Submitted to: International Symposium on Adjuvants for Agrochemicals
Publication Type: Proceedings
Publication Acceptance Date: 8/30/1995
Publication Date: N/A
Citation: Interpretive Summary: Physical and chemical properties of plant surfaces and spray solutions are critical to whether droplets are captured by plants in spray applications for crop protection. Droplets may be reflected, or bounce, upon impact with some plants, particularly those having waxy leaves, and possibly lost. Surfactants, or other surface-active substances, added to spray solutions may reduce bounce and improve retention. However, surfactants differ in their ability to improve droplet capture, their net effect dependent on properties of the plant surface and spray solution. In large-volume spray applications, surfactants may improve droplet capture and plant-surface coverage, but at the risk of excessive runoff, as opposed to smaller-volume sprays which are retained as discrete droplets. Retained droplets, upon evaporation, leave deposits of active ingredients and solution additives. Deposits vary from broken, ring-like structures to single masses in form. Surfactants may modify deposit form, in some cases spreading material over large areas. These interplays of pest-control formulation and plant, sometimes overlooked due to their existence at a microscopic level, are important to achieving efficient crop protection and assuring a safe, high quality supply of food and fiber with minimum environmental impact.
Technical Abstract: Spray droplet:plant interaction is central to droplet retention: Those adhering contribute to crop protection/bioregulation, while those reflected reduce efficiency and add to pollution. Impaction is complex, with surface morphology and solution surface tension (ST) important factors. Droplet reflection lessens, in order, from leaves of cabbage, wheat, soybean and foxtail, with none from pear, correlating highly with dynamic surface tension. Surfactants differ in ability to reduce ST and improve retention by a given plant, their net effect dependent on the plant surface and spray volume. In high-volume sprays, reduced ST often increases droplet retention, spread and coalescence, but allows runoff. Low-volume sprays are retained as discrete droplets: Surfactants increase droplet:plant interface depending on solution properties and surface microstructure/chemistry. ST has little effect on interface area for easily wetted sugar beet leaves, while efficient surfactants increase interface on hard-to-wet kohlrabi. After droplets adhere, the liquid phase evaporates, leaving an apparently dry deposit of active ingredient (AI) and nonvolatile additives. Marked concentration, pH, ionic-strength and AI- distribution changes occur between bulk and constituent additives in drying. Evaporation is rapid at the droplet:air:leaf-surface interface, droplet contents depositing heavily at such sites. Deposits vary from narrow annuli to single masses, crystalline to amorphous in form. Reduced spray volume at constant dose raises the amount of AI per unit interface. Surfactants modify deposit form, silicone surfactants spreading material over large areas.