Evaporation Time and Spread Area of Adjuvant-amended Droplets on Waxy and Hairy Leaves

Understanding the evaporation of pesticide droplets and wetting of leaf surfaces can increase foliar application efficiency and reduce pesticide use. Evaporation time and wetted area of single pesticide droplets on hairy and waxy geranium leaf surfaces were measured under the controlled conditions for five droplet sizes and three relative humidity levels. Stereoscopic sequential images of the droplet evaporation processes were taken for five droplet sizes, three relative humidity (RH) conditions and 9 different sprays. The sprays were combinations of water, an alkyl polyoxyethylene surfactant, a fungicide and three insecticides. The evaporation time and wetted area of droplets were significantly changed by adding the surfactant to the sprays, but not by adding the fungicide or insecticide. Droplet evaporation times on waxy leaves were longer than those on hairy leaves. Evaporation times increased exponentially as droplet diameter and RH increased with limited variability of regression coefficients independent of spray type and leaf surface. The wetted area of droplets also increased exponentially as droplet diameter increased but it was not significantly affected by RH. On the waxy geranium leaf surfaces, the contact area of pesticide droplets decreased throughout the evaporating process and at all RH conditions, while, on hairy leaf surfaces for the same size droplets and at the same RH conditions, the contact area continued to spread until evaporation was nearly completed. Therefore, droplet size, surface characteristics of the target, relative humidity, and chemical composition of the spray mixtures (water alone, pesticide, additives) should be included as important factors that affect the efficacy and efficiency of pesticide applications.