‘Porous surfaces like paper and cloth curb Covid spread better’

MUMBAI: Surfaces such as paper and cloth, which expedite evaporation of respiratory droplets, can curtail the spread of Covid-19, according to a recent study by a team of researchers from the Indian Institute of Technology-bombay (IIT-B).

The researchers, all from the department of mechanical engineering, found Sars-cov-2, the virus that causes Covid-19,can survive for four days on glass and seven days on plastic as well as stainless steel. In comparison, the virus survived for only three hours on paper and two days on cloth. Their paper was published in Physics of Fluid, a peer-reviewed letter by AIP Publishing, on Tuesday.

Covid-19 can be transmitted via droplets that settle on surfaces of objects, known as fomite transmission. “This study was to understand the evaporation of respiratory droplets on porous surfaces, which are responsible for fomite transmission of the virus,” said co-author Rajneesh Bhardwaj, associate professor of mechanical engineering, IIT-B.

In a previous study, the same researchers had shown that Sars-cov-2’s survival period correlated to the drying time of a respiratory droplet on an impermeable (non-porous) surface along with a residual film left on it by the droplet. This follow-up study demonstrated how the mass loss of respiratory droplet and evaporation mechanism of a thin liquid film are different for porous and non-porous surfaces.

“The researchers from IIT-B have worked out a good theory using simple models. While it is known that droplets evaporate faster on porous surfaces due to capillary action, this is the first time it has been put in context of spread of a virus. This lays foundation for further systematic study to understand the spread on different surfaces,” said Amitabh Bhattacharya, associate professor, applied mechanics department, Iit-delhi, who was not part of the study.

Based on their findings, the researchers have made recommendations to curb the spread of the virus. “We recommend that furniture in hospitals and offices, made of impermeable material, such as glass, stainless steel, or laminated wood, be covered with porous material, such as cloth, to reduce the risk of infection upon touch,” said co-author Sanghamitro Chatterjee, a postdoctoral researcher. Another co-author, Janani Srree Murallidharan, assistant professor, IIT-B, said, “Other recommendations include covering seats in public places, such as parks, shopping malls, restaurants, and waiting halls, with cloth.”

For the study, the team used proxy droplets to visualise evaporation on porous and impermeable surfaces, and compared the measurements with computerbased models. The researchers found that 99.9% of a droplet’s liquid content evaporated within the first few minutes on both impermeable and porous surfaces. Then, a microscopic thin residual liquid film remained on the exposed solid parts where the virus could survive.

“The film left on the surface of the porous material evaporates faster, therefore, harbouring the virus for lesser time,” said Amit Agrawal, co-author and institute chair professor at department of mechanical engineering.