Viral transport in evaporating sessile model respiratory droplets

Viral particles, or virions, remain infectious in the dry residue of respiratory droplets for times as long as hours. This is counterintuitive, since salt concentration increases dramatically as the drop’s water evaporates, making the drop a harsh environment for virions. It has been hypothesized that the drop components (mainly salt and the glycoprotein mucin) segregate during the evaporation process, with virions being transported away from salt deposits. This would protect them from the salt’s damaging effects. Thus, understanding where virions reside in a drop residue is essential to disentangle the physico-chemical mechanisms that drive their inactivation. However, determining the virion location in an environment as heterogeneous and complex as a respiratory drop is challenging. Here we show by electron microscopy that virions in the drop’s dry residue are found mainly forming aggregates in protein-rich regions, away from salt crystals. We complement our experimental observations with a theoretical description of the flow inside the drop, which allows to elucidate the transport mechanisms yielding the observed viral spatial distribution. We anticipate our results to be relevant to explain the discrepancies in the infectivity decay rates measured in respiratory drops between previous reported studies.