Dewetting of surfactant solutions close to receding three-phase contact lines

Abstract

The wetting and dewetting behaviour of aqueous surfactant solutions is important for many natural processes and industrial applications like printing and coating processes. In order to control and optimize these processes, in-depth knowledge of the topic is required. Preliminary studies showed a significant influence of surfactants on the wetting and dewetting behaviour of aqueous solutions. This influence is also present for concentrations below the critical micelle concentration and increases with higher surfactant concentration. One hypothesis proposes that Marangoni effects close to the receding three-phase contact line of aqueous surfactant solutions are the cause for the altered dewetting behaviour. As of yet, this hypothesis could not be tested, since the dynamics and hence the flow field of aqueous surfactant solutions close to receding three-phase contact lines were not measured. Therefore, in this work, the dynamic dewetting behaviour of aqueous surfactant solutions close to receding three-phase contact lines depending on the surfactant concentration was studied and the aforementioned hypothesis tested. For this purpose, two complementary experimental setups were developed making it possible to measure flow fields on the 10 µm scale close to receding three-phase contact lines. For one thing, a fast laser scanning confocal microscope was developed and built. It utilizes two instead of the usual one scan unit. Two-dimensional images can be recorded with a frame rate up to 1 kHz in the horizontal direction and up to 200 Hz in the vertical direction. Thus, it is significantly faster than comparable commercially available state-of-the-art microscopes. Additionally, astigmatism particle tracking velocimetry was used by modifying a commercial microscope. Since this method, in contrast to the confocal microscope, enables the direct measurement of three-dimensional flow fields, it was used to test the hypothesis. This way, flow fields close to the receding three-phase contact line of drops consisting of aqueous surfactant solutions were measured with varying concentrations below the critical micelle concentration. The resulting flow fields were compared to an existing hydrodynamic theory for water. I discovered that especially close to the liquid/air interface of the drop at a distance of up to 112 µm from the three-phase contact line the flow fields of surfactant solutions differed from the expected flow field of water. The measured flow fields can only be explained if the free surface is not considered to be stress-free. This stress along the free surface corresponds to a Marangoni stress caused by a spatially differently distributed surfactant concentration. Thus, the initially presented hypothesis is verified. The Marangoni stress, as well as the gradient of surface tension along the free surface, were calculated based on the measured flow fields. Both parameters increase with increasing surfactant concentration. Even though the surface surfactant concentration close to the contact line only differs by 0.01 % from the equilibrium value, the observed influence on the flow fields is substantial. The measured effects occur even for absolute surfactant concentrations below 1 ppm. This indicates that even small impurities, which are in practice often unavoidable, have a significant influence on the dynamic dewetting behaviour of liquids.