Electrification by Collisions of Ice Particles on Ice or Metal Targets
To clarify the problem of charge separation from colliding ice particles, experiments have been carried out in a wind tunnel at Weissfluhjoch (Switzerland) with frozen droplets of 20 μm diameter impinging on various targets. Ice impacting on ice at 10m/s and at a temperature of —45°C produced a charge separation of the oder of 2 fC (= 2 × 10–15 C) per collision. The sign of the charging could be reproducibly positive or negative, depending on the preparation of the target and the course of the experiment. Thermal effects were important only insofar as an ice target after sublimation tended to acquire negative charge through the collisions, whereas after deposition positive charging was favoured.
A clue to the mechanism of charge separation was obtained from collisions between ice particles and metal targets. Metals characterized by a large workfunction (C, Pt, Au) became negatively charged, whereas metals of smaller workfunction (Ba, Li, Mg, Pb, Zn, Cd, Cu, Ni, Al) acquired positive charge from impinging ice particles. Measurements using rotating cylinders composed of two different metals confirmed that the two charge separation currents as well as the measured surface potential difference of the metal pair corresponded to the work-functions involved. A linear correlation between charge separation and surface potential difference was obtained, with a slope of 15 fC/V per collision.
It was inferred from these results that the ice surface is able to exchange electrons from surface states. The workfunction of ice was determined to be around 4.4eV. It can be modified within a few tenths of an eV by doping or surface treatments such as scratching, ageing, sublimation, deposition. It is concluded that charge separation occurring between colliding ice particles is due to such surface effects, which may be still difficult to predict from natural situations.
Applied electric fields changed the charging in a nonlinear manner, especially when a metal target was used whose workfunction was much different from that of ice.
KeywordsWind Tunnel Impact Velocity Charge Separation Polarization Charge Metal Target
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