How Well Do Damp Masks Work To Contain Droplets?

Images of the high-speed images showing what happens when droplets hit an increasingly moist surgical mask; the respiratory droplets form small spheres on the surface of the mask, providing additional resistance to the droplets struck by possible penetration. Credit: Bagchi et al

Research shows that moist masks still stop the penetration of respiratory droplets.

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

This study only examined the effects of wet masks on drip penetration; the researchers note that people should follow public health guidelines for changing their mask when wet, as wet masks are harder to breathe through, are less efficient at filtering inhaled air, and can hang more around the rim of the mask. ventilate than dry masks.

“While the efficacy of various dry face masks has been studied, a comprehensive study of wet masks is lacking. Still, users wear masks for a long time and during this time the mask matrix gets wet due to respiratory droplets released when breathing, coughing, sneezing, etc.”, wrote the team of engineers from the University of California San Diego, Indian Institute of Science and the University of Toronto The researchers presented their findings Nov. 21 at the 74th annual meeting of the American Physical Society’s APS Division of Fluid Dynamics. The same paper will be published Dec. 7 in Physical Review Fluids.

Primary components of the experimental setup, not to scale. Credit: Bagchi et op.

They found that, perhaps counterintuitively, wet masks actually make it more difficult for these respiratory droplets to penetrate and escape the mask, shattering into smaller, atomized particles; research has shown that these smaller particles are more likely to spread the SARS-CoV-2 virus by lingering in the air longer than the larger droplets that fall to the ground. In modeling the physics behind why this happens, they found that there are two very different mechanisms in place for hydrophobic masks like regular surgical masks, versus hydrophilic masks like the cloth variants.

To study exactly how wetness affects droplet penetration, the researchers generated fake respiratory droplets using a syringe pump, which slowly pushed fluid through a needle and onto one of three types of mask materials: a surgical mask and two cloth masks of different thicknesses. The researchers recorded what happened when the droplets hit the mask using a high-speed camera that captured the impact at 4,000 frames per second, and continued to study as the mask became moist.

They found that droplets from coughing or sneezing have to travel at a higher speed to be pushed through a mask when wet, compared to when it’s dry. On low absorbency hydrophobic masks, such as surgical masks, the respiratory droplets form small beads on the surface of the mask, providing additional resistance to the droplets struck against potential penetration.

The hydrophilic cloth masks do not show these beads; instead, the cloth absorbs the liquid, with the wetted area expanding as the mask absorbs more volume. The porous matrix of these cloth masks is filled with liquid and therefore the droplets must displace a greater volume of liquid to penetrate the mask. Due to this extra resistance, the penetration is weaker.

“In summary, we showed that wet masks are better at limiting ballistic respiratory droplets than dry masks,” said Sombuddha Bagchi, lead author of the paper and a PhD student in mechanical engineering at the Jacobs School of Engineering at UC San Diego.

“However, we also need to pay attention to lateral leakage and breathability of wet masks, which were not examined in our study,” added Abhishek Saha, a co-author and professor of Mechanical and Aerospace Engineering at UC San Diego.

The team of engineers – which also included Professors Swetaprovo Chaudhuri of the University of Toronto and Saptarshi Basu of the Indian Institute of Science – were well versed in this kind of experimentation and analysis, although they were used to studying the aerodynamics and physics of droplets for applications such as propulsion systems, combustion or thermal sprays. They turned their attention to the physics of respiratory droplets last year when the COVID-19 pandemic began, and have been studying the transport of these respiratory droplets and their role in the transmission of Covid-19-type diseases ever since.

In March 2021, the same team published a paper in Science Advances detailing the effectiveness of one-, two-, and three-layer dry masks in preventing respiratory droplets from entering the mask. Using a similar method to this wet mask experiment, they showed that three-layer surgical masks are most effective at preventing large droplets from coughing or sneezing from being nebulized into smaller droplets. These large cough drops can penetrate the single- and double-layer masks and atomize into much smaller droplets, which is especially crucial as these smaller aerosol droplets can remain suspended in the air for a longer period of time.

Reference: “Penetration and Secondary Atomization of Drops Affected on Wet Face Masks” by Sombuddha Bagchi, Saptarshi Basu, Swetaprovo Chaudhuri, and Abhishek Saha, November 23, 2021, Physical Review Fluids.
DOI: 10.1103 / PhysRevFluids.6.110510