In crop protection, it is well know that droplet size determine spray efficacy.The optimisation of both spray deposition and retention leads to a dilemma:should small droplets be used to increase retention or large droplets be preferred to avoid drift? An ideal droplet should have a short time of flight to minimise its distance travelled while impacting the target with a moderate kinetic energy. This paper aims to determine an optimum range of droplet sizes for boom-sprayer applying herbicide using a modelling approach. The main parameters of spray deposition and retention models are systematically varied and the effects on drift potential and droplet impaction outcomes are discussed.The results of the numerical simulations showed that droplets with diameter ranging between 200 µm and 250 µm offer high control of deposition by combining a low drift potential and a moderate kinetic energy at top of the canopy. A fourfold reduction of the volume drifting further than 2 m from the nozzle was observed for a spray with a volume median diameter of 225 µm when the relative span factor of the droplet spectrum was reduced from 1.0 to 0.6. In the latter * Corresponding author Email address: nicolas.decock@ulg.ac.be (Nicolas De Cock)Preprint submitted to Journal of L A T E X Templates August 3, 2017scenario, an increase from 63 to 67% of the volumetric proportion of droplets adhering to the wheat leaf was observed. Therefore, strategies for controlling the droplet size distribution may offer promising solutions for reducing adverse impact of spray applications on environment.

A model predicting the spray droplet interception and retention by a single virtual plant has been developed. The model was based on three main experimental inputs: the 3D architecture of a barley plant, the spray quality and the droplet impact behavior. Two contrasted formulation scenarios, limits of the common range covered by pesticide application in terms of surface tension, were tested by changing the droplet behavior at impact in the model. Simulations were undertaken for studying the variability of spray retention resulting from spray quality, applied volume and plant size for a difficult-to-treat target. Results showed that the spray retention efficiency ranged from 6.8% to 96.6% of a theoretical full adhesion scenario, where all intercepted droplets were captured, according to spray quality for the two formulation scenarios tested. Average retention increased with increasing spray fineness, applied volume per hectare and plant size. Variability of deposits, evaluated using the coefficient of variation of simulated retentions, was found to be a function of the mean droplet density according to CVα , where CV is the coefficient of variation and N the number of droplet per square centimeter. Variability was also found to be a function of the plant size according to a relation CV α S -0.5 , where S is the total leaf area of the plant model. The variability of deposits increased with decreasing spray fineness, applied volume per hectare and plant size because of the reduced number of droplets contributing to retention. Wetting properties greatly influenced retention but surprisingly poorly influenced the variability of deposits. Such a modeling approach that is capable of an independent investigation of the influence of various parameters on spray retention can be used to improve understanding of application methods and adjuvants that could help minimizing development of resistance in problematic weed species.

The present work proposes an extension of the existing analytical development on the radial spread of a liquid jet over a horizontal surface to the case of a thin radial flow. When the gap, H, between the jet nozzle and the plate is reduced the discharging area may be smaller than the inlet area leading to an increase of the main flow velocity downstream of the thin cylindrical opening. This increase of velocity, defined here as H the distance between the nozzle and the plate, U 0 jet mean velocity, U 1 the main stream velocity, h(r) the liquid film thickness, U (r) the interface velocity and δ(r) the boundary layer thickness.