Insights on drying and precipitation dynamics of respiratory droplets from the perspective of COVID-19

Abstract

We isolate a nano-colloidal droplet of surrogate mucosalivary fluid to gain fundamental insights into airborne nuclei’s infectivity and viral load distribution during the COVID-19 pandemic. The salt-water solution containing particles at reported viral loads is acoustically trapped in a contactless environment to emulate the drying, flow, and precipitation dynamics of real airborne droplets. Similar experiments validate observations with the surrogate fluid with samples of human saliva samples from a healthy subject. A unique feature emerges regarding the final crystallite dimension; it is always 20%–30% of the initial droplet diameter for different sizes and ambient conditions. Airborne-precipitates nearly enclose the viral load within its bulk while the substrate precipitates exhibit a high percentage (∼80–90%) of exposed virions (depending on the surface). This work demonstrates the leveraging of an inert nano-colloidal system to gain insights into an equivalent biological system.

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

“…Several researchers (e.g., Refs. 76 and 77 ) studied the effect of evaporation of organic solvents and saltwater droplets on surfaces, while a few others (e.g., Ref. 78 ) studied the evaporation of saliva droplets on surfaces in context with COVID-19.…”

“…It is observed that the acoustic streaming causes an increase in evaporation rate that is comparable to forced convection at a relative velocity of around 0.1 m/s. Basu et al 77 studied the drying and precipitation characteristics of the saltwater droplets in a similar levitator using a shadowgraph technique. He et al 79 used an inverted microscope and a CMOS camera in their experiments and found that during the evaporation, a droplet of size ranging from 5 to 100 μ m shrinks to a few micrometers in size (referred to as residues).…”

How coronavirus survives for hours in aerosols

“…32 Even bigger-size droplets may undergo evaporation before landing on a surface/ground and a small droplet-nuclei may be formed, which remains suspended in air, and the virus may still survive therein. 33 Notably, previous virus titer measurements [dose 50% tissue-culture infectious dose (TCID 50 ) per milliliter] 34 disclosed that the coronavirus can sustain for hours in aerosols. It is, therefore, clear that the six-feet social distancing norm alone is not sufficient to curb disease spread via the airborne route.…”

Airborne virus transmission via respiratory droplets: Effects of droplet evaporation and sedimentation

“…Airborne transmission of virus-containing saliva droplets produced by speaking, coughing, or sneezing is one of the well-known [ 1 , 2 ] mechanisms that play a crucial role in the spread of numerous infectious diseases, such as influenza [ 3 , 4 ] and SARS-CoV-2 [ [5] , [6] , [7] , [8] ]. When a saliva droplet evaporates to a so-called droplet nucleus, which is a small particle with much reduced water content [ 9 ], it can remain suspended in air for a long time. According to current WHO guidelines, the term droplet nucleus refers to droplets with radii smaller than [ 10 ].…”