  Spreading of respiratory diseases, such as COVID-19, from contaminated
surfaces is dependent on the drying time of the deposited droplets containing
the virus. The evaporation rate depends on environmental conditions, such as
ambient temperature and relative humidity and physical properties (e.g.,
droplet volume, contact angle and composition). The respiratory droplets
contain salt (NaCl), protein (mucin), and surfactant
(dipalmitoylphosphatidylcholine) in addition to water, which are expected to
influence the evaporation in a big way. A diffusion-based theoretical model for
estimating the drying time is developed which takes into account the dynamic
contact angle of saliva droplets laden with salt and insoluble surfactants. The
effect of the initial volume, contact angle, salinity, surfactant
concentration, ambient temperature and relative humidity on the drying time of
droplets is investigated.
