Unusual properties of foams at low pressures

In addition to a high coarsening rate, foams at low pressures may show special properties. These include a bulk modulus going to zero, a Poisson's ratio approaching zero, and a zero binding energy, implying foam separation into individual bubbles at a critical pressure. We provide examples of calculations made for particular two-dimensional clusters with an ideal gas, which illustrate these features. We also discuss the difficulties in performing experiments at the low pressures at which the unusual properties show up.