The paper presents the results of terrestrial modeling and space experiments carried out to investigate diffusion of the surface- active substance (surfactant) from a drop of binary mixture to the surrounding liquid. The objective of these studies is to estimate the role of the capillary effect in the formation of mass transfer under conditions of maximum suppression of the gravitational motion and to identify parameters, which can be used to describe surfactant dissolution from the drop and the evolution of the concentration field round the drop. The application of the interferometric techniques allowed us to visualize the structure of flows and surfactant concentration fields and to watch their evolution. Laboratory experiment with a drop in a thin horizontal layer has shown that surfactant diffusion is accompanied by an intensive, non-stationary Marangoni convection. The observed motion is caused by the vertical surfactant gradients generated at the free boundary of the drop under the action of the buoyancy force, while its non-stationary behavior is attributed to an essential difference in the diffusion time and viscosity. By contrast, the process of surfactant diffusion under microgravity conditions gave rise to a relatively weak and rapidly decaying solutal Marangoni convection despite repeated attempts to create favorable conditions for its development. The main reason for the observed phenomenon was the existence of long-lived concentration fields, which were generated near the interface due to a large characteristic time of the surfactant diffusion and negligible gravitational mechanisms of motion.
|
