and emulsions depends on the response of the thin liquid In concentrated fluid dispersions the liquid films are under dyfilms and the Plateau borders during shear and dilation (1). namic conditions during film rupture or drainage. Aqueous foam Several researchers have determined the static and dynamic films stabilized with sodium decylsulfonate and aqueous emulsion interfacial properties of gas/liquid and liquid/liquid interfilms stabilized with the nonionic Brij 58 surfactant were formed faces in the presence of surfactants using a number of methat the tip of a capillary and the film tension was measured under ods (2, 3). The assumption in these studies is that the behavstatic and dynamic conditions. In the stress relaxation experiments ior of the film can be derived from the properties of its single the response of the film tension to a sudden film area expansion interfaces.
was studied. These experiments also allowed the direct measurement of the Gibbs film elasticity. In the dynamic film tension A versatile interfacial and film tensiometer has been develexperiments, the film area was continuously increased by a conoped in our laboratory (4-6). In this technique, a curved, stant rate and the dynamic film tension was monitored. The measpherical cap-shaped fluid interface of liquid film is formed sured film tensions were compared with the interfacial tensions of at a capillary tip and the interfacial tension (IFT) of the the respective single air/water and oil/water interfaces, which single interface or the film tension of the film can be deterwere measured using the same radius of curvature, relative expanmined by measuring the capillary pressure of the interface sion, and expansion rate as in the film studies. It was found that or film. The IFT or film tension is related to the capillary under dynamic conditions the film tension is higher than twice the pressure and radius of the interface or film curvature by the
single interfacial tension (IFT) and a mechanism was suggested to
Young-Laplace equation. The IFT and film tension can be explain the difference. When the film, initially at equilibrium, is measured not only in equilibrium but in dynamic conditions expanded and the interfacial area increases, a substantial surfactant depletion occurs inside the film. As a result, the surfactant as well. The automated apparatus makes it possible to change can be supplied only from the adjoining meniscus (Plateau border) the interfacial area in virtually any mode (expansion or conby surface diffusion, and the film tension is controlled by the traction with various rates).
diffusion and adsorption of surfactant in the meniscus. The results
In real emulsions or foams, the size distribution of the have important implications for the stability and rheology of foams bubbles or drops is generally polydisperse. Hence, the thin and emulsions with high dispersed phase ratios (polyhedral strucliquid films formed between the bubbles or drops are typiture). α§ 1997 Academic Press cally not flat, as in a homogeneous dispersion (Fig. 1a), but