In the same environmental conditions, a saliva droplet on our smartphone screen takes three times more time to dry than that on a normal glass surface, IIT Hyderabad researchers have found.
The disease spread is minimal after droplets containing the virus dry, the interdisciplinary study adds.
At a time when the Covid-19 cases are rising again in the country, an IITIITH study has predicted the lifetime of SARS-COV-2 droplets in different conditions like ambient temperature, relative humidity, and volume and composition of droplets.
“Normal glass surfaces are hydrophilic, allowing the saliva droplet spread and thereby making its evaporation speedier. Whereas the mobile screens, screen guards are hydrophobic, repelling the wet droplets and thus allowing them to sit there for long,” Prof Kirti Chandra Sahu of the Department of Chemical Engineering told DH.
Infections like Covid-19 spread primarily through respiratory droplets (of size>10 μm) or discharge from the nose of an infected person during coughing/sneezing.
“While the lifetime of a small saliva droplet of size one nanolitre is less than one minute, a normal size saliva droplet of 10 nano-litres takes more than 15 minutes to evaporate at room temperature and relative humidity of 50 per cent. For high relative humidity, say of greater than 90 per cent, the droplet stays live on the surface for significantly longer, about an hour,” Prof Sahu said.
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“Of course, viruses could still be alive in the dried droplets, which requires further research involving biologists,” the researcher adds.
The IITH study focused on various factors affecting the drying time of saliva droplets, “whereas earlier studies were based on water droplets.”
Saliva droplets consist of salt, protein (mucin), and surfactant (dipalmitoylphosphatidylcholine) in addition to water.
Through a theoretical and numerical study, Prof Sahu along with Dr Saravanan Balusamy, Dr Sayak Banerjee of the Department of Mechanical and Aerospace Engineering showed that these ingredients delay the evaporation of respiratory droplets significantly as compared to pure water droplets. Their work is published in International Communications in Heat and Mass Transfer, a leading journal in this field.
Some of the key findings are:
An increase in humidity makes the droplets take longer than an hour compared to a droplet under a lower humid environment that will dry-up in minutes.
Lower ambient temperature also increases the droplet's drying time.
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The angle made by a droplet on the surface (known as “contact angle”) plays an important role in its drying time and hence highly hydrophilic surfaces may be less susceptible to prolonged contamination.
The longest drying times are observed with the combination of low ambient temperature and high relative humidity, while the drying time progressively reduces as the humidity falls and the temperature rises.
Researchers also found that for a fixed initial volume of the droplet, increasing the initial contact angle increases the drying time.
Fluid dynamics plays an important role in the study of these droplets migrating in the air or deposited on surfaces.
IITH study underlines the importance of proper sanitization of the contact prone surfaces and air-conditioned indoor areas, and advocates wearing face masks and maintaining social distance as the best measures to prevent air transmission of Covid-19.