Two-step model for contact charge accumulation

In contact electrification of metal against insulator, multiple contacts, not a single contact, are always needed for reaching the charge saturation level. Many experiments have been done to find out the reason for this. The results are the same: multiple contacts are needed for reaching the saturation level. Two-step model is proposed as a tentative explanation of charge accumulation. Charge transfer involves two steps: contact of the two surfaces and their separation. Separation plays a major role. In this two-step process, interface states are formed when contact is established. They are then populated with electrons from one or both sides. During separation, before the interface states are eliminated, all electrons from these interface states are transferred to either the metal or the insulator. Should more or fewer electrons be transferred to the insulator, it will thus gain or lose some electrons. This two-step cycle is repeated to transfer charge into polymer until the equilibrium is established.