Nanotextured Surfaces Lead to Differential Wettability of Compound Droplets

Abstract Water interaction with nanotextured surfaces is critical in diverse natural and technological processes. In general, water is not perfectly pure and can often contain low amounts of oily components. Here it is revealed how, contrary to wetting behavior on non‐porous surfaces, a tiny amount of oil can critically influence water wettability on nanopore structures. Surfaces coated with nanoporous thin films are used to show that water droplets undergo a nanopore‐triggered instantaneous meaningful spreading effect when adding parts‐per‐million (ppm) amounts of oil. Remarkably, minute oil fractions affect water wettability on the nanosurface more than large oil fractions. Moreover, the binary droplets exhibit contact angles even smaller than those of pure components. The physical reason behind the peculiar wetting response of nanopore surfaces arising from traces of oil in water can be attributed to spontaneous transport phenomena in the pore network. The ppm level oil‐in‐water composed droplets also show enhanced mobility on the nanoporous surfaces, as evidenced by very small sliding angles. This feature is indeed leveraged to demonstrate a self‐slippery droplet‐surface system. The study unveils that the presence of ultra‐low concentrations of oil can cause distinctive water‐nanosurface interplays, providing a fresh framework in both water‐related nature phenomena and technical applications.