In the search for the best face mask to shield you against droplets, there is a simple test according to a new proof-of-concept devised by scientists.
A team from Duke University has put up a simple, low-cost laser setup. They then conducted a simple experiment across fourteen different kinds of face masks. The details of their study, as well as its results, are published in the journal, Science Advances.
Where are the droplets?
To observe where the droplets went, the researchers used a box with a laser, a lens, and a cell phone camera to capture the dissipation of the particles. The lens expands the laser beam vertically, passing through slits in the enclosure. Water particles are sprayed from a bottle and into the box.
For the samples, it included cotton masks, a surgical face mask, a valved N95 mask, as well as a bandana and a fleece. The test with no mask whatsoever was the control for the experiment.
While the green laser emitter used in the study was a two-watt, 532-nanometer, scientific pump laser, researchers said that suitable green lasers of similar powers are available for less than US$100 can be used. Although scientific lasers have better specifications such as higher beam pointing, intensity stability, and better beam profile, researchers noted that these "advantages" are irrelevant in the application.
"The fundamental question is, how well does a specific mask type prevent droplets from spreading," said Martin Fischer, lead researcher of the study and an imaging specialist.
The Best Choices, and the Worst
In the results, fitted N95 face masks were able to block the most water droplets, which simulates small droplets ejected when a person speaks. It is followed by surgical masks, then polypropylene masks.
The rest of the masks tested also showed some capability to block droplets, but not as good as those mentioned before. Also, the N95 face mask with valves was not able to prevent as much as its valve-less counterpart, mainly because of the installed exhaust.
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Bandanas were among the worst choices for protection against droplets in the tested materials. Meanwhile, according to the Duke University experiment, wearing a fleece is actually worse than wearing no protection at all. Researchers believe that the materials in the fleece broke large water droplets into smaller ones.
The experiment design was relatively easy to set up, with the required materials for the testing being ubiquitous or easily acquired. However, the researchers noted that this same simplicity comes with several limitations, as well as their recommendations for overcoming them.
Their choice of mask test setup was only for demonstration purposes. A number of other factors—like physiological difference, mask fit, the position of the head, and even speech pattern—could affect how the droplets are expelled and blocked.
Another limitation is that the use of a cell phone camera can only do so much. While there is no specific value for the camera sensitivity in relation to droplet sizes, the researchers estimated the light scatter through the use of the Lorenz-Mie theory (LMT). The LMT is a function to express scattered fields of lights through an infinite series of partial waves.
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Watch the Duke University experiment on face masks below:
