(186ag) Stability of Static and Dynamic Foams

of the liquid it contains, is examined. As liquid content rises,

channels between bubbles (where most of the liquid is found) tend to

deform (i.e. sag) under gravity, the deformation being opposed by

surface tension forces that resist stretching of foam films. However

with increasing deformation and film stretch, the forces opposing

stretching tend to soften. Catastrophic collapse of the static

structure then occurs, and a convective instability then onsets. The

stability of a moving foam (in particular a foam constrained to

migrate along a channel) is also examined. Viscous drag forces

(primarily located on the channel walls) deform the migrating foam

structure out of equilibrium: increasing the migration speed,

increases the deformation. Beyond a critical migration speed, the foam

structure breaks up by shrinking certain foam films away to nothing,

thereby inducing bubbles to exchange neighbours. This neighbour

exchange instability is an inherently dynamic effect: it is not

possible to maintain films in their shrunken state for arbitrarily

long times even with quasistatic changes in the migration speed.