MECHANISMS CONTROLLING LEAF RETENTION OF AGRICULTURAL SPRAY SOLUTIONS

Active ingredient deposition as a whole during foliar spray application was studied by means of microscopic and macroscopic methods. High-speed photography of the impact of spray droplets showed that rebound from reflective plant surfaces was reduced only with surfactant concentrations well above the critical micelle concentration. These results correlate with laboratory spraying experiments to determine quantitatively the retention of spray solutions. The dynamic interaction between droplet and foliage seemed to be specifically dependent on surfactant structure, concentration and the distribution of isomers or by-products in the technical product. Rough surfaces with epicuticular wax crystals were difficult to wet; the orientation of the leaf, however, had little influence on retention. Apolar artificial surfaces retained water rather well and are therefore unsuitable as model surfaces for reflective leaves. Measurement of dynamic surface tension at 100 msec gave a satisfactory correlation with retention values in the case of liquids containing surfactant but failed to predict the good adhesion of polyvinyl alcohol solutions.