Contact electrification of silica and soda glass

Charge transfer from metals to silica (and SiO2 grown on silicon) is predominantly negative and significantly dependent on the workfunction of the contacting metal. Charge transfer to soda glass, on the other hand, is strongly positive and does not vary much with metal workfunction; these features are unchanged by a wide range of surface treatments and are clearly intrinsic to the material itself, not an accidental result of some particular surface condition. The authors present strong evidence indicating that the positive charging of glass is a consequence of the presence of sodium (that is, in ordinary glass; other metals such as silver can be similarly effective). One consequence of this is that the charging of glass can be strongly modified by exposure to water, which leaches out the metallic ions. The presence of mobile sodium ions also causes pronounced charge accumulation when contacts are repeated to the same spot, an effect which is virtually absent from silica. Their observations suggest that the amount of charge transfer to glass must be limited by some factor other than the density of electron donor sites, e.g. by transfer of electrons or ions during separation.