Evaporation-induced flows in confined droplets of diluted aqueous NaCl solution

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The flows in a naturally evaporating liquid droplet confined by two flat plates were investigated experimentally. A diluted sodium-chloride (NaCl) aqueous solution was used to form the confined droplets. The evaporation of water at the free surface of the droplet built up a concentration gradient inside the solution, which induced a natural convection flow. Three-dimensional trajectories of tracer particles in the droplet were tracked, and the corresponding velocity fields were obtained by using a digital in-line holographic microscopy technique. Furthermore, this study investigated the effects of the confined droplet’s aspect ratio and the liquid’s molar concentration on the evaporation-induced flows in the droplet.