On the rebound of soapy drops

Pure water is known to bounce on super-hydrophobic materials, and we discuss here whether this remains true if the surface tension of water is lowered by either alcohol or surfactants. After determining the threshold in surface tension below which drops stick to the substrate, we show that a decrease of surface tension makes the rebound slower, a consequence of the reduced stiffness of this kind of spring. We also report that water with "slow" surfactants can still bounce despite a static surface tension smaller than the rebound threshold, which is interpreted as an effect of dynamic surface tension. The liquid is substantially deformed at impact, which impoverishes the surfactants at the its surface and thus can trigger repellency for a wetting liquid. A water-alcohol mixture with low surface tension (first row) does not bounce on a water-repellent material, while soapy water with even lower surface tension can (second row), due to the slow adsorption of surfactants at the interface.