Oscillatory Marangoni convection around bubbles and drops in heterogenous solutions of surfactants

Experimental investigation was performed to study the concentration convection around stationary gas bubbles and insoluble drops in a thin liquid layer placed in a vertical Hele-Shaw cell. The bubbles or drops, squeezed between the two parallel cell walls took the shape of short cylinders with free lateral sur-faces. The cell was filled in with an aqueous solution of a surface-tension active fluid (suifactant) with vertically stratified concentration. A special wire frame prevented bubblesfrom rising up under the buoyancy force, thus modelling the microgravity conditions. A convective motion in the mixture develops at the bubble or drop interface, due to the solutocapillary Marangoni forces. Owing to a small thickness of the liquid layer (similar to 1mm), the arising flows and surfactant concentration distributions are nearly two-dimensional so that it is possible to investigate their structure and evolution by interferometric technique. The experiments revealed the development of oscillatory convection around the drop interface, which was similar to that observed in bubble tests. The period and duration of oscillations were determined in relation to time, surfactant concentration gradient and concentration Marangoni number. The analysis of bubble and drop behavior showed that the existence of self-oscillatory modes is related to the specific interaction between the solutocapillary and soluto-gravitational mechanisms of motion.