The Microphysics of Surrogates of Exhaled Aerosols from the Upper Respiratory Tract

Airborne transmission plays a significant role in the transmission of respiratory diseases such as COVID-19, for which the respiratory aerosol droplets are responsible for the transportation of potentially infectious pathogens. However, the aerosol physicochemical dynamics during the exhalation process are not well understood. The representativeness of respiratory droplet surrogates of exhaled aerosol and suspension media for aerosols currently used for laboratory studies remains debated. Here, we compare the evaporation kinetics and equilibrium thermodynamics of surrogate respiratory aerosol droplets including sodium chloride, artificial saliva (AS) and Dulbecco’s modified Eagle’s medium (DMEM) by using the Comparative Kinetics Electrodynamic Balance. The potential influences of droplet composition on aerosol hygroscopic response and phase behaviour, and the influence of mucin are reported. The equilibrium hygroscopicity measurement was used to verify and benchmark the prediction of evaporation kinetics of complex solutions using the Single Aerosol Particle Drying Kinetics and Trajectory model. We show that the compositionally complex culture media which differs from sodium chloride and artificial saliva (mucin-free solutions). The DMEM evaporation dynamics contained three distinctive phases when drying at a range of humidities, including a semi-dissolved phase when evaporating at the environmental humidity range. The effect of mucin on droplet evaporation and phase behaviour at low RH were compared between AS and DMEM solutions. In both cases, mucin delayed the crystallisation time of the droplets, but it promoted phase change (from homogenous to semi-dissolved/spherical with inclusions) to occur at higher water activities.