Perturbations of Charged-Droplet Trajectories Caused by Induced Target Corona: LDA Analysis

During electrostatic spraying, droplet deposition can inherently become limited by gaseous discharges induced to flow between earthed target points and the nearby charged-spray cloud as a result of electric-field intensification at the points caused by the cloud's space charge (c. 25 μZC/m
³
). This problem is distinct from back ionization, and it has profound consequences for electrostatic crop spraying of foliar tips. Laser Doppler anemometry was developed to determine the spatial extent to which the counter-flow of ions from earthed points interacts with the spray cloud and how the trajectories of approaching charged droplets are consequently altered. In the vicinity of cylindrical, spherical, and disk targets the two-dimensional velocity fields were measured for conductive droplets of 30-50 Atm diameter of up to -6 mC/kg charge level, delivered by aerodynamic entrainment at 3 m/s. While electrostatic forces were shown to enhance droplet motion toward the various smooth targets, introduction of a 2-cm target point not only negated the electrostatic deposition benefit, but its action in some locations increased droplet repulsion from the targets.