The role of structure in rupturing Newton-black soap films: dynamics of a molecular bilayer

Newton-black soap films can be considered as a bilayer of surfactant molecules, containing only hydrated water and counterions. Their thickness is about 5 nm, they can exist freely, without support, and they form spontaneously out of thicker soap films. These properties make them extremely interesting to study. Their behaviour can directly be compared with thicker liquid films which are not of molecular dimensions in the direction perpendicular to the film surface. Rupture experiments on these films enable us to follow the growth of a hole in the lamella. The Newton-black film distinguishes itself from thicker films in its elastic properties whereas thicker films behave as Newtonian, purely viscous materials. Also the rim, around the hole in the soap film which contains the material that originally filled the hole, has a highly irregular structure in the Newton-black film. This is in contrast with thicker films, where the rim is a smooth toroid. In this paper we investigate the influence of the structure of the Newton-black film on these characteristics features. A physical model is introduced both for the elasticity and the irregular structure of the rim. From the experiments a drastic change in elastic modulus and frequency of the irregularities is found when the temperature is changed. Within the framework of the models a change of structure of the Newton-black film appears obvious. Thus, it is the structure of the Newton-black film which induces unique properties. Changes in the structure of these ordered lamellae cause large deviations in the dynamical behaviour. (C) 1999 Elsevier Science B.V. All rights reserved.