The efflorescence-deliquescence behavior of saliva droplets and its implication for viability of airborne microorganisms

The aerosol-based transport of viruses and bacteria through the transmission of aerosolized expiratory secretions is one of the main routes for the spreading of infectious diseases such as SARS-CoV-2. A number of studies have confirmed that environmental factors such as temperature and relative humidity can affect the inactivation and transmission of respiratory pathogens. However, there remain significant uncertainties in understanding aerosol micro-physics occurring under different environmental conditions to quantify the survival of microorganisms carried by aerosols. Here we study the size and phase changes of levitated saliva droplets composed of various salts and mucin under well-defined atmospheric conditions. An electrodynamic balance (EDB) is utilized for recording the evaporation and condensation kinetics of single, levitated saliva droplets with a time resolution of seconds. Efflorescence and deliquescence behaviors of droplets are monitored using light scattering and Mie theory. Compared with pure water droplets, a saliva droplet remains stably levitated for hours when the droplet approaches crystallization having reached a final size during evaporation. The morphology of crystallized particles will be imaged using a scanning electron microscope (SEM). The organic-based phase is expected to shield pathogens from inactivation by forming a solid or semisolid shell hindering the diffusion of solutes. This work highlights the importance of accounting for changes in the micro-environment of aerosols undergoing evaporation and condensation in a realistic environment which is needed to study the viability of airborne viruses and other microorganisms.