Images from the high-speed footage show what happens when droplets hit a surgical mask, as the respiratory droplets form small beads on the mask's surface, providing additional resistance for the impacted droplets against possible penetration. Bagchi et al have credit.
After studying the effectiveness of different layers of masks in stopping respiratory droplets from escaping face masks, a team of international researchers has now turned their attention to modeling what happens to droplets when they come in contact with wet masks. The results show that damp masks are still effective at stopping the droplets from escaping and being atomized into smaller, easier-to-spread aerosolized particles.
The study only looked at the effects of wet masks on droplet penetration, so people should change their masks if they are wet.
A comprehensive investigation of wet masks is lacking, while the efficacies of various dry face masks have been explored. The team of engineers from the University of California San Diego, Indian Institute of Science, and University of Toronto wrote that the mask matrix becomes wet when users wear masks for long periods of time. The researchers presented their findings at the American Physical Society's 74th Annual Meeting. The paper will be published in December.
They found that wet masks make it more difficult for the respiratory droplets to get out of the mask, splintering into smaller, aerosolized particles, which can spread the disease. They discovered that there are two very different mechanisms for the different types of masks.
The researchers created mock respiratory droplets using a needle and a pump, which slowly pushed liquid through a needle and onto one of three types of mask materials. The researchers used a high-speed camera to record the impact of the droplets on the mask, and continued to study it as the mask became damp.
The primary components of the setup are not scaled. Bagchi et at.
When it's dry, the droplets from a cough or sneeze have to be pushed through a mask at a higher rate than when it's wet. The respiratory droplets form small beads on the mask's surface, providing additional resistance against possible penetration.
The cloth masks absorb the liquid with the wetted area spreading as the mask absorbs more volume. The porous matrix of these cloth masks becomes filled with liquid, and the droplets are required to move a larger volume of liquid to penetrate the mask. The penetration is weaker due to this additional resistance.
"We showed that wet masks are better at limiting respiratory droplets than dry masks," said Sombuddha Bagchi, the first author of the paper and a mechanical engineering PhD student at the Jacobs School of Engineering at UC San Diego.
"However, we also need to pay attention to side leakage and breathability of wet masks, which were not investigated in our study", said Abhishek Saha, a co-author and professor of Mechanical and Aerospace Engineering at UC San Diego.
The team of engineers, which also includes Professors Swetaprovo Chaudhuri from University of Toronto, and Saptarshi Basu of the Indian Institute of Science, were well-versed in this type of experiment and analysis, though they were used to studying the aerodynamics and physics of droplets They have been studying the transport of respiratory droplets and their role in the transmission of COVID-19 type diseases since last year.
The team published a paper in Science Advances detailing the effectiveness of dry masks of one, two, and three layers. Three-layer surgical masks are the most effective at stopping large droplets from a cough or sneeze from getting atomized into smaller droplets, according to the results of a wet mask experiment. The large cough droplets can penetrate through the single- and double-layer masks and atomize to much smaller droplets, which is crucial since these smaller aerosol droplets are able to linger in the air for longer periods of time.
Penetration and secondary atomization of droplets impacted on wet facemasks has been reported.
How well do wet masks hold droplets? A study shows that damp masks still stop respiratory droplets.
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