Density functional theory (DFT) calculations suggested that 0.048 electrons migrate from hydrogen to oxygen of the surface molecule, which may generate the local electric field estimated to be ~1.6×10 7 V/cm in a coarse grain model . The remarkable surface self-electrification enables droplet electricity harvesting -a promising hydro-voltaic technology . A mere 100 µl water droplet impacting the surface from a 15 cm height generates over 140 V voltage, a 200 μA current, and a maximum power of 50 Wm -2 , surpassing conventional droplet generator designs .