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 molecu les, containing only hydrated water and counterions. Their thickness is abo ut 5 nm, they can exist freely, without support, and they form spontaneousl y out of thicker soap films. These properties make them extremely interesti ng to study. Their behaviour can directly be compared with thicker liquid f ilms which are not of molecular dimensions in the direction perpendicular t o 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 it self from thicker films in its elastic properties whereas thicker films beh ave as Newtonian, purely viscous materials. Also the rim, around the hole i n 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 con trast with thicker films, where the rim is a smooth toroid. In this paper w e investigate the influence of the structure of the Newton-black film on th ese characteristics features. A physical model is introduced both for the e lasticity and the irregular structure of the rim. From the experiments a dr astic change in elastic modulus and frequency of the irregularities is foun d when the temperature is changed. Within the framework of the models a cha nge 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 d ynamical behaviour. (C) 1999 Elsevier Science B.V. All rights reserved.