The following article originally appeared on the National Institute of Standards and Technology's website.
Editor's note: The Centers for Disease Control and Prevention recommends wearing face coverings in public settings. Since COVID-19 spreads mainly from person to person through respiratory droplets produced when an infected person coughs, sneezes or talks, a breathable well-fitting face covering may help slow the spread of the virus. Cloth face coverings are not as effective at capturing virus particles as N95 masks or surgical masks, but these should be reserved for healthcare workers.
As a fluid dynamicist and mechanical engineer at the National Institute of Standards and Technology, I've devoted much of my career to helping others see things that are often difficult to detect.
I've shown the complex flow of air that occurs when a dog sniffs. I've helped develop ways to detect drugs and explosives by heating them into a vapor. I've explored how drug residue can contaminate crime labs. I've even shown how to screen shoes for explosives.
Most of these examples fit into a common theme: detecting drugs and explosives through the flow of fluids that are usually invisible. When I'm in the laboratory, I use a number of advanced fluid flow visualization tools to help better understand and improve our ability to detect illicit drugs and explosives on surfaces, on people, and in the environment.
When COVID-19 emerged as a threat to our global community, I pondered how I could use these unique visualization tools to help. These measurement systems excel at showing how air moves around, so it was clear to me that I could use these tools to create qualitative video content that illustrates the importance of wearing a face covering and the pros and cons of various kinds of homemade face coverings in an easily understandable way.
Building the Lab
I was allowed to bring parts of my scientific flow visualization equipment home with me during the quarantine. I have a fairly elaborate wood shop in my home (woodworking is an addiction—I mean, hobby—of mine), and this is where I set up my flow visualization gear for these experiments.
Schlieren imaging is one of the primary tools I use for airflow visualization experiments, and is a true workhorse in my NIST lab, drawing a lot of attention during VIP tours because of the striking visuals it provides. The schlieren technique allows us to see changes in temperature in air.
So, if you place your hand in the test section, you will see the warm air rising from your hand. If you ignite a lighter in the test section, you will see a strong buoyant plume of hot air rising from the small flame. And if you put your face in the test section and cough—you guessed it—you will see the warm air exit your lungs and shoot out of your mouth and nose as an air jet.
Our schlieren system is a sophisticated optical device, utilizing several lenses, optical components and a 45.72 centimeter (18-inch) first-surface concave spherical mirror. These systems can be finicky to assemble and align and almost always require large, heavy laser tables for vibration stability, ease of alignment and structural rigidity.
Obviously, I couldn't bring my 725.74-kilogram (1,600-pound) laser table home with me, so I tried something I’d never done before—build a schlieren system with tripods and wood. After a few days of construction and alignment, it worked! Check out this time-lapse video of my shop being transformed into a home laboratory!
Testing the Coverings
With a fully functional schlieren optical system, a high-speed camera and a six-second commute from home to my "lab," I was ready to begin collecting data on the qualitative effectiveness of various homemade face coverings.
Gail Porter (director of the NIST Public Affairs Office), Jennifer Barrick (NIST Public Affairs Office) and Amy Engelbrecht-Wiggans (NIST Material Measurement Laboratory) provided all the homemade face coverings for this effort. Gail and I would talk often and narrow down the best features of certain face-covering designs, and then she would create new coverings and drop them off on my back deck.
I'm actually pretty good with a sewing machine, but Gail's and Amy's skills are off the charts. I looked at 26 different face coverings, each with a different geometry, fabric or material combination, and tying mechanism. Leon Gerskovic (NIST Public Affairs Office) was my cinematography mentor and guided me through the entire effort.
After weeks of data collection, over 50 GB of video data (looking at yourself coughing over and over gets a little strange after a while), and literally hundreds of fake coughs, we had a clear message—"cover smart, do your part, slow the spread."
We learned that even the simplest face coverings (bandanas, ski neck warmers, etc.) stopped much of your cough from landing on someone else. We also learned that a good seal around the nose, chin, and cheeks helps to prevent your cough from leaking out of the covering. And pulling your face covering below your nose is not good—you would be surprised how much air comes out of your nose when you cough.
Additionally, we found that fabrics with very tight and nonporous weaves actually increase air leaking out by the nose and chin. So, while these tight fabrics may filter droplets at a greater efficiency, they are not breathable and could possibly defeat the purpose of the face covering. Another interesting observation was the impressive reduction in airflow velocity while talking with all the face coverings—a good thing considering that most people out in public should be talking far more than coughing.
My hope is that the video content that was generated by these efforts provides a helpful illustration for why we all should cover up in public spaces and while near others. The good news is that even the most basic face coverings qualitatively appear to reduce the distance the air exiting your lungs during talking and coughing travels.
The bad news is that many people will be watching a video of me coughing over and over again—mildly embarrassing, but I'm getting used to it. I have some plans to continue visualizing face coverings using mannequins and fog droplets, so stay tuned!
About the author: Matthew Staymates is a mechanical engineer and fluid dynamicist at NIST. His research interests focus on improving trace drug and explosives detection systems, along with developing next-generation detection technologies.