Impaction of spray droplets on leaves: influence of formulation and leaf character on shatter, bounce and adhesion

TL;DR

This study investigates how spray formulation and leaf surface properties affect droplet behavior upon impact, combining experiments with models to predict shatter, bounce, and adhesion on real leaves with natural variability.

Contribution

It introduces impact criteria that account for leaf angle and droplet trajectory, validated with experimental data on real leaves and commercial spray nozzles.

Findings

Droplet shatter increases with size and velocity.

Droplets bounce more on hard-to-wet surfaces with high surface tension.

Discrepancies suggest air layer effects and wet surface conditions influence outcomes.

Abstract

This paper combines experimental data with simple mathematical models to investigate the influence of spray formulation type and leaf character (wettability) on shatter, bounce and adhesion of droplets impacting with cotton, rice and wheat leaves. Impaction criteria that allow for different angles of the leaf surface and the droplet impact trajectory are presented; their predictions are based on whether combinations of droplet size and velocity lie above or below bounce and shatter boundaries. In the experimental component, real leaves are used, with all their inherent natural variability. Further, commercial agricultural spray nozzles are employed, resulting in a range of droplet characteristics. Given this natural variability, there is broad agreement between the data and predictions. As predicted, the shatter of droplets was found to increase as droplet size and velocity increased, and the surface became harder to wet. Bouncing of droplets occurred most frequently on hard to wet surfaces with high surface tension mixtures. On the other hand, a number of small droplets with low impact velocity were observed to bounce when predicted to lie well within the adhering regime. We believe this discrepancy between the predictions and experimental data could be due to air layer effects that were not taken into account in the current bounce equations. Other discrepancies between experiment and theory are thought to be due to the current assumption of a dry impact surface, whereas, in practice, the leaf surfaces became increasingly covered with fluid throughout the spray test runs.