Self-stabilization of droplet clusters levitating over heated salt water

• Published in: International journal of thermal sciences 2022-12, Vol.182, p.107822, Article 107822
• Main Authors: Fedorets, Alexander A., Shcherbakov, Dmitry V., Levashov, Vladimir Yu, Dombrovsky, Leonid A.
• Format: Article
• Language: English
• Subjects: Droplet cluster; Experiment; Levitation; Modeling; Salt water; Self-stabilization
• ISSN: 1290-0729; 1778-4166
• DOI: 10.1016/j.ijthermalsci.2022.107822

When the water surface is heated locally, a self-ordered regular cluster of levitating droplets is formed above it. A condensational growth of water droplets leads to coalescence of the cluster with water layer. At the same time, the intended use of water droplets as biochemical microreactors is possible only when the cluster is stable. A method of self-stabilization of a droplet cluster levitating over the locally heated water surface is proposed for the first time. The physical analysis of the problem shows that the desired result can be reached by dissolving a small amount of sodium chloride in a layer of water. This is explained by simultaneous action of two processes with opposite effects on evaporation. On the one hand, when water evaporates, a thin layer of increased salt concentration that prevents evaporation is formed at its surface. On the contrary, diffusion of salt in water decreases the surface concentration of salt. Different combinations of water heating intensity and average salt concentration are expected to result in the stable size of nearly identical levitating droplets, when there is a balance between the droplet evaporation and the condensation of vapor from the ascending vapor-air flow. The predicted phenomenon has been observed experimentally. In a series of laboratory experiments, a range of parameters has been obtained for which self-stabilization of the droplet cluster takes place. The theoretical analysis of necessary conditions for spontaneous stabilization of a droplet cluster uses a steady-state solution for temperature and salt concentration in the water layer. The localization of a small cluster makes it sufficient to derive a solution to the one-dimensional problem. The obtained analytical solution to this problem takes into account the temperature dependence of the salt diffusion coefficient. The resulting threshold values of salt concentration agree well with the experimental data. •The phenomenon of self-stabilization of droplet clusters is discovered.•The conditions of self-stabilization over the layer of salt water are determined.•A theoretical model of the self-stabilization phenomenon is proposed.•The minimum concentration of salt for the cluster stabilization is predicted.