Thinning of Soap Films: The Role of Surface Viscoelasticity

Abstract

The thinning velocity of circular horizontal soap films made from dilute surfactant solutions (around the CMC) was studied experimentally. We used two different surfactants, one ionic (DTAB) and the other nonionic (C 10E 5). We demonstrate the important role of surface elasticity in the thinning process. The elasticity values obtained from the thinning velocities (ϵ r) have been found to depend strongly on the surfactant concentration of the solution. We compare these elasticities to those obtained independently from measurements in which the surface of the solution is slowly compressed and expanded around equilibrium (ϵ ω). The elasticity ϵ ω varies not only with the surfactant concentration, but also with the surface expansion or compression rate ω. Contrary to current expectations, the elasticity ω f playing a role in the soap film thinning is not equal to the value of ϵ ω for the surface expansion rates encountered in the thinning process; ϵ f is closer to ϵ ω values corresponding to much faster rates. We explain this discrepancy by showing that surfactant exchanges with the bulk responsible for the rate dependences of ϵ ω are ineffective in soap film thinning, because these exchanges are slower than surface convection.