Reactive spreading and recoil of oil on water

Droplets of oil containing oleic acid were observed to spread, then recoil, on an aqueous solution of sodium hydroxide. Surfactant is produced at the interface during spreading, and for reagent concentrations of order O(1 mM) spreading is observed to be much faster than in the absence of a chemical reaction [radius R(t)proportional to t(alpha) with 0.64 < 0.89]. After t similar to 10 s, drops reach a maximum radius R-max similar to 3-5 times the initial radius. Spreading is faster and R-max is larger for higher concentrations of reagents. The drops are then observed to recoil (with apparent power-law behavior -0.34 <-0.14), due to diffusion of surfactant away from the oil/water interface, with the rate of recoil being controlled by the NaOH concentration. (C) 2006 American Institute of Physics.