Charging of dust particles on surfaces

Experimental investigations have been made of the charge on dust particles resting upon a metal surface in vacuum. The surface is agitated so that the particles drop through a small hole and a Faraday cup beneath measures the charge on each particle. The surfaces are metals (Hf, Zr, V, W, Co, Ni, Pt, and stainless steel) and the dust grains are both metallic conductors (Zn, V, and stainless steel) and insulators (silica and alumina) in the size range of 50-200 mum. The contact charge is consistent with a model based upon the grain capacitance and the effective contact potential between the grain and surface. An electric field above the surface induces an additional charge on metallic grains consistent with Gauss's law. The induced charge on insulating grains increases with repeated contact. UV irradiation may increase or decrease the charge depending upon the relative importance of photoemission and photoconductivity. (C) 2001 American Vacuum Society.