We report the observation of two diffusive relaxation modes in a very swollen microemulsion, measured by quasielastic light scattering experiments. In addition to a short-time diffusion process, we observe a long-time diffusive relaxation mode with unusual scaling behavior: the diffusion constant $D$ is an exponential function of the characteristic length scale $\xi$, $D\propto \exp (-\xi )$. This observation provides experimental evidence for thermally activated topological relaxation of random fluid phases, as predicted by Milner et al. [J. Phys. (Paris) 51, 2629 (1990)]. We describe the activation energy of such a process by some simple microscopic model for the surfactant layer and give an estimation of the membrane elastic and Gaussian moduli $\kappa$ and $\overline{\kappa }$, and its spontaneous curvature.

• Received 3 August 2000

DOI:https://doi.org/10.1103/PhysRevLett.86.3340