will be dry with large bubbles and the bottom of the foam will be wet with small bubbles. This is due to the drainage of liquid and the coarsening of gas from small to large bubbles.
Drainage and coarsening. The study of foam drainage has a long history that culminates in a non-linear partial differential "drainage equation" for the change of liquid fraction with time and position. The drainage equation is ultimately a continuity equation expressing the conservation of liquid in foams. But drainage is not the only phenomena present in aqueous foams. Gas diffusion is equally important and, as verified experimentally by the Durian group (PRL, 2002), has a major impact enhancing drainage in freely evolving foams. We investigate steady state foams in the light of the connection between drainage and coarsening.
Steady state foams. We generate a column of foam and maintain it in steady state by continuously bubbling gas in the bottom of the column. In this regime, the bubbles move upwards at a fixed velocity and the bubble size and liquid fraction profiles do not change with time. We study the capillary and viscous drag contributions to the liquid fraction profile and the effect of coarsening on the net transport of gas in the system.
Electrical conductivity of foams.