Experimental study on the impact outcomes of droplets on rice foliage: influence of liquid properties and leaf angles

As one of the most important cereal crops, the superhydrophobicity of rice leaves makes spray droplets more prone to bounce or shatter, which reduces the plant protection product deposition rate. To clarify the impact mechanism of droplets on rice foliage, the effects of droplet diameter, velocity and liquid physicochemical properties on the outcomes of droplets impact were investigated using high-speed photography. The critical boundaries of droplets impact outcomes (adhesion/bounce, bounce/shatter or adhesion/shatter) on horizontal rice leaves were determined according to the 90th percentile values of the Weber number (We).

The results showed that the addition of xanthan gum (XG) could inhibit droplet shattering while adding Vibrant reduced droplet bouncing on horizontal rice leaves, respectively. When both XG and Vibrant were added to the liquid, the bounce and shatter of the droplet impaction were completely inhibited, and all the generated droplets adhered on the horizontal leaves. For droplets impacting on inclined leaves, the embodied outcomes of droplet impact on inclined rice leaves for each liquid solution presented the same classification with that of impact on horizontal leaves. Compared with the impact results on horizontal leaves, the water and XG solutions exhibited an increased tendency of droplet bounce, only a limited number of droplets were able to adhere on the inclined leaves, and the bounce tendency became more pronounced as the leaf inclination angle increased.

High viscosity and low surface tension could inhibit droplets shatter and bounce, respectively. Therefore, the XG + Vibrant droplets with high viscosity and low surface tension exhibited a strong adhesion property. The inclined angle leaves did not affect the impact outcomes of XG + Vibrant droplets, and all test droplets adhered after impact. This study can provide a guidance for improving the adhesion and deposition of spray droplets on rice leaves, which is crucial to maximize plant protection application efficacy and reduce environmental pollution. © 2025 Society of Chemical Industry.