Foams flow by-stick and slip

摘要

FROM the top of a beer to the meringue on a pie, from shaving cream to a bubble bath, we encounter foams everywhere. But consider their properties; they are fascinating materials. They are made only of gas and liquid, materials that flow freely, yet foams can be rigid solids like the whipped cream on a dessert. This solid-like behaviour results from the foam microstructure, gas bubbles in a fluid, with interfaces stabilized by surfactant. But foams also flow easily, as anyone savouring the whipped cream of the dessert will affirm. What makes foams solid? How do they flow? Important new insight has come from a recent experiment by A. D. Gopal and D. J. Durian and a computer simulation by Durian. The results emphasize the critical importance of the disordered packing of the bubbles, and show that foam flow occurs through topological rearrangements of bubble positions, sharing many features with the highly non-linear dynamics of the avalanches observed in granular flow and the intermittent 'stick-slip' dynamics of earthquakes.