Retention and Spread Capability of Impacted Droplets with Surfactant and Hydrocolloid Based Adjuvants

HighlightsRetention and spread of droplets with seven adjuvants were investigated on flat leaves.Surface tension of spray solutions, leaf wettability, and roughness strongly affected droplet retention and spread.Complete droplet retention occurred on hydrophilic leaves with all adjuvants and concentrations.High retention and spread on superhydrophobic leaves were achieved with the organo-silicone adjuvant.No retention occurred on superhydrophobic leaf surfaces with the hydrocolloid polymer adjuvant.Abstract. To optimize spray application efficiency, it is necessary to understand how the addition of adjuvants modifies the deposition properties of spray droplets on leaf surfaces due to variations in adhesive characteristics, such as roughness and wettability. Retention and spread of droplets with seven commercially available adjuvants were tested and compared at different concentrations. Tests were conducted with three leaf surfaces ranging in roughness and wettability from very smooth and hydrophilic to very rough and superhydrophobic. The adjuvants were formulated with non-ionic surfactant, crop oil, seed oil, organo-silicone, hydrocolloid polymer, or combinations of these agents as primary ingredients. Droplets of approximately 340 µm diameter were emitted from a streamed mono-sized generator. Droplet impact and spread were recorded with a 3D imaging system consisting of three high-speed digital cameras and analyzed using 3D motion analysis software. Retention and spread were determined by comparing droplet liquid volumes and droplet cross-sectional areas before and after impact, respectively. The surface tension of spray solutions and wettability of leaf surfaces strongly affected droplet retention. Droplets with lower surface tension were more likely to achieve high retention than those with higher surface tension. Droplet retention generally decreased with increasing leaf roughness-wettability. All droplets with and without adjuvants achieved 100% retention on the hydrophilic leaf surface. Addition of non-ionic, oil, or organo-silicone based adjuvants in the spray solution improved droplet retention on hydrophobic leaves, while the organo-silicone based adjuvant achieved the highest retention on superhydrophobic leaf surfaces. Droplet retention with the hydrocolloid polymer adjuvant was generally comparable to the other six adjuvants on the hydrophilic leaves and on the hydrophobic leaves with intermediate roughness and wettability but failed to achieve any retention on the superhydrophobic leaves. To improve droplet retention and adhesion, selection of adjuvants representing the non-ionic, oil, organo-silicone, blended, or hydrocolloid-based additives for enhancing spray solution performance must comply with the leaf surface characteristics. Keywords: Droplet rebound, High-speed imaging, Leaf wettability, Pesticide droplet deposition, Topography parameters.