(Click the image to view it in full.) Although face shields block the initial forward movement of the jet, the ejected droplets move relatively easily around the visor and spread over a large area depending on slight environmental disturbances. Photo credit: Florida Atlantic University College of Engineering and Computer Science
The visualization study by the FAU College for Engineering and Computer Science shows why face protection alone does not work.
When the guidelines from the U.S. Centers for Disease Control and Prevention (CDC) aren’t enough to convince you that face shields alone shouldn’t be used to stop the spread of disease COVID-19, then maybe a new visualization study.
To raise public awareness of the effectiveness of face shields alone, as well as face masks with exhalation valves, researchers from the College of Engineering and Computer Science at Florida Atlantic University used qualitative visualizations to test how face shields and valved masks inhibit the spread of aerosol – large droplets. Widespread public use of these alternatives to regular masks could potentially adversely affect mitigation efforts.
For the study, just published in the journal Physics of liquidsResearchers used flow visualization in a laboratory setting using a laser light sheet and a mixture of distilled water and glycerin to create the synthetic mist that was the contents of a cough stream. They envisioned droplets being ejected from a mannequin’s mouth while simulating a cough and sneeze. By placing a plastic face shield and an N95 rated face mask and valve, they were able to map the droplet paths and demonstrate their performance.
To demonstrate the performance of the face shield, in addition to a vertical laser blade, the researchers used a horizontal laser blade that revealed how the droplets cross the horizontal plane. The researchers not only observed the droplets spreading forward, but also found that the droplets also spread in the opposite direction. Photo credit: Florida Atlantic University College of Engineering and Computer Science
The results of the study show that although face shields block the initial forward movement of the jet, the ejected droplets move relatively easily around the visor and spread over a large area depending on slight environmental disturbances. Visualizations for the face mask equipped with an exhalation port show that a large number of droplets flow unfiltered through the exhalation valve, which significantly reduces its effectiveness as a source control means.
“From this latest study, we observed that face shields can block the initial forward movement of the exhaled jet. However, aerosolized droplets ejected with the jet can move around the visor relatively easily,” said Manhar Dhanak Ph.D., department head , Professor and director of SeaTech who co-authored the paper with Siddhartha Verma, Ph.D., lead author and assistant professor; and John Frankenfeld, a technical specialist, all in FAU’s Department of Ocean and Mechanical Engineering. “Over time, these droplets can spread over a wide area both laterally and longitudinally, albeit with decreasing droplet concentration.”
The researchers also tested an N-95 rated face mask with exhalation valves and found that the exhalation port significantly reduced the effectiveness of the mask as a source control means, as large numbers of droplets passed unfiltered and unobstructed through the valve. Photo credit: Florida Atlantic University College of Engineering and Computer Science
To demonstrate the performance of the face shield, in addition to a vertical laser blade, the researchers used a horizontal laser blade that revealed how the droplets cross the horizontal plane. The researchers not only observed the droplets spreading forward, but also found that the droplets also spread in the opposite direction. In particular, face shields impede the forward movement of exhaled droplets to some extent, and valved masks do this to an even lesser extent. However, as soon as they are released into the environment, the aerosol-sized droplets are widely distributed depending on slight environmental disturbances.
Like the N-95 rated face mask used in this study, other types of masks such as: B. Certain fabric-based masks that are commercially available are equipped with one or two exhalation ports located on either side of the face mask. The N95 rated face mask with the exhalation valve used in this study had a small amount of exhaled droplets emerging from the gap between the top of the mask and the bridge of the nose. In addition, the exhalation port significantly reduced the effectiveness of the mask as a source control means, as a large number of droplets passed unfiltered and unhindered through the valve.
“There is an increasing trend for people to replace normal cloth or surgical masks with clear plastic face shields and use masks that have exhalation valves,” Verma said. “A driving factor for this increasing acceptance is better comfort compared to normal masks. Face shields, however, have noticeable gaps in the bottom and sides, and masks with exhalation ports contain a one-way valve that restricts airflow when you breathe in but allows air to escape freely. The inhaled air is filtered through the mask material, but the exhaled breath goes through the valve unfiltered. ”
The researchers say the key takeaways from this latest study are that face shields and masks with exhalation valves may not be as effective as regular face masks in limiting the spread of aerosol droplets. Despite the increased comfort offered by these alternatives, it may be preferable to use well-constructed, high quality cloth or surgical masks with a simple design rather than face shields and masks with exhalation valves. Widespread public acceptance of the alternatives in place of regular masks could potentially adversely affect ongoing COVID-19 mitigation efforts.
“The research conducted by Professors Dhanak and Verma into the importance of appropriate face coverings in stopping the spread of COVID-19 literally lit the world,” said Dr. Stella Batalama, Dean of the FAU College of Engineering and Computer Science. “While the widespread acceptance of the need for face coverings has increased steadily, there is an increasing trend for people to replace normal cloth or surgical masks with clear plastic face shields and with masks equipped with exhalation valves. This latest research provides important evidence to further support the CDC guidelines and inform the public to make better choices in facial coverings for their benefit and public safety. ”
Reference: “Visualization of the droplet distribution for face protection and masks with exhalation valves” by Siddhartha Verma, Manhar Dhanak and John Frankenfield, September 1, 2020, Physics of liquids.
DOI: 10.1063 / 5.0022968
