Foams and compressed emulsions exhibit a fascinating mechanial behavior; while they consist entirely of fluids, their macroscopic rheological behavior is elastic-like within the time scales accessible to oscillatory rheology.  They show the viscoelastic behavior typical for soft glassy materials, which suggests that a slow structural relaxation should dominate their rheology at low frequencies. In this talk we will present experimental results on these systems and discuss differences to the behavior of suspensions of hard particles.  We combine linear and nonlinear viscoelastic measurements by performing constant-rate sweeps, keeping the strain rate constant as the oscillation frequency is varied.  The viscoelastic response of these systems at different extents of oscillatory strain rate supports the validity of Strain-Rate Frequency Superposition (SRFS) [*] for these systems; the shape of the structural relaxation remains unchanged within a wide range of applied strain rates.  However, the same techniques also allow us to detect a secondary relaxation process that appears to be intimately linked to the main structural relaxation itself, with a time scale that shows the same strain rate dependent scaling behavior.  We present a simple model description that qualitatively accounts for the observed behavior.

[*] see /cond-mat/0608151