Relaxation time of the topological T1 process in a two-dimensional foam.

The elementary topological T1 process in a two-dimensional foam corresponds to the flip of one film with respect to the geometrical constraints, and is a process by which the structure of an out-of-equilibrium foam evolves. We study both experimentally and theoretically the T1 dynamics in a dry two-dimensional foam. The dynamics is controlled by the surface viscoelastic properties of the films (surface shear plus dilatational viscosity, mu_{s}+kappa, and Gibbs elasticity ), and is independent of the shear viscosity of the bulk liquid. Moreover, the dynamics of the T1 process provides a tool for measuring the surface rheological properties: we obtained =32+/-8 mN/m and mu_{s}+kappa=1.3+/-0.7 mPa.m.s for sodium dodecyl sulfate, and =65+/-12 mN/m and mu_{s}+kappa=31+/-12 mPa.m.s for bovine serum albumin, in good agreement with literature values.