Rheology and Glassy Dynamics of Foams

After a brief`warm-up' discussion of osmotic pressure of foams, the basic phenomena of foam rheology are reviewed, focusing on linear vis-coelastic spectra (elastic and loss moduli) with brief mention of nonlinear eeects. Theoretical models for some of these properties are then described, starting with Princen's model for the elastic modulus G 0 of an ordered foam in two dimensions. There is a basic connict between this model, which predicts a step-function onset of the modulus when droplets rst contact at volume fraction = 0 , and the experimental data (which show G 0 ? 0). The three dimensional ordered case is reviewed next, focusing on anhar-monic deformation theory which predicts a logarithmic softening of the modulus near 0 ; this is still not soft enough to explain the observations. The 3D disordered case is then addressed; a combination of disorder and the anharmonic eeect nally seems able to explain the data. We then consider the problem of the frequency-dependent loss modulus G 00 (!) which describes dissipation in a foam. Somewhat alarmingly, the data suggest behaviour incompatible with linear response theory; reconciliation is possible if one invokes some very slow relaxation processes at timescales longer than experiment. We brieey describe the search for foam-speciic slow relaxation mechanisms of surfactant and water transport, which so far has yielded no viable candidates. Since similar anomalies in G 00 (!) are observed in several other systems, they are instead tentatively ascribed to a generic phenomenon: glassy dynamics. A recent model for the rheology of \soft glassy matter" is then reviewed; though phenomenological, this suggests that glassy dynamics may be a useful concept in foam rheology.