Bipolar Spray Charging for Leaf-Tip Corona Reduction by Space-Charge Control

Previous studies in the development of electrostatic crop-spraying technology have confirmed a process limitation due to gaseous discharges induced to flow from earthed plants under action of intensely charged pesticide spray clouds. This paper reports the development and evaluation of a bipolar spray-charging strategy introduced in an effort to further increase the electrodeposition efficiency of charged airborne spray droplets by minimizing corona discharge at prominent leaf tips of such target plants. It was hypothesized that a properly constituted bipolar spray cloud would eliminate this detrimental corona neutralization of the approaching spray by reducing to below threshold the space-charge electric field imposed at the periphery of the target-plant canopy, while still maintaining an adequate local space-charge field within the electrically shielded plant canopy to enhance droplet deposition. Veriable-frequency bipolar square-wave functions ranging from 12 to 36 Hz were input to an induction-charging nozzle to produce spray clouds having the desired space-charge characteristics. Results of experimental charge-and mass-transfer analyses document that the bipolar strategy successfully eliminated induced target-corona but failed to provide any electrodeposition benefit within shielded target zones. As compared with uncharged sprays, however, unipolar-charging enhanced the droplet deposition within similar targets by factors of 1.3- and 1.5-fold for corona and noncorona target conditions, respectively.