MECHANISMS, EXPERIMENT, AND THEORY OF LIQUID SHEET BREAKUP AND DROP SIZE FROM AGRICULTURAL NOZZLES

Spray nozzles operate by discharging a liquid sheet or jet which subsequently breaks up into droplets. Droplets which are too small can become entrained in ambient air currents and carried off target, while larger droplets often reduce coverage and efficacy. Many agricultural chemicals are formulated as an oil-in-water emulsion. In this study, single-phase (water) and two-phase (oil-in-water) emulsion were investigated photographically, experimentally, and theoretically to isolate the relevant mechanisms of sheet disintegration and representative droplet size. Three distinct mechanisms of sheet breakup were observed and parameterized by different scaling of the Weber number. Mechanisms include wave growth, rim breakup, and hole growth. Wave and rim breakup were found to dominate in single-phase sprays, while formation of holes within the liquid sheet and hole growth was dominant when an immiscible second phase was introduced. Existing models for wave growth and rim breakup leading to atomization, along with a novel model for hole expansion and subsequent sheet destruction, compare favorably with droplet diameters obtained experimentally. Although the exact mechanism for hole creation is not definitively established, several possibilities are discussed and inferred from experimental observations. It appears that a second immiscible phase, if low- or nonwetting, creates the necessary precursor for hole formation within the liquid sheet leading to sheet breakup. If this mechanism is indeed correct, it should be possible to control the spray droplet size distribution from spray nozzles given the size and wettability (hydrophobic nature) of solid or liquid immiscible particles/droplets within a two-phase system.