EXPERIMENTAL AIR-ASSISTED ELECTROHYDRODYNAMIC SPRAYING

A linear electrohydrodynamic (EHD) atomizer, operated at -25 kV, was used to quantify the effects of droplet size, charge to mass ratio, forward speed and the addition of air assistance, on spray deposition in natural and artificial crop canopies. Spraying was done at 0.5, 1.0 and 2.0 m s-1 forward speed, using three Risella EL oil:butanol EHD formulations (80:20, 70:30 and 60:40) producing droplets with volume median diameters of approximately 124, 71 and 48 mum and charge to mass ratios of approximately 2.0, 7.5 and 9.5 mC kg-1, respectively. The two levels of air assistance used (11.3 m s-1, 0.39 m3 s-1 and 21.2 m s-1, 0.72 m3 s-1) were produced by a commercial air-curtain device. Spray deposition was compared with that obtained from medium-quality flat-fan hydraulic nozzles applying 238 l ha-1 without air assistance. Spray drift was measured 2 m downwind over a winter wheat crop, in wind speeds of 1.5 and 2.5 m s-1. Using artificial targets, total spray deposition and canopy penetration from the charged sprays increased as target density decreased. Larger droplets with a smaller charge to mass ratio penetrated the canopy better than smaller, more highly charged sprays but gave poorer abaxial surface deposits. Air assistance increased canopy penetration but reduced abaxial surface deposits. In a winter wheat crop at growth stage (GS) 22, decreasing charge to mass ratio resulted in decreased plant deposits and increased ground contamination, although in all cases this was significantly less than that with hydraulic nozzle applications. Similar trends were measured at GS 37-39, where the addition of air assistance increased canopy penetration by highly charged sprays and decreased the soil contamination found with the larger-droplet, lower-charged applications. There was no consistent effect of forward speed. Spray drift from EHD sprays increased with increasing charge to mass ratio and decreasing droplet size, and was significantly greater than that from the hydraulic nozzle spray at both wind speeds. The use of air assistance reduced spray drift from the smallest droplets by approximately 93%.