Constraints on contact charging of insulators. I. Spatial localisation of insulator states

It is a commonplace observation that an insulator may be charged by another insulator, to much the same extent as by a metal. Yet the phenomenon is somewhat surprising: since charge is by definition immobile in the insulator one may as: 'how does charge pass from one insulator to another?'. If electron states in insulators are strongly localised in space, the states on the surface of one insulator will not overlap the states on the surface of a second insulator, except occasionally by chance. A possible resolution of this apparent paradox is that contact between insulators normally involves a degree of rubbing or sliding, deliberate or inadvertent, and this will obviously help to overcome any constraints on charge transfer imposed by localisation. The authors have investigated charge transfer between two insulators, using equipment designed to eliminate sliding during contact. The experiments clearly show that insulators charge each other even under these conditions, i.e. with sliding eliminated as far as possible. If sliding were indeed entirely eliminated it would be difficult to reconcile these observations with the electron-transfer model of contact charging. However, they show that a certain amount of relative 'sliding' is unavoidable, if only because the contact is inevitably accompanied by deformation of the contacting surface. This residual 'sliding' may be sufficient to overcome the constraints of localisation. Whatever the reasons, it is clear that charge transfer between two insulators is not limited by the spatial localisation of the electron states.