Electrification Associated with Droplet Accretion on Ice

Abstract

Droplets in the size range of 40 to 120 µm diameter were accelerated in a small wind tunnel and accreted onto an ice target connected to an electrometer that measured the charging effect. The experiments were performed with environment temperatures between ?6 and ?14C and impact velocities between 1 and 30 m sec?1, both in the absence of an external field and with fields up to 2.5 ? 105 V m?1. It was found that in the absence of field, significant charging occurs only above a threshold velocity, of the order of 10 m sec?1 for 100-µm droplets. The sign and magnitude of the effect were critically dependent on the liquid composition, the sign being negative for NACl solutions and positive for (NH4)2SO4 solutions and pure water. The magnitude increased with droplet size and with impact velocity, reaching a maximum in the order of 1?3 fC (average charge per droplet) at about 25 m sec?1, for 100-µm droplets. No evidence of ice splintering was found, and the effect appeared associated with splashing. An explanation based on the mechanism of double layer shearing (which implies that the Workman-Reynolds effect is active) is proposed. External fields induce charge separations proportional to their intensity, with proportionality constants increasing with droplet size and impact velocity; this effect becomes predominant for fields in the order of 104 V m?1 or higher. Consequences of the experiments for the theory of thunderstorm electrification are drawn. A separate series of experiments, dealing with the electrification associated with the splashing of 370- and 500-µm droplets, is reported in an Appendix.