Evaporating droplets on inclined plant leaves and synthetic surfaces: experiments and mathematical models

TL;DR

This study investigates the evaporation behavior of surfactant droplets on plant leaves and synthetic surfaces, demonstrating that standard models generally apply except in specific high-concentration cases, with experiments and mathematical modeling supporting the findings.

Contribution

It provides experimental data and mathematical models for droplet evaporation on inclined surfaces, highlighting conditions where standard models are valid or need modification.

Findings

Most evaporation follows standard modes and models

High surfactant concentration on wheat causes nonstandard evaporation

Mathematical models accurately describe most experimental results

Abstract

Hypothesis: Evaporation of surfactant droplets on leaves is complicated due to the complex physical and chemical properties of the leaf surfaces. However, for certain leaf surfaces for which the evaporation process appears to follow the standard constant-contact-radius or constant-contact-angle modes, it should be possible to mimic the droplet evaporation with both a well-chosen synthetic surface and a relatively simple mathematical model. Experiments: Surfactant droplet evaporation experiments were performed on two commercial crop species, wheat and capsicum, along with two synthetic surfaces, up to a $90\,^{\circ}$ incline. The time-dependence of the droplets' contact angles, height, volume and contact radius was measured throughout the evaporation experiments. Mathematical models were developed to simulate the experiments. Findings: With one clear exception, for all combinations of surfaces, surfactant concentrations and angles, the experiments appear to follow the standard evaporation modes and are well described by the mathematical models (modified Popov and Young-Laplace-Popov). The exception is wheat with a high surfactant concentration, for which droplet evaporation appears nonstandard and deviates from the diffusion limited models, perhaps due to additional mechanisms such as the adsorption of surfactant, stomatal density or an elongated shape in the direction of the grooves in the wheat surface.