Effect of salting-out on Marangoni spreading at a miscible interface

Marangoni spreading is frequently observed in nature and is utilised in industrial processes, including contaminant removal, drug delivery, and fabrication of complex structures. The spreading of a hydrophilic organic droplet on the free surface of an aqueous solution represents a convenient reference process. Spreading on an infinitely miscible interface is known to reach a quasi-steady state, with the spreading diameter determined by molecular diffusion into the bulk. However, the coupled effects of the spreading flow, the interface’s solubility, and the evaporation of the volatile droplet are not well understood. In this work, we experimentally investigate the Marangoni spreading of a hydrophilic organic droplet on the surface of a saline solution. Widely available salts are often used to reduce the solubility of non-electrolytes in water by exploiting the salting-out effect, which controls the spreading of the organic droplet on the free surface of the saline. Quasi-static spreading-diameter measurements are used to quantify this spreading. The free surface’s height, measured using the transmission speckle method, indicates that a high-concentration saline solution renders the interface temporarily non-miscible. Schlieren images capture the structure of the evaporative field, demonstrating that the evaporation is significantly reduced by mixing between the spreading droplet and the bulk fluid. It can, however, be retained through partial mixing due to the salting-out effect. A scaling law is deduced to interpret variations of the spreading diameter controlled by the salting-out effect. This work presents an effective experimental method for determining the salting-out constant, a crucial parameter in regulating interfacial reactions.