Evaporation rate and development of wetted area of water droplets with and without surfactant at different locations on waxy leaf surfaces

The evaporation and formation of deposit patterns from single droplets deposited at various locations on waxy leaves were investigated under controlled conditions. Leaf locations included the interveinal area, midrib and secondary vein on both adaxial and abaxial surfaces. Tests were conducted with 300 and 600 μm diameter droplets containing water and a non-ionic surfactant. The ambient temperature was 25 °C and relative humidity (RH) was 60%. Evaporation time and wetted area varied with the area where the droplets were deposited on the leaf surfaces. The variation in evaporation time for 300 μm diameter droplets without surfactant on the interveinal area, midrib and secondary vein of adaxial surface was 30% whilst the variation of wetted area was 39%. The wetted area was significantly larger on the adaxial surface than on the abaxial surface but the evaporation time between both surfaces was not significantly different. For the whole leaf, the average evaporation time of 300 μm diameter droplets decreased by 44% and the average wetted area increased by 202% when 0.25% non-ionic surfactant was added to the spray solution. The total mean evaporation time increased 279% and the wetted area increased 166% without the surfactant, 452% and 229% with the surfactant when droplet diameter was increased from 300 to 600 μm. The largest deposit area was measured on the midrib of adaxial surface with added surfactant. The 300 μm diameter droplets had longer evaporation times per droplet volume and greater wetted area per droplet volume than the 600 μm diameter droplets, thereby supporting the hypothesis that increased pesticide application efficiency could be achieved by finer spray. This study also demonstrated that the ratio between spray coverage area and the amount of spray liquid required could be increased by the use of surfactants, thereby offering possibilities of reduction in spray application rates and of increase in application efficiency.