Liquids spreading and recoiling on liquids: The influence of interfacial chemical reactions H.A. Stone, School of Engineering & Applied Sciences, Harvard University, Cambridge, MA 02138 Flows driven by chemical reactions at interfaces: We have been investigating problems involving chemical reactions at the interface between two liquids, where the reaction causes a significant modification to the flow. In particular, we have focused on liquid lenses floating at an interface. Droplets of oil containing oleic acid first spread, then recoil, on an aqueous solution of sodium hydroxide (figure left). Spreading and retraction: Surfactant (sodium oleate) is produced at the interface during spreading, and for reagent concentrations about 1 mM spreading is much faster than in the absence of a chemical reaction (the rate of spreading is quantified by measuring the radius as a function of time). After about 10 seconds, drops reach a maximum radius, which is about 3-4 times the initial radius. Spreading is faster and the maximum radius is larger for higher concentrations of reagents. The drops are then observed to recoil, which we explain by the diffusion of surfactant away from the oil/water interface, with the rate of recoil being controlled by the NaOH concentration in the subphase (a sketch of these processes is given in the figure on the right). We are currently exploring other problems where flow is altered by an interfacial chemical reaction.