How long does a droplet that carries Covid-19 virus survive? It depends on factors like the ambient temperature, volume, angle at which it is spread and humidity.

Fluid dynamics plays an important role in the study of the life of these droplets migrating in the air or deposited on surfaces.

Also read: Coronavirus survives less time on cloth, paper than on glass: Study

A study at the Indian Institute of Information Technology (IIT-H) has found that the spread of the virus is minimal after drying of the droplets containing the virus.

Another key takeaway from the study is that droplets survive much longer on the screens of smartphones than on normal glass surfaces.

“The drying time for droplets on a smartphone screen is three times longer than that on a normal glass surface,” the study, conducted by an interdisciplinary research group, said.

Researchers at IIT-H conducted the study to predict the life of the SARS-COV-2 droplet in different environmental conditions.

Respiratory infections such as the SARS-CoV-2 virus spread primarily by respiratory droplets of saliva or discharge from the nose of an infected person during coughing or sneezing.

Saliva droplets consist of salt, protein (mucin), and surfactant (dipalmitoylphosphatidylcholine) in addition to water.

“These ingredients delay the evaporation of respiratory droplets significantly compared to pure water droplets,” the researchers said.

Any increase in humidity will make the droplet take longer than an hour to dry up compared to a droplet under a lower humid environment that will dry-up in minutes.

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“Lower ambient temperature also increases the drying time of the droplet. The angle made by a droplet on the surface (which is known as contact angle) plays an important role in the drying time of the droplet,” the study finds.

The multi-disciplinary study was undertaken by Saravanan Balusamy, Sayak Banerjee (Department of Mechanical and Aerospace Engineering) and Kirti Chandra Sahu (Department of Chemical Engineering).

The findings of the research appeared in the International Communications in Heat and Mass Transfer, a leading journal in this field.

“The time taken by saliva droplets to dry up also depends on the nature of the surface material on which it falls as it dictates how far the droplet spreads out,” Sayak Banerjee said.

“While the lifetime of a small saliva droplet of size 1 nanolitre is less than a minute, a normal size saliva droplet of 10 nanolitres takes more than 15 minutes to evaporate at room temperature and relative humidity of 50 per cent,” Kirti Chandra Sahu, said.

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 temperature rises.

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