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University of California San Diego

After studying the effectiveness of different layers of masks in preventing respiratory droplets from escaping from the mask, a group of international researchers now turn their attention to modeling what happens when droplets come into contact with a wet mask. Their results showed that a moist mask can still effectively prevent these droplets from escaping from the mask and being atomized into smaller, more easily spread aerosol particles.

This study only investigates the impact of wet masks on droplet penetration; the researchers pointed out that if the mask gets wet, people should follow the public health guidelines to replace the mask, because wet masks are more difficult to breathe, and the efficiency of filtering inhaled air is lower. In comparison, the exhaust volume at the edge of the mask is larger.

"Although the effects of various dry masks have been explored, a comprehensive investigation of wet masks is lacking. However, users wear masks for a long time, and during this period, due to respiratory droplets released by breathing, the mask matrix becomes wet, coughing, Sneezing, etc.," wrote a team of engineers from the University of California, San Diego, the Indian Institute of Science, and the University of Toronto. The researchers presented their findings at the 74th APS Fluid Dynamics Division Annual Meeting of the American Physical Society on November 21. The same paper will be published in Physical Review Fluids on December 7.

They found that, perhaps counter-intuitively, wet masks actually make it more difficult for these respiratory droplets to penetrate and escape from the mask, breaking up into smaller atomized particles; Smaller particles stay in the air longer, so they are more likely to spread the SARS-CoV-2 virus. When modeling the physics of this happening, they found that hydrophobic masks (such as general surgical masks) and hydrophilic masks (such as cloth masks) have two distinct mechanisms.

In order to accurately study how humidity affects droplet penetration, the researchers used a syringe pump to generate simulated respiratory droplets that slowly push the liquid through a needle onto one of three mask materials: surgical masks and two cloth masks of different thicknesses . The researchers used a high-speed camera to capture the impact at 4,000 frames per second, recorded what happened when the droplet hit the mask, and continued the study when the mask got wet.

They found that droplets from coughing or sneezing must pass through the mask at a higher rate when wet than when it is dry. On hydrophobic masks with low absorption rate, such as surgical masks, respiratory droplets form small beads on the surface of the mask to provide additional resistance to the affected droplets and prevent possible penetration.

Hydrophilic cloth masks do not show this kind of beading; instead, the cloth absorbs liquid, and as the mask absorbs more volume, the wetted area spreads out. The porous matrix of these cloth masks is filled with liquid, so droplets need to replace a larger volume of liquid to penetrate the mask. Due to this additional resistance, the penetration is weak.

"All in all, we show that wet masks can limit ballistic respiratory droplets better than dry masks," said Sombuddha Bagchi, PhD in mechanical engineering, the first author of the paper. Student at the Jacobs School of Engineering, University of California, San Diego.

"However, we also need to pay attention to the side leakage and breathability of wet masks. We did not study these issues in our research," added Abhishek Saha, professor and co-author of mechanical and aerospace engineering at the University of California, San Diego.

The team of engineers-which also includes Professor Swetaprovo Chaudhuri of the University of Toronto and Saptarshi Basu of the Indian Institute of Science-are proficient in this type of experiment and analysis, even though they are accustomed to studying the aerodynamics and physics of droplets used in propulsion systems , Combustion or thermal spray applications. Last year, when the COVID-19 pandemic began, they turned their attention to the physics of respiratory droplets. Since then, they have been studying the transmission of these respiratory droplets and their role in the spread of COVID-19-type diseases. .

In March 2021, the same team published a paper on Science Advances, detailing the effectiveness of one-, two- and three-layer dry masks in preventing respiratory droplets from penetrating the mask. Using a method similar to the wet mask experiment, they showed that three-layer surgical masks are the most effective in preventing large droplets produced when coughing or sneezing from atomizing into smaller droplets. These large cough droplets can penetrate single-layer and double-layer masks and atomize into smaller droplets. This is particularly important because these smaller aerosol droplets can stay in the air for a longer time. Further explore the three-layer mask is the most effective for large respiratory droplets. More information: the penetration and secondary atomization of droplets affecting wet masks, accepted papers, journals.aps.org/prfluids/acce... 0f4bae7b00a7446193d3 California The University of San Diego provides a quotation: How much droplets do wet masks contain? Research shows that wet masks can still prevent the penetration of respiratory droplets (November 22, 2021) Retrieved on November 26, 2021 from https://phys.org/news/2021-11-masks-droplets-damp-respiratory-droplet .html This document is subject to copyright. Except for any fair transaction for private learning or research purposes, no part may be copied without written permission. The content is for reference only.

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