Abstract
The SARS-CoV-2 virus-laden drops from exhalation and sneezing have complex physicochemical and physiological properties. The virion typically ranges from 60–140 nm in diameter. The virus-carrying fluid is not pure water. A viral droplet comprises water (about 95%), proteins, mucins (glycoproteins), salts, and surfactants (DPPC, dipalmitoyl phosphatidylcholine). The virus-carrying fluid can impact SARS-CoV-2 infectibility. A flow regime analysis of droplet evaporation shows the gas-phase transport will enter transitional or non-continuum flow regimes. A field volume/phase averaging of two immiscible fluids inside the drops is proposed. Based on a simplified 1-D model, the virus’s diffusion time scale is two to four orders of magnitude longer than the liquid water’s self-diffusion time scale for low virus loadings. With only water evaporating from the drops, the virus number density, species concentrations, and pH value change significantly. When the droplet reaches 96% of its lifetime, the molar concentration will increase to 125 times its initial value. An initial pH value of 6.45 will decrease to 4.35 — a more acidic condition favoring the virus infectivity. For future studies, we recommend that droplet lifetime analysis consider the coupled gas-phase transport (including the continuum effects) with the liquid-phase transport (including non-Fickian diffusion in micro/nanoscale).
