Contact electrification-why is it variable?

Research into the causes of triboelectrification is hampered by the well known problem of irreproducibility. Charge transfer by contact of a metal to a polymer, for example, may vary widely from one contact to another, even on the surface of a single polymer sample. Such variation may, in principle, be a result of place-to-place variations in the properties of the polymer; or it might be a result of changes in the properties of the metal (e.g. work function); or to variations in the process of charge transfer (e.g. variable amounts of back-tunnelling, or material transfer). The authors have made a detailed study of the variation in charge transfer for a variety of metal and polymer samples in order to decide among these possibilities. They find clear evidence that the charge transfer is associated with variations in the properties of the polymer surface. There are strong correlations between the fluctuations in the charge transfer from different metals, and between neighbouring places on the polymer surface. However, the variability of charge transfer is not a consequence of adventitious contamination, as is often suggested. Their observations show that the mean charge and the variation about the mean are little influenced by the surface treatment; moreover, if the charge transfer for a particular metal/polymer combination is scaled to the mean charge the spread about the mean is closely similar, both in magnitude and form, for a wide range of polymers. They conclude that the fluctuations in charge transfer must be much more closely connected with the basic mechanism of charge transfer than has hitherto been supposed.