Terrestrial simulation of drop saturation by a surfactant under microgravity conditions

Abstract

The paper presents the results of experimental study of mass transfer in a drop of weakly soluble fluid during its saturation with a surfactant from its water solution forming a thin (∼ 1 mm) horizontal layer. The use of the interferometer allowed us to visualize the concentration fields and to define the structure of flows under the conditions of maximum suppression of the buoyancy convection. It has been found that despite the small layer thickness, in the case of initially homogeneous solution penetration of the surfactant into the drop leads to the formation of the local density inhomogeneities at both sides of the interface and to the development of a slow gravitational flow. An increase in the initial concentration of the surfactant in the solution causes a small-scale non-stationary solutocapillary motion at the drop surface. On the other hand, solubility of the drop in water also increases which leads to the formation of a counter-diffusion flux and eventually to the disappearance of the interface. In the case of initially inhomogeneous distribution of the surfactant, a competition between the gravitational and capillary mechanisms of convection gives rise to a large-scale oscillatory flow around the drop.