Limitations of classical methods for measuring surface tension in visco-elastic liquids

摘要

The surface tension of a water/detergent mixture and a visco-elastic waterborne paint sample were measured using the classical ring method in which a thin free liquid film (lamella) is formed between the ring and the parent liquid. Lamella tension was measured during the stretching of the lamella. Measurements of water/surfactant mixtures have shown essentially classical behavior of lamella tension (independence of tension on deformation). Measurements of a waterborne paint formulation, however, have shown that after stretching of the lamella, the lamella tension decreases. The lamella tension of the paint sample increases after contraction of the lamella. Comparison of experimental results with rheological properties of the paint have shown that bulk visco-elastic properties of liquid are found in both the bulk sample and the lamella. The conclusion is that for the study of surface properties of visco-elastic liquids such as waterborne paints, the applicability of classical methods is limited. Therefore, it was necessary to develop new methods and approaches to study the properties of these materials. These methods are described in this paper.