Stability of Respiratory-Like Droplets under Evaporation*

Enveloped viruses contained in airborne respiratory droplets have been seen to lose infectability fastest at intermediate ambient relative humidities H_r. However, the precise physico-chemical mechanisms that generate such least-favorable conditions for the virus are not fully understood yet. Analyzing the evaporation dynamics of  respiratory-like droplets in air, we reveal that at high H_r, the salt dissolved in respiratory drops inhibits their evaporation indefinitely. Conversely, at low H_r the drop evaporates leaving a porous solid residue inside which virions may remain dormant for long times. The optimal H_r for the fastest reduction of the infectability lies in between, in line with the empirical evidence for the corona virus transmission probability. Finally, we use electron microscopy to examine the structure of the solid residue left upon the drop evaporates completely.