Spraying of agricultural chemicals through spray nozzles is a standard delivery practice. 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 droplets which are too large often reduce coverage and efficacy. Many agricultural chemicals are dissolved in an oil phase and subsequently formulated as an oil-in-water emulsion which is then sprayed. 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, 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. A hypothetical case is presented when breakup due to hole growth competes with wave growth for a two phase oil-in-water formulation. Although the exact mechanism for hole creation is not definitely established, several possibilities are discussed and inferred from experimental observations. It appears that a second immiscible phase, if low- or non-wetting, 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 wettablity (hydrophobic nature) of solid or liquid immiscible particles/droplets within a two phase system.

Mechanisms, experiment, and theory of liquid sheet breakup and drop size from agricultural nozzles

246th National Meeting (2013)

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