  Saliva is primarily composed of water, but additionally includes a variety of
organic and inorganic substances such as salt, proteins, peptides, mucins,
virions, etc. The presence of such solutes affects the evaporation time of
respiratory droplets that are sedimenting in air, and thereby the airborne
transmission of infections. From solutions of the coupled heat-conduction and
water-diffusion equations within the droplet and in the ambient vapor phase, we
find that the solute-induced water vapor-pressure reduction considerably slows
down the evaporation process and dominates the solute-concentration dependence
of the droplet evaporation time. The evaporation-induced solute-concentration
gradient near the droplet surface, which is accounted for using a two-stage
evaporation model, is found to further intensify the slowing down of the drying
process. On the other hand, the presence of solutes is found to reduce
evaporation cooling of the droplet, which causes a slight decrease in the
evaporation time. Overall, the first two effects are dominant, meaning that the
droplet evaporation time increases in the presence of solutes. The
solute-concentration dependence of the water diffusivity inside the droplet
does not significantly change the evaporation time. Finally, crust formation on
the droplet surface is found to increase the final equilibrium size of the
droplet.
