Comparing the Evaporation of Sessile and Free Surrogate Respiratory Fluid Droplets: Kinetics, Dried Particle Morphology, and Residue Distribution

The interplay between evaporation and conditioning environment (i.e. sessile or free) in determining the dispersal of non-volatile components, such as pathogens, in respiratory droplets is essential for accurate pathogen viability and surface deposition modelling relevant to respiratory disease transmission. This study provides direct comparison of the evaporation dynamics and dried morphology of near-uniform picolitre droplets, containing a range of surrogate respiratory fluid formulations, in both sessile and free environments, under typical ambient conditions. Free droplets typically follow the classical R-squared evaporation law. In contrast, sessile droplet evaporation on hydrophobic substrates is strongly composition-dependent. Aqueous-inorganic solutions evaporate with a receding contact line, whereas formulations containing artificial saliva exhibit stronger surface adhesion, leading to a pinned contact line. Artificial saliva sessile droplets leave circular residues, while free droplets form distorted-spherical agglomerates. The evaporation kinetics of aqueous-inorganic sessile droplets agree with an isothermal, gas-diffusion-limited model, extending the compositional-complexity over which this framework has been validated for picolitre-droplets. In comparison, free droplet evaporation modelling requires accurate coupling of heat and mass transfer. Both types crystallise at solute concentrations above the bulk solubility limit. Sessile droplets show greater variability in crystallisation time, concentration and dried-particle morphology, indicating a heterogeneous crystallisation component, despite the hydrophobic surface.