  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.

Fluid Dynamics