DYNAMIC SURFACE TENSION IN RELATION TO DROPLET IMPACTION INTERACTIONS WITH LEAF SURFACES

Spray droplets of a wide range of sizes and velocities are reflected upon impaction on leaves of many plants. Several physical properties of the droplet liquid, such as surface tension and extensional viscosity, may affect reflective potential. Equilibrium surface tension, a commonly measured liquid property, is not a reliable indicator of reflection tendency. However, dynamic surface tension has been found to be strongly related to droplet reflection. An oscillating jet method was developed enabling measurement of dynamic surface tension at a range of short surface ages, even as short as 1ms. Some surfactants are unable to reduce surface tension rapidly at short air-liquid interface ages typical of droplet impaction processes. Hence, they may be unable to effectively limit reflection and improve retention. A thin-film diffusion model was developed which can be used to calculate dynamic surface tension at a range of surface ages, given liquid properties, surfactant diffusivity, apparent interfacial film thickness, and surfactant concentration. Correlating measured dynamic surface tension data with the diffusion model provides estimates of apparent film thickness and diffusion of surfactant/water mixtures. These activity properties are useful in understanding surfactant effects on high shear rate physical processes such as droplet atomization and leaf-surface impact, where short surface ages are critical. This paper describes the oscillating jet system, the diffusion model, and presents dynamic surface tension results and model fit for several surfactants.