Abstract
To control the evolution of a pandemic such as COVID-19, knowing the conditions under which the pathogen is being transmitted represents a critical issue, especially when implementing protection strategies such as social distancing and wearing face masks. For viruses and bacteria that spread via airborne and/or droplet pathways, this requires understanding how saliva droplets evolve over time after their expulsion by speaking or coughing. Within this context, the transition from saliva droplets to solid residues, due to water evaporation, is studied here both experimentally, considering the saliva from 5 men and 5 women, and via numerical modeling to accurately predict the dynamics of this process. The model assumes saliva to be a binary water/salt mixture and is validated against experimental results using saliva droplets that are suspended in an ultrasound levitator. We demonstrate that droplets with an initial diameter smaller than 21 μm will produce a solid residue that would be considered an aerosol of <5 μm diameter in less than 2 s (for any relative humidity less than 80% and/or any temperature greater than 20°C). Finally, the model developed here accounts for the influence of the saliva composition, relative humidity and ambient temperature on droplet drying. Thus, the travel distance prior to becoming a solid residue can be deduced. We found that saliva droplets of initial size below 80 μm, which corresponds to the vast majority of speech and cough droplets, will become solid residues prior to touching the ground when expelled from a height of 160 cm.
Highlights
To control the evolution of a pandemic such as COVID-19, knowing the conditions under which the pathogen is being transmitted represents a critical issue, especially when implementing protection strategies such as social distancing and wearing face masks
Water evaporation from saliva droplets must be considered as it reduces the droplet size over time, and affects its transport properties
The theoretical model developed for the evaporation of water/NaCl droplets is intended to address most of the effects listed above
Summary
Measurements on saliva droplets expelled by coughing or sneezing usually consist of imaging the whole flow field using high-speed cameras (Bourouiba et al, 2014; Gupta et al, 2009) This approach allows the velocity and the number of expelled droplets to be obtained, as well as visualize their displacement over time (Roth et al, 2021). This dif ferential pressure tends to flatten the droplet, which takes an oblate spheroid shape This deformation is apparent for millimetric size drop lets, presented, but it is less pronounced for diameters in the range of a few hundred μm, as shown in the top row images from Fig. 1C. As the particle size shrinks, the acoustic pressure gradient is insufficient to block the movement of the particle in the radial direction or to prevent it from rotating
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