  Pathogens contained in airborne respiratory droplets have been seen to remain
infectious for periods of time that depend on the ambient temperature and
humidity. In particular, regarding the humidity, the empirically least
favorable conditions for the survival of viral pathogens are found at
intermediate humidities. However, the precise physico-chemical mechanisms that
generate such least-favorable conditions are not understood yet. In this work,
we analyze the evaporation dynamics of respiratory-like droplets in air,
semi-levitating them on superhydrophobic substrates with minimal solid-liquid
contact area. Our results reveal that, compared to pure water droplets, the
salt dissolved in the droplets can significantly change the evaporation
behaviour, especially for high humidities close to and above the deliquesence
limit. Due to the hygroscopic properties of salt, water evaporation is
inhibited once the salt concentration reaches a critical value that depends on
the relative humidity. The salt concentration in a stable droplet reaches its
maximum at around 75% relative humidity, generating conditions that might
shorten the time in which pathogens remain infectious.
