Bubble in flow field: A new experimental protocol for investigating dynamic adsorption layers by using capillary pressure tensiometry

Abstract

For many years the model of a dynamic adsorption layer (DAL) is well established as explanation for the behavior of rising bubbles in surfactant solutions. This model explains the velocity profile and the evolution of the shape of a rising bubble based on the hypothesis of the balance between the drag force and the structure of the adsorbed layer governed by Marangoni convection. However, direct measurements of interfacial properties of the bubble during rising are a real challenge. Here we present a new experimental protocol called “bubble in flow field” suitable for direct measurements of dynamic interfacial properties of a bubble surface using the capillary pressure tensiometry under liquid flow conditions. The experimental results for pure water demonstrate the applicability of the technique for measuring the surface tension of bubbles in a liquid flow field. The operational conditions including capillary tip geometry, liquid flow rate, bubble size and distance of the flow outlet to the bubble pole are optimized experimentally to achieve significant flow effects on a fixed bubble, while surface tension measurements are still reliable. The results for surfactant solutions (C12DMPO and CTAB) indicate respective effects depending on the flow conditions around a freshly formed bubble and evidence the formation of a dynamic adsorption layer. The adsorption equilibrium state is reached faster for higher liquid flow rates. However, the effect of the flow field is negligible on pre-aged bubble surfaces, i.e. no net desorption effects caused by the liquid flow are observed.