Electrostatic charging of two insulating powders

The electrostatic charging characteristics of a mixture of two insulating powders with radii R and r such that R≫r are studied using a charge spectrometer which allows complete characterization of the charge and radius distribution function. The data are consistent with a natural extension of the surface-state theory which assumes, to first order, that the smaller radii powder particles share a fixed amount of charge determined by surface states on the larger radii particles. The challenges this result poses to microscopic theories of insulator contact charge exchange are discussed.