Fast thermal dynamics in aqueous foams

Sudden rearrangement of the tightly packed bubbles in a foam can be induced by application of shear and by diffusion of gas from smaller to larger bubbles. Using diffusing-wave spectroscopy, we measure dynamics at times that are short in comparison with the rate of these events, where the relative bubble configurations are fixed. We find a distinctly nonexponential contribution to the autocorrelation function that spans several decades in delay time prior to the full decay caused by rearrangements. The short-time dynamics are independent of changes in the rearrangement rate brought about by alteration of the coarsening rate or by application of shear and are therefore attributed to thermal fluctuations of the bubbles. The magnitude of these microscopic fluctuations can be understood in terms of the macroscopic shear modulus.