Estimating the dynamic impact behavior of spray droplets on rice foliage

Rice, as a hydrophobic plant, exhibits minimal adhesion of pesticide droplets on its foliage, posing a significant challenge in reducing droplet runoff and minimizing pesticide usage during chemical application in paddy fields. To reveal the droplet impaction mechanism on rice foliage, a computational fluid dynamics (CFD) model of droplet behavior prediction was established via the volume of fluid (VOF) method. The predictive model numerically computed the process of a single droplet impacting the horizontal rice leaf surface, and simulated the impact behavior of gas-liquid two phases on the solid surface. By exploring the dynamic collision behavior law of droplets on the target blade, the collision outcomes (adhesion, bound and shatter) of droplets on the target leaf surfaces were predicted. Simulation results showed that the droplet size and impact velocity were the main factors affecting the impact outcomes. Additionally, smaller and lower velocity droplets are more prone to retention on the target. Droplets with diameter of 100, 200, and 300 μm adhered with the impact velocity of 1.0, 0.7, and 0.5 m/s, and bounced within the impact velocity range of 1.1–6.9, 0.7–4.5, and 0.5–3.5 m/s, respectively. The critical $K_{crit}$ value of droplet breaking was 42.81. Subsequently, a logistic regression model was established due to critical parameters of CFD simulations. To validate the model, high-speed photography was used to track the collision behavior of droplets on rice leaves. Droplet impact test demonstrated a good agreement between predicted simulation and experimental results, proving the authenticity of using the CFD model to simulate motion of droplets impacting on the rice foliage. This is of practical significance for guiding chemical application in paddy fields.