IMPACT AND ADHESION OF SURFACTANT-AMENDED WATER DROPLETS ON LEAF SURFACES RELATED TO ROUGHNESS

Understanding the effects of the microscopic-scale surface roughness of targeted leaves on spray droplet retention and spread can help develop optimal strategies to improve pesticide application efficiency. Improved spray deposition would minimize the amounts of chemicals to be applied and reduce off-target losses and production costs while protecting the environment. A 3D optical surface profiler was used to measure the arithmetic mean roughness height (S-a), providing a reliable metric for microscopic-scale leaf surface roughness. The relationship between leaf surface roughness and droplet deposition was examined by correlating droplet adhesion with the S-a of six leaf types for spray solutions at five surfactant concentrations (0.0% to 0.75%) and eight initial droplet horizontal travel speeds (0.5 to 4.5 m s(-1)). Leaf roughness and wettability, in terms of S-a and contact angle (theta(c)), ranged from smooth and easy-to-wet to rough and difficult-to-wet (37 degrees < theta(c) < 159 degrees and 1.11 mu m < S-a < 2.47 mu m). Deposition of 310 mu m droplets was determined by comparing initial droplet volume to the residual liquid volume after impact. Droplet size and initial travel velocity were controlled by a streamed mono-sized droplet generator mounted on a variable-speed linear track. Droplet motion and impacts were recorded with a 3D stereoscopic system consisting of three ultrahigh-speed video cameras and analyzed using 3D motion analysis software. Initial droplet horizontal flight velocity had little to no influence on impact speed; however, droplet impact angle decreased with increasing initial flight speed. Complete deposition occurred on smooth leaves (S-a < 1.51 mu m) regardless of surfactant concentration and initial droplet horizontal flight velocity. For rough leaves (S-a > 1.6 mu m), deposition decreased with increasing S-a and initial droplet horizontal flight velocity, while it increased as surfactant concentration increased. Thus, spray deposition on rough leaf surfaces could be greatly improved with slow initial droplet flight speeds and high surfactant concentrations. In contrast, greater deposition on smoother leaf surfaces could be achieved at any initial droplet flight velocity and with low concentrations of non-ionic surfactant, or possibly none.