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 [1, 2, 3]. This is surprising, since salt concentration increases dramatically as the drop’s water evaporates, making the drop a harsh environment for virions [4]. A plausible hypothesis to explain the slow decay of viral infectivity is that the main solution components (mainly virions, salt and protein [5]) segregate during the evaporation, with virions being transported away from salt deposits, protecting them from the salt’s damaging effects [1]. Understanding where virions reside in a drop residue is essential to disentangle the physico-chemical mechanisms that drive their inactivation [6]. However, determining the virion location in an environment as heterogeneous and complex as a sessile drop is experimentally challenging. Here we use electron microscopy in the drop’s dry remains to show that virions are found mainly forming aggregates in protein-rich regions of the residue, away from salt crystals. Previous works have also attempted to detect the presence of virions in drop residues, with inconclusive results due to experimental limitations [5, 7, 8]. Additionally, neither the physical mechanism behind the observed viral distributions were properly described in previous works. In this work, 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. The theoretical transport model, supported by experiments, allows us to rationalize the spatial distribution of salt, protein, and virions inside evaporating respiratory drops. We anticipate our results to be an essential ingredient to explain the discrepancies between the infectivity decay rates measured in respiratory drops, which nowadays exhibit important discrepancies [9].