How well do wet masks work to contain droplets?

High-speed footage showing what happens when droplets hit an increasingly wet surgical mask; respiratory droplets form small beads on the face of the mask, providing additional resistance to impacted droplets against possible penetration. Credit: Bagchi et al

A study shows that wet masks still prevent the penetration of respiratory droplets.

After studying the effectiveness of different layers of masks in preventing respiratory droplets from escaping face masks, a team of international researchers has now focused on modeling what happens to droplets when they come into contact with wet masks. Their results show that wet masks are still effective in preventing these droplets from escaping the mask and being atomized into smaller, easier to spread aerosol particles.

This study only examined the effects of wet masks on droplet penetration; researchers note that people should follow public health advice to change their mask if it is wet, as wet masks are more difficult to breathe, less effective at filtering inhaled air, and can ventilate more around the edge of the mask than dry masks.

“While the effectiveness of various dry masks has been explored, a full investigation of wet masks is lacking. Yet, users wear masks for long periods of time, and during this time, the mask matrix becomes wet due to the respiratory droplets released by breathing, coughing, sneezing, etc. Wrote the engineering team from the University of California at San Diego, Indian Institute of Sciences and University of Toronto. The researchers presented their findings on Nov. 21 at the American Physical Society’s 74th annual meeting of the APS division of fluid dynamics. The same article will be published in Physical examination fluids December 7.

Experimental setup of the wet mask

Main components of the experimental set-up, not to scale. Credit: Bagchi et al.

They found that, perhaps counterintuitively, wet masks make it harder for these respiratory droplets to penetrate and escape into the mask, breaking into smaller particles and aerosols; research has shown that these smaller particles are more likely to spread the SARS-CoV-2 viruses by staying in the air longer than the large droplets that fall to the ground. By modeling the physics of why this happens, they found that two very different mechanisms are present for hydrophobic masks like common surgical masks, compared to hydrophilic masks like fabric varieties.

To study the exact impact of humidity on droplet penetration, the researchers generated false respiratory droplets using a syringe pump, which slowly pushed the liquid through a needle and onto a of the three types of mask materials: a surgical mask and two fabric masks of different thicknesses. The researchers recorded what happened when the droplets hit the mask using a high-speed camera capturing the impact at 4,000 frames per second, and continued to study it as the mask was getting wet.

They found that droplets from a cough or sneeze must travel at a higher speed to be pushed through a mask when it is wet, compared to when it is dry. On hydrophobic masks with low absorption capacity, such as surgical masks, respiratory droplets form small beads on the face of the mask, providing additional resistance to impacted droplets against possible penetration.

The hydrophilic fabric masks do not have this bead; instead, the fabric absorbs the liquid, with the wetted area expanding as the mask absorbs more volume. The porous matrix of these fabric masks fills with liquid, and the droplets must therefore displace a greater volume of liquid to penetrate the mask. Due to this extra strength, the penetration is lower.

“In summary, we have shown that wet masks are able to restrict respiratory ballistic droplets better than dry masks,” said Sombuddha Bagchi, lead author of the article and a doctoral student in mechanical engineering at the Jacobs School of Engineering of the ‘UC San Diego.

“However, we also need to be careful of side leakage and breathability of wet masks, which were not investigated in our study,” added Abhishek Saha, co-author and professor of mechanical and aerospace engineering at UC. San Diego.

The engineering team, which also includes Professors Swetaprovo Chaudhuri from the University of Toronto and Saptarshi Basu from the Indian Institute of Science, were familiar with this type of experiment and analysis, although they were used to to study the aerodynamics and physics of droplets. for applications such as propulsion systems, combustion or thermal spraying. They turned their attention to respiratory droplet physics last year when the COVID-19[female[feminine pandemic began, and since then have been studying the transport of these respiratory droplets and their roles in the transmission of Covid-19-type diseases.

In March 2021, this same team published an article in Scientists progress detailing the effectiveness of one, two and three layer dry masks in preventing respiratory droplets from entering the mask. Using a methodology similar to this wet mask experiment, they showed that three-layer surgical masks are most effective at preventing large droplets from a cough or sneeze from atomizing into smaller droplets. These large cough droplets can penetrate through single and double layer masks and atomize into much smaller droplets, which is especially crucial as these smaller aerosol droplets can stay in the air for longer. periods.

Reference: “Penetration and secondary atomization of droplets impacted on wet masks” by Sombuddha Bagchi, Saptarshi Basu, Swetaprovo Chaudhuri and Abhishek Saha, November 23, 2021, Physical examination fluids.
DOI: 10.1103 / PhysRevFluids.6.110510


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