Evolution of grain boundaries in two-dimensional foam

The temporal evolution of disorder around grain boundaries between domains of ideally six-fold coordinated two-dimensional foam has been studied experimentally, using a foam comprising bubbles bridging between a soap solution and a cover glass. The disorder, quantified by the second central moment of the distribution of topological classes of the cells (μ₂), generally increases. In certain cases, in which the evolution can be followed over longer times, μ₂ eventually falls. This may be connected with the transient peaks for μ₂ found in previous studies of relatively ordered soap froths. The absolute values of μ₂ depend upon the boundary conditions imposed upon the foam, a rigid wall leading to higher values than a deformable boundary. The disorder about the grain boundaries propagates into the adjacent regions of ordered foam with constant speed, the roughness of the interface increasing with time.