To investigate what drives the asymmetry of the electrowetting phenomenon we calculate the numbers of interfacial water-water hydrogen bonds (HBs) (Figure , b and Figure ). We use the geometric criterion to determine whether a water molecule forms hydrogen bonds i.e., with a distance between the oxygen atoms of the donor and the acceptor less than 0.35 nm, and a corresponding H-O•••O angle of less than 30°. The HBs are calculated only between water molecules that are included in the first and second layers relative to the interface. The boundaries of these layers are identified using the density profiles reported in Figure .