FAST THERMAL DYNAMICS IN AQUEOUS FOAMS

Sudden rearrangement of the tightly packed bubbles in a foam can be in duced 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 distinct ly nonexponential contribution to the autocorrelation function that sp ans several decades in delay time prior to the full decay caused by re arrangements. The short-time dynamics are independent of changes in th e rearrangement rate brought about by alteration of the coarsening rat e or by application of shear and are therefore attributed to thermal f luctuations of the bubbles. The magnitude of these microscopic fluctua tions can be understood in terms of the macroscopic shear modulus. (C) 1997 Optical Society of America.