Submitted to: Pesticide Formulation and Application Systems Symposium Proceedings
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/5/2001
Publication Date: N/A
Citation: Interpretive Summary: Dynamic surface tension at young surface ages is a key physical characteristic of spray mixtures. Two processes that are critical to spraying plant foliage are linked to this characteristic. An agricultural nozzle changes a solid stream of spray liquid into large numbers of spray droplets in less than 5 milliseconds. Some plant leaves tend to reflect droplets that land on them. Typical spray droplets remain on the leaf surface only about 1 to 3 milliseconds before they bounce off the leaf, possibly onto the ground. Adjuvants in spray mixtures must be active at these short time frames if they are to influence the droplet size distribution of the resulting spray cloud and reduce rebound. Most commercial pest control agents are formulated with several adjuvants to improve their handling and storage characteristics. It would be valuable to be able to measure standard physical characteristics of possible formulations that could predict what effect the spray mixture would have on droplet size formation and/or on droplet rebound. This paper presents data on dynamic surface tension at young surface ages for 40 common agricultural products and surfactants. The effect of molecular weight and concentration is shown for several surfactants. The data presented will be valuable to formulation chemists who create pest control agents for the US farmers. Spray mixture properties that affect droplet formulation and impaction on leaf surfaces should be included at the formulation level. Thus manufacturers could produce spray mixtures that are easier to control and improve spray cover on the target plant.
Technical Abstract: Dynamic surface tension data for a selection of surfactants, pesticides and other miscellaneous liquids are presented. Surface tension was measured using the Oscillating Jet technique at surface ages ranging from 0.5 to 7 ms. This millisecond timescale is relevant to such spray application processes as atomization and droplet impaction. The oldest age at which surface tension could be measured varied with the liquid tested. The waveforms of some liquids were very stable, while others became distorted soon after leaving the orifice. Similarly, different formulations of the same surfactant varied in their stability. For most of the liquids tested, surface tensions decreased with both increasing surface age and increasing concentration. At concentrations less than 0.1%, surface tension of most surfactants, measured at surface ages on the order of 1 to 3 ms, was nearly equal to that of water. The effect of varying the molecular weight of surfactant (within a given surfactant chemistry) varied with the surfactant. The method used to determine surface tension was accurate - as indexed by the small deviation of the dynamic values from the experimental values of the pure solutions ethanol and isopropanol.