Abstract
Retention of agricultural sprays on plant surfaces is an important challenge. Bouncing of sprayed pesticide droplets from leaves is a major source of soil and groundwater pollution and pesticide overuse. Here we report a method to increase droplet deposition through in-situ formation of hydrophilic surface defects that can arrest droplets during impact. Defects are created by simultaneously spraying oppositely charged polyelectrolytes that induce surface precipitation when two droplets come into contact. Using high-speed imaging, we study the coupled dynamics of drop impact and surface precipitate formation. We develop a physical model to estimate the energy dissipation by the defects and predict the transition from bouncing to sticking. We demonstrate macroscopic enhancements in spray retention and surface coverage for natural and synthetic non-wetting surfaces and provide insights into designing effective agricultural sprays.
Highlights
Retention of agricultural sprays on plant surfaces is an important challenge
One of the most important inefficiencies arises from the low retention of sprayed liquids on plant surfaces due to their hydrophobic/superhydrophobic properties—droplets from sprays impacting plant surfaces can bounce or roll off plant surfaces
Surface tension of all the used solutions was measured to be within 13% of that of water (Supplementary Fig. 1)
Summary
Retention of agricultural sprays on plant surfaces is an important challenge. Bouncing of sprayed pesticide droplets from leaves is a major source of soil and groundwater pollution and pesticide overuse. One of the most important inefficiencies arises from the low retention of sprayed liquids on plant surfaces due to their hydrophobic/superhydrophobic properties—droplets from sprays impacting plant surfaces can bounce or roll off plant surfaces Such plants are common, and they usually get their non-wetting properties from the presence of waxy features on their surface[6,7,8]. One common approach to improve drop retention is to add surfactants to the sprayed liquid in order to reduce the surface tension and promote spreading of droplets on the surface[26]. It was suggested that the observed retention arises from non-Newtonian properties, the extensional viscosity of the polymer solution Ze. Stretching the fluid during expansion and retraction unfolds and deforms high molecular weight polymers leading to significant energy dissipation that can prevent droplet rebound. The underlying physical mechanism is still being investigated[32,33,34]
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