Foam is a dense random packing of gas or liquid bubbles in a small amount of immiscible
liquid containing surfactants. Most applications of foam, such as shaving
cream, firefighting, and froth flotation, make use of its unusual rheological
properties. We are running molecular dynamics simulations on a simple bubble-scale
model developed by D. J. Durian [Phys. Rev. Lett. 75, 4780 (1995); Phys.
Rev. E 55, 1739 (1997)], with the aim of relating macroscopic rheological behavior
to microscopic bubble motion. For example, a quiescent foam is a zero-temperature
glassy solid because the energy required to change the relative positions of bubbles
is much larger than thermal energy kT. If a foam is steadily sheared, however, it flows as bubbles
change their relative positions in rearrangement events. Our aim is to determine
whether the effects of steady shear on bubble motion can be characterized by an
effective temperature, Teff. We are also studying the statistics avalanche-like
rearrangement event dynamics and the effect of random bubble packing on the response
to step and oscillatory strains.
