Stability of Respiratory-Like Droplets under Evaporation

TL;DR

This study investigates how respiratory-like droplets evaporate under different humidity conditions, revealing that salt content influences evaporation rates and potentially affects pathogen viability.

Contribution

It provides new insights into the physico-chemical mechanisms affecting droplet evaporation, especially the role of salt concentration at various humidities.

Findings

Salt alters evaporation behavior compared to pure water.

Water evaporation is inhibited at high humidity due to salt hygroscopicity.

Maximum salt concentration occurs around 75% relative humidity.

Abstract

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.