Chapter 11: Evaporation and Precipitation Dynamics of a Respiratory Droplet

Respiratory droplets are the primary mode of transmission for several diseases, including COVID-19. These droplets ejected through the exhalation process during coughing, sneezing, and speech consist of a complex mixture of volatile and non-volatile substances. While transmitted and translated in air, these complex liquid droplets undergo a series of coupled thermophysical processes. The distance these droplets can travel and the number of active pathogens they carry depend on the residue's droplet lifetime and morphology. Thus, the evaporation and precipitation processes in these are critical in assessing the potential threat they possess in the possible transmission of this disease. In this chapter, we summarize synergistic experimental and modeling approaches through which a critical insight into the dynamics of the airborne surrogate respiratory droplets can be obtained. In the experimental section, we propose acoustic levitation as a suitable tool to study the respiratory droplet without any substrate or container, which affects the drying characteristics for commonly studied sessile droplets. The experimental results also become a benchmark for the mathematical model presented in the second part of the chapter. The mathematical description of the various coupled subprocesses is identified and subsequently solved. The experimental and modeling results highlight some of the critical features of these respiratory droplets.