Transient analysis of charged droplet motion and corona-induced velocity perturbations during electrostatic deposition processes

The time-varying characteristics of charged-droplet motion have been investigated. A three-dimensional electrostatically shielded target, having an interior or exterior point as well as a solid spherical target with a topside point, underwent spraying characterized by a rising and then decaying space-charge field as the target was traversed at 5 km/h by -4 C/kg spray. During the 1 s spraying event two orthogonal components of the transient droplet velocity were monitored at various target regions. For both target systems electrostatic attractive forces could be detected, especially in the 350 ms time period immediately following turbulent intersection by the spray cloud. During this period electrostatic forces were sufficient to cause a 15-20 cm/s attractive velocity as compared with -2 to 5 cm/s for uncharged spray in the interior regions of the shielded parallel-plate target, even in the presence of target points. Space-charge fields driving the droplet-deposition process decayed with an exponential time constant of 0.22-0.28 s