Bioinspired Hand-Operated Smart-Wetting Systems Using Smooth Liquid Coatings.

Manually controllable "hand-operated" smart systems have been developed in many fields, including smart wetting materials, electronic devices, molecular machines, and drug delivery systems. Because complex morphological or chemical control are generally required, versatile strategies for constructing the system are technologically important. Inspired by the natural phenomenon of raindrops rarely bouncing and usually spreading on a puddle, we introduce a droplet-impact-triggering smart-wetting system using "non-smart" smooth liquid coating materials. Changing the droplet impact energy by changing the volume or casting height causes the droplet to completely bounce or spread on the liquid surface, regardless of the miscibility between the two liquids, owing to the stability of air layer. As the bouncing of a droplet on a liquid interface is not usually observed during wetting, we first analyze how the droplet bounces, then prove that the wettability is triggered by the droplet's impact energy, and finally introduce some applications using this system.