A review of liquid sheet breakup: Perspectives from agricultural sprays

During the spray process, many factors control the disintegration of the liquid sheet at the exit of a nozzle operating in an ambient atmosphere. Understanding the role of these parameters has significant implications on the spray quality. In particular, in agricultural applications, the formation of finer spray droplets during breakup increases the risk of potential off-target movement of the spray as finer droplets can be carried by wind, travel long distances, and deposit on neighboring crops and vegetation. Herein, we provide a review of liquid sheet breakup with a focus on factors that influence the average droplet size distribution in agricultural sprays, seeking to reduce off-target spray drift. In Section I, we introduce key droplet size metrics relevant to spray drift as well as the general physical mechanisms of liquid sheet breakup. Next, we discuss factors that influence both spray sheet breakup and droplet size distributions, separated into non-chemical (process-dependent) factors in Section II and chemical (formulation-dependent) factors in Section III. Non-chemical factors include the geometry of the nozzle, operating pressure, gas aeration, and the temperatures of the sprayed liquid and the ambient gas. Chemical factors include viscosity, surface tension, and the presence of inhomogeneities in the spray media, such as oil emulsions and solid particles. In Section IV, we further examine the role of surfactants in these spray breakup events, with an extended focus on those containing emulsified oils typical of drift-reduction adjuvants, additives used to enhance the effectiveness of pesticides. The role of surfactant transport on resultant drop sizes, and potential characterization of these transport processes with time-dependent versions of the bridging, entering, and spreading coefficients are provided. Finally, methods are given for measuring dynamic surface tension and interfacial tensions at appropriate length and timescales for calculating the dynamic coefficients. •Droplet size in agricultural sprays should be 100–400 μm to reduce the drift and maintain a good deposition and coverage.•The process-dependent (non-chemical) and formation-dependent (chemical) factors that impact spray droplet sizes are reviewed.•The key role of emulsified oil and water- and oil-phase surfactants in spray breakup is highlighted.