Evaporation-induced flows inside a confined droplet of diluted saline solution.

Flow patterns inside a droplet of diluted aqueous NaCl solution confined by two flat substrates under natural evaporation were investigated both experimentally and numerically. We focused on natural convection-driven flows inside confined droplets at high Rayleigh numbers (i.e., the ratio of buoyancy to diffusion, Ra), where the convection of solutes is strongly dominant, compared to diffusion. The evaporated water at the free surface of the droplet builds up a concentration gradient inside the solution, which induces the Rayleigh convection flow. Three-dimensional trajectories of tracer particles in the droplet were tracked, and axisymmetric flow motions induced by the Rayleigh convection were experimentally measured by using a digital in-line holographic microscopy technique. In addition, the effects of the confined droplet's aspect ratio and the liquid's molar concentration on the evaporation-induced flows were investigated. The convection velocity is found to be increased as molar concentration increases, because Rayleigh convection becomes significant at high the molar concentration is high (i.e. high Ra). Our numerical simulation based on the Boussinesq approximation fairly well predicted the velocity profiles of evaporating confined droplets at low concentrations. Consequently, evaporation kinetics inside the confined droplets can be controlled with varying droplet's aspect ratio and the liquid's molar concentration, which provides helpful information for the design of biochemical microplating with limited resources and for tuning self-assembly micro/nanoparticle clusters.