Droplet migration and interactions from coupled mass transport

Mass transport of solutes often results in non-uniform concentration profiles. Nevertheless, the presence of such solute gradients and subsequent fluid flows and colloidal migration/interactions in many circumstances have not been fully appreciated. Here, we present studies on internal flows of neighboring droplets, where local solute gradients arise from coupled mass transport between droplets. First, we show that such local solute gradients can induce interfacial Marangoni flow that drives bulk fluid motion. Observed flow profiles inside droplets depend on the direction of interfacial mass transport and suggest interesting interactions between neighboring droplets. In addition, we combine experiments and COMSOL simulation to identify two distinct mechanisms that drive flows in neighboring droplets with coupled mass transport. Further investigations reveal that the transition between these two mechanisms is related to the separating distance between droplets. Finally, we design droplet pairs with a source-sink configuration in their chemical compositions. Visualization of internal flows indicates distinct droplet-droplet interactions as the separating distance varies.