Dissolution of a heavy liquid droplet deposited onto free surface of unlike liquid solvent

Abstract

Dissolution dynamics and kinetics of a heavy liquid droplet deposited onto the free surface (or inserted at a certain depth under the surface) of a massive liquid solvent have been studied experimentally. As the solvent are selected distilled water, as well as concentrated solutions of sugar or common salt. As the soluble media were chosen the colored substances of different types (aniline ink, Indian ink, medical solutions of brilliant greenery or iodine), which allowed us to observe and register on the camera all the evolution stages of the heterogeneous system (a colored drop spreading in a transparent solvent) within the convective mixing regime. If the temperature of the solvent is high enough (T>40oC), the convective mode is not realized, and the droplet is almost immediately dissolved into the homogeneous solution, at a low temperature (in our case it is room temperature of 18-20 oC), the spreading configuration is very complicated: after deposition on the surface, the droplet spreads over it, then in the central part of the spot appear one or several vertical channels (the number of channels depend on the size and mass of the initial droplet, as well as the dynamics of surface spreading which is controlled by the surface tension), along which the solution penetrates into the interior of the solvent. When penetrating the solvent, the channels begin branching and forming secondary flows, and so on, until the bottom of the cell is reached by initial droplet material. If a liquid stream is introduced to the solvent under pressure (we use injection from a syringe needle), a single well defined vertical channel is formed, but its head at some depth undergoes reflection with the formation of backward flows and complex spreading patterns in the upper volume of the solvent.