Alcohol vapor-induced water drop bouncing on hydrophilic surfaces*

It is generally conceived that an impacting water drop of low Weber number completely bounces off superhydrophobic surfaces. Here we show that the impacting drop can jump off hydrophilic surfaces when Isopropyl alcohol (IPA) vapor is subjected to the surfaces. IPA molecules consist of hydrophilic hydroxy head and hydrophobic alkyl tail similar to surfactant molecules. The supply of IPA vapor leads to the adsorption of the hydrophilic head of IPA molecules onto hydrophilic solid surfaces, and it allows the hydrophobic tail of IPA molecules to be oriented toward the gas phase, which induces the droplet bouncing. Experiments revealed that a critical Weber number exists for the drop bouncing and the surface wettability of the IPA-adsorbed surface turns into its original hydrophilic state by removing the surrounding IPA vapor. We visualize such novel drop behaviors with a high-speed camera and analyze their prominent features by combining experiments and theory.