Experimental and Molecular Dynamic Simulation of Droplet Deposition on Superhydrophobic Plant Leaf Surfaces

Pesticide droplet deposition on targeted plant leaf surfaces is of great importance but remains a significant challenge, especially on leaf surfaces of superhydrophobic plants. The loss of sprayed pesticide droplets leads to the overuse of pesticides and environmental pollution. Therefore, in this study, we aimed at developing a system that was capable of enhancing droplet deposition on the surfaces of superhydrophobic plant leaves via hydrogen bonding between a bio-based surfactant and glycerol at low concentration (0.25%). The system based on the sorbitol-alkylamine surfactant (denoted as SSAS-C-12) with a small amount of glycerol (0.001%) could efficiently inhibit droplet bouncing and splashing on different superhydrophobic/hydrophobic plant leaf surfaces. The results obtained indicated that the addition of glycerol did not change the surface tension, viscosity, contact angles on the plant leaf surfaces, and aggregate morphology of the SSAS-C-12 solutions. Diffusion-ordered nuclear magnetic resonance spectroscopy revealed that glycerol accelerated the diffusion of SSAS-C-12 molecules. More specifically, SSAS-C-12 molecules could diffuse and adsorb on plant leaf surfaces within a short period of time. Other surfactants (denoted as DSSAS-C-12 and BAPO-C-12) with varying numbers of hydroxyl groups were used to verify the enhancement of the deposition on superhydrophobic plant leaf surfaces caused by hydrogen bonding. It was revealed that a decrease in the number of hydroxyl groups in the surfactant molecules led to a decrease in the number of hydrogen bonds between the glycerol and surfactant molecules. Moreover, the diffusion rates of the DSSAS-C-12 and BAPO-C-12 molecules in solution were low, causing the surfactant molecules to not reach the solid-liquid interface in time. Consequently, the droplets containing surfactant molecules (of DSSAS-C-12 or BAPO-C-12) bounced and broke up on the surfaces of plant leaves. Finally, we used molecular dynamics (MD) simulations to explore the energy and molecular distribution of different surfactant-glycerol mixtures. The energy evolution of the SSAS-C-12-glycerol system and the distribution of surfactant molecules relative to the distance from the solid surface in the MD simulations showed that the addition of glycerol twisted the headgroup in SSAS-C-12 via hydrogen bonding with glycerol. In this case, SSAS-C-12 molecules experienced rapid diffusion and adsorption on the solid interface. Therefore, this study not only provided a constructive way to overcome the bouncing behavior of droplets but also prompted us to verify whether all hydrogen bonding interactions among different molecules could display similar control efficiencies through the rational selection of additives.