Quantification of drifting droplets in sprays experiencing crosswind shear via digital image analysis techniques

In agricultural spray applications drift risk and coverage potential are strongly dependent on the droplet size distribution (DSD) produced during the atomization process. This DSD is typically measured in a controlled spray chamber or a wind tunnel environment. When measuring sprays in this manner the spray experiences an idealized condition wherein the atomization process is unperturbed by interactions with the ambient air. This contrasts with the conditions a spray experiences during field application, in which the spray experiences crosswind during atomization. High speed shadowgraphy was used to visualize a spray containing a viscosity modifier oriented in a low speed wind tunnel to simulate this crosswind condition. The spray thus oriented was observed to exhibit bag breakup, wherein portions of the lamella are inflated by the crosswind, creating a thin liquid membrane, supported by thicker adjacent portions of the lamella, which ruptures into a cloud of fine droplets that are entrained in the crossflowing air as drifting droplets. Digital inline holography was used to quantify the entrainment rate of these droplet via simultaneous measurement of their three-dimensional size and position as they passed through a measurement volume downwind of the lamella. This measurement was used to quantify the efficacy of numerous commercially available drift reduction technologies in crosswind conditions. It was found that the addition of viscosity modifiers reduced the entrainment rate of drifting droplets, and the further addition of an oil-based adjuvant that forms a spontaneous emulsion further reduced the entrainment rate in tank mixes with low surfactant loading. The improved performance with the oil-based emulsion is hypothesized to be a result of the emulsified oil droplets inhibiting the formation of bag membranes in crosswind.