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

TL;DR

This study investigates the relaxation dynamics of the T1 process in 2D foams, revealing that surface rheological properties control the process and can be measured through it, independent of bulk viscosity.

Contribution

It provides a combined experimental and theoretical analysis of T1 dynamics, highlighting the role of surface viscoelasticity and establishing a method to measure surface rheological properties.

Findings

T1 relaxation dynamics are governed by surface viscoelasticity.

Surface properties, not bulk viscosity, control T1 dynamics.

The method yields surface elasticity and viscosity consistent with literature.

Abstract

The elementary topological T1 process in a two-dimensional foam corresponds to the "flip" of one soap film with respect to the geometrical constraints. From a mechanical point of view, this T1 process is an elementary relaxation process through which the entire structure of an out-of-equilibrium foam evolves. The dynamics of this elementary relaxation process has been poorly investigated and is generally neglected during simulations of foams. We study both experimentally and theoretically the T1 dynamics in a dry two-dimensional foam. We show that the dynamics is controlled by the surface viscoelastic properties of the soap films (surface shear plus dilatational viscosity, ms+k, and Gibbs elasticity e), and is independent of the shear viscosity of the bulk liquid. Moreover, our approach illustrates that the dynamics of T1 relaxation process provides a convenient tool for measuring the surface rheological properties: we obtained e = 32+/-8 mN/m and ms+k = 1.3+/-0.7 mPa.m.s for SDS, and e = 65+/-12 mN/m and ms+k = 31+/-12 mPa.m.s for BSA, in good agreement with values reported in the literature.