ELECTROSTATIC EFFECTS IN INERTIAL IMPACTORS

A focusing impactor is used to study the influence of particle charge q on the capture efficiency versus Stokes number curve eta(S). For hig hly charged particles, image attraction to the collector leads to cons iderable particle capture even at S = 0. This produces long subcritica l tails in the eta(S) curve (poor sizing resolution) and reduces the v alue S of S at which eta = 1/2. A similar behavior, now due to Browni an motion, arises for singly charged ultrafine particles, with a negli gible influence of the image force. Both types of tails are reduced or eliminated by a repulsive electric field E between the collector and the nozzle, though at the price of increasing S. For repelling fields E large enough for the subcritical tails to disappear, S is a functi on only of the ratio ZE/U between the electrical and hydrodynamic velo cities of the particles. The functional dependence S(ZE/U) is charact erized experimentally for a nearly incompressible thin-plate orifice n ozzle flow at Reynolds number Re = 68. Calibration aerosols include si ngly or doubly charged oil droplets with diameters between 16 and 155 nm. Also polystyrene latex spheres (PSL) 74 nm in diameter generated f rom a water suspension by electrospray, whose charge q was narrowly co ntrolled in the range 540 > q/e > 390 with a differential mobility ana lyzer. The analysis of the rate of deposition in the stagnation point region in the limit S = 0 predicts the repulsive fields required to of fset the tails, in fair agreement with those observed both for singly and doubly charged as well as highly charged particles. The conclusion is reached that impactors whose detector is an electrometer can attai n rather high resolutions in all the range of sizes and charges explor ed. (C) 1997 Elsevier Science Ltd.