  The occurrence of concentration and temperature gradients in saline
microdroplets evaporating directly in air makes them unsuitable for nucleation
studies where homogeneous composition is required. This can be addressed by
immersing the droplet in oil under regulated humidity and reducing the volume
to the picoliter range. However, the evaporation dynamics of such a system is
not well understood. In this work, we present evaporation models applicable for
arrays of sessile microdroplets with dissolved solute submerged in a thin layer
of oil. Our model accounts for the variable diffusion distance due to the
presence of the oil film separating the droplet and air, the diffusive
interaction of neighboring droplets, as well as the variation of the solution
density and water activity due to the evolving solute concentration. Our model
shows excellent agreement with experimental data for both pure water and NaCl
solution. With this model, we demonstrate that assuming a constant evaporation
rate and neglecting the diffusive interactions can lead to severe inaccuracies
in the measurement of droplet concentration particularly during nucleation
experiments. Given the significance of droplet evaporation in a wide array of
scientific and industrial applications, the models and insights presented
herein would be of great value to many fields of interest.
