While most respiratory droplets evaporate in a matter of seconds, experiments have shown COVID-19 can survive on a variety of surfaces for several hours, and sometimes days.

To find out how, researchers used to computer models to simulate the behavior of thin liquid films.

Their simulations -- detailed Tuesday in the journal Physics of Fluids -- revealed the importance of London-van der Waals forces, which cause thin liquid films to cling the surface of different materials and protect COVID-19 particles from desiccation.

"To describe this thin film, we used tools that are otherwise seldom used by researchers within the engineering realm," study co-author Rajneesh Bhardwaj said in a news release

"Specifically, we developed a computational model for the evaporating mass rate of the film as a function of disjoining and Laplace pressures inside the film, using the Hertz-Knudsen law, a well-established kinetic theory of gases," said Bhardwaj, a professor at the Indian Institute of Technology in Bombay.

Bhardwaj and research partner Amit Agrawal, physics professor at IIT Bombay, found the drying time of thin liquid films was influenced by contact angle and surface type.

The discovery helps explain why the survival of COVID-19 particles varies by material, the researchers said.

"Our model for the thin film transport shows that survival or drying time of a thin liquid film on a surface is on the order of hours and days, similar to what has been observed in measurements of ... [the lowest concentration of virus that still infects cells]," said Agrawal.

"It captures the relatively longer survival time on plastic and glass compared to metals," Agrawal added.

Since the beginning, scientists have been working hard to figure out how COVID-19 particles behave in the air and on surfaces, in order to inform public health recommendations aimed at slowing the transmission of the coronavirus.

Previous studies have shown the movement of COVID-19 particles through the air is influenced by air flow and humidity.

For the latest study, researchers studied the survival of virus particles under ambient conditions.

"Our biggest surprise was that the drying time of this nanometric film is on the order of hours," said Bhardwaj. "It suggests the surface isn't completely dry, and the slowly evaporating nanometric film is providing the medium required for the survival of the coronavirus."

Researchers suggest their study's results highlight the importance of disinfecting frequently touched surfaces, like door knobs.

But while the CDC continues to recommend frequent hand washing, the agency and other health organization have claimed surfaces aren't a primary driver of virus transmission.

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