generated. Triboelectric contact If two different insulators are touched together, such as when a piece of rubber is touched against a piece of glass, then the surface of the rubber will acquire an excess negative charge, and the glass will acquire an equal positive charge. If the surfaces are then pulled apart, a very high voltage is produced. This so-called "tribo" or "rubbing" effect is not well understood. It may be caused by electron-stealing via quantum tunneling, or by transfer of surface ions. Friction is not required, although in many situations it greatly increases the phenomenon. Certain phenomena related to frictionally generated electrostatic charges have been known since antiquity, though of course the modern theory of electricity was developed after the Scientific Revolution. Solid-solid contact The mechanism of contact electrification (CE) between solid-solid has been debated for more than 2600 years. A most controversial topic in CE is the identity of the charge carriers: electron transfer, ion transfer or even materials species transfer. Recent studies by using Kelvin probe force microscopy suggest that electron transfer is the dominating charge carrier in CE for solid-solid cases. When the interatomic distance between two atoms belonging to two materials is shorter than the normal bonding length (typically ~0.2 nm), the electrons will transfer at the interface. It implies that a strong electron cloud overlap (or wave function overlap) between the two atoms/molecules in the repulsive region will reduce the interatomic potential barrier (Fig. 1), and result in electron transition between the atoms/molecules. The contact/friction force in CE is to induce strong overlap between the electron clouds (or wave function in physics, bonding in chemistry). Liquid-solid contact Besides ion transfer at liquid-solid interface, electron transfer occurs as well. As for the liquid-solid case, molecules in the liquid would have electron cloud overlap with the atoms on the solid surface at the very first contact with a virginal solid surface, and electron transfer is required in order to create the first layer of electrostatic charges on the solid surface. Then, ion transfer is the second step, which is a redistribution of the ions in solution considering electrostatic interactions with the charged solid surface (Fig. 2). Both electron transfer and ion transfer co-exist at liquid-solid interface. Electrolytic-metallic contact If a piece of metal is touched against an electrolytic material, the metal will spontaneously become charged, while the electrolyte will acquire an equal and opposite charge. Upon