Lifetime of sessile saliva droplets in the context of SARS-CoV-2

Abstract

Spreading of respiratory diseases, such as COVID-19, from contaminated surfaces is dependent on the drying time of the deposited droplets containing the virus. The evaporation rate depends on environmental conditions, such as ambient temperature and relative humidity and physical properties (e.g., droplet volume, contact angle and composition). The respiratory droplets contain salt (NaCl), protein (mucin), and surfactant (dipalmitoylphosphatidylcholine) in addition to water, which are expected to influence the evaporation in a big way. A diffusion-based theoretical model for estimating the drying time is developed which takes into account the dynamic contact angle of saliva droplets laden with salt and insoluble surfactants. The effect of the initial volume, contact angle, salinity, surfactant concentration, ambient temperature and relative humidity on the drying time of droplets is investigated.

Fluid dynamics of respiratory droplets in the context of COVID-19: Airborne and surfaceborne transmissions

“…A few notable contributions to the prediction of the lifetime of respiratory droplets can be found in Refs. 63 , 73 , and 74 . A computational model based on kinetic theory was used to investigate the drying time and time-varying thickness of a liquid film over solid surfaces.…”

“… 73 A diffusion-based theoretical model was used in Refs. 63 and 74 , to estimate the drying time. Unlike, the pinned droplet with a constant contact angle as assumed by Bhardwaj and Agrawal, 63 Balusamy et al 74 considered the dynamic contact angle of saliva droplets laden with salt and insoluble surfactants (see Fig.…”

“…In Ref. 74 , the molality ( M ) of the saliva is fixed at 0.154 mol/kg. represents the diffusion coefficient of water vapor ( ), which is given by 63 The saturation vapor density, C sat (in kg/m 3 ), is calculated using Raoult's law for water-salt-surfactant mixtures, wherein the saturation vapor density of pure water, and X w denotes the mole fraction of water in the solution.…”

“…III A 1 ) and studied the effect of various surfaces owing to different contact angles, ambient temperature, and relative humidity. The effect of the composition of saliva droplets, [i.e., the presence of salt, protein (mucin), and surfactant (dipalmitoylphosphatidylcholine)] on the evaporation rate and the residue formation was also investigated by Balusamy et al 74 It was found that a droplet can survive longer on hydrophobic surfaces as compared to a hydrophilic surface and that its lifetime increases as the contact angle increases. Also increasing the concentration of the ingredients (salt, surfactant, protein, etc.)…”

“…Next, we present how environmental conditions, such as ambient temperature and humidity, influence the lifetime of sessile respiratory droplets. Balusamy et al 74 presented the regime maps depicting the lifetime of a typical saliva droplet ( nl, M = 0.154 mol/kg and ) on surfaces with contact angles and (see Fig. 17 ).…”