New foams: Fresh challenges and opportunities

Some recent trends in foam research have been directed towards the rapid production of highly monodisperse bubbles, of diameter on a scale around or below 100 μm, with surprising consequences. Such foams remain wet under gravity and order spontaneously and rapidly, forming substantial microcrystals. Previously, ordered foams have resulted only from slow processes of deliberate fabrication, or from the influence of walls in confined geometry. This opens up a wide range of new topics of interest, analogous to those of metallurgy: equilibrium crystal phases, their stability, their defects and interfaces, and phase transitions between them. The ordered structures associated with confined geometries also offer opportunities for microfluidics. Other new kinds of foam incorporate particulate matter, and are intermediate between foam and granular matter. Remarkable properties are beginning to emerge for these as well, including “superstability”, that is, a high degree of stability with respect to both coarsening and rupture.