The effect of surfactant monolayers on vortex rings formed from an impacting water drop

The impact of a falling drop with a flat liquid surface can result in the formation of a subsurface vortex ring that penetrates a significant distance into the liquid bulk. Herein an experimental study of these vortex rings is presented for water drops striking a water surface. The effect of surfactant monolayers was investigated by measuring the vortex velocity for water surfaces that were free of surfactants, as well as surfaces covered with surfactant monolayers. A soluble and an insoluble monolayer were investigated, Triton X-100 and oleyl alcohol, respectively. Oleyl alcohol was investigated for concentrations ranging from 0 to 0.495 μg/cm2, and Triton X-100 was investigated for concentrations ranging from 0 to 4.0 mg/L. For both surfactants the vortex velocity displayed a maximum at intermediate surfactant concentrations. In all cases the drop fluid was free of surfactants. A possible mechanism based on capillary wave damping is presented to explain these results. The relevance of this work to rain over lakes and oceans is discussed.