Impact and Adhesion of Surfactant-Amended Water Droplets on Leaf Surfaces Related to Roughness

Abstract

Highlights The relationship between impacting droplet deposition and leaf surface roughness was examined. Complete deposition occurred on smooth leaves with roughness heights shorter than 1.5 micrometers. Droplet deposition on rough leaves was best achieved at low initial travel speeds and high surfactant concentrations. This research provides understanding of increasing droplet adhesion on plants for pesticide spray applications. Abstract. 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 (Sa), 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 Sa 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 Sa and contact angle (θc), ranged from smooth and easy-to-wet to rough and difficult-to-wet (37° 1.6 μm), deposition decreased with increasing Sa 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.