Marangoni spreading and contracting three-component droplets on completely wetting surfaces

When a droplet comes in contact with a completely wetting surface, the liquid typically spreads until it covers the entire substrate. However, nonuniform evaporation of a multicomponent droplet can generate surface tension gradients that alter this behavior. Here, we explore the rich dynamics of fully miscible, three-component droplets composed of water, ethanol, and propylene glycol on completely wetting glass substrates. These droplets initially spread rapidly but then stop and contract. We experimentally and theoretically investigate this behavior throughout the ternary parameter space at different relative humidities. Evaporation changes the composition of the droplet over space and time, resulting in a reversal of Marangoni flows that ultimately determines the dynamic droplet shape. We illustrate the utility of such dynamics by collecting, aggregating, and removing contaminants from a 4-cm(2) area using a single µL-scale droplet.