As a fluid dynamicist and mechanical engineer at the National Institute of Standards and Technology (NIST), 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.
I was allowed to bring parts of my scientific flow visualization equipment home with me during the quarantine. I have a fairly elaborate woodshop 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!
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!
Check out this post by Heather Evans about NIST's other efforts to help fight the coronavirus.
Thank you for this. It's very reassuring.
This is awesome! What a fun and informative project. For the home sewers, would Gail and Amy be willing to provide links to the tested mask designs (assuming they are among the many on the web)?
NIST does not endorse any particular face covering pattern. There are many good patterns available for free on the web and I have personally made at least 8 different kinds from these tutorials. This video show qualitative air flow differences among different face covering designs and materials, not which are the best available. We recommend that you try several different tutorials. Chose the one that best matches your sewing skills and the materials you have available. Then tweak it if necessary to get, as Matt says above a breathable, multi-layer, comfortably snug fit. Happy sewing!
Can you share information after you've found your favorite tutorial or pattern? Thank you for asking this question.
Free patterns for the fitted mask can be found here in childrens, womens, and mens sizes: https://www.craftpassion.com/face-mask-sewing-pattern/
Video on how to sew face masks can be found here, with the fitted design starting at around 24:30: https://www.youtube.com/watch?v=M3Dm1NKoqfA
This is some amazing science.
It's a Particulate Mask, Matt. They don't stop virion - either direction.... May catch millions of virion in droplets, but also allows millions to pass through! Simply not worth quantifying? The Particulate Mask is simply a catch-all for teaching others to do something against one's will out of fear. So far so good! I find people are afraid of a cough, cautious with designated distance - emotionally angry when someone does cough and physically and verbally violent when they're in the same area. Interesting that the medical community indicates that children two years and under should not wear a mask at all. Oxygen levels are important at that age. What of an adult with cardiac or pulmonary issues - regardless of age? So, Okay..... Home bound they are, but for what and why? A virus? Or an election year?! Sorry, Matt. Great effort though!
This NIST project does not show the transmission of viruses. And homemade face coverings are clearly not as effective as N95 masks and other personal protective equipment at filtering viruses. What this video does show is the dramatic difference with and without a face covering in the trajectory for the bulk of airflow created while breathing, talking, and coughing. As the author states, this shows qualitatively how wearing a well-fitting face covering can protect others.
Eric Putt's response above is classic over extension argument. Matt's study doesn't say anything about viral spread, it just shows the effect of different media in the cough scenario. Your response is perfect. There is so little actual data and so much recycled conjecture, breaking this down to an unbiased easy to understand visual is pretty genius.
He specifically mentioned at the first image that "it does not show viruses." Just airflow.....
So, are you saying that unless you can trap 100% of virions, it's not worth wearing? By the same logic, if seatbelts only saved lives in 50% of crashes, then we shouldn't bother wearing them at all?
Sorry, Mr. Putt. You had a good argument going then you mentioned politics ("...election year...") and that brings your veracity into question. Politics should have NO bearing on the discussion. The aerodynamics of airflow is the bottom line. The physics involved in disease transmission does not care a whit about politics, personal rights, government control or whatever issue is being used as an excuse. A point that your, and many other anti-mask arguments I've seen, omit is that, while masks may only block few or very few particles, not wearing a mask blocks NO particles.
I've been wondering about those flows that come out of the top and sides of the mask and how outgoing flow interacts with someone else's incoming flow.
This is so important when we evaluate the distance to keep apart outdoors, let alone indoor distances.
I suppose it would be too tricky to illustrate how someone in front of you influences the flow from your mouth and nose, with a mask, when they are breathing in during your exhalation.
Nice and clearly done study!
Awesome presentation, very informative.
Nice and informative. Well done. Thank you.
This is really cool! Excellent representation on how homemade masks work. We need more videos like this from NIST! Great way to communicate with non scientific community of followers.
Thanks, I shared this visual with fellow employees and family. Being a Microbiologist I totally get it but some people need the visual.
Would the author be willing to add singing to the demonstration? There is so much concern in churches regarding the safety of indoor singing that could be spreading the virus.
Yes please, I would like to know that also!
It was really interesting to see how these Schleiren Images are created, I have been wondering for a while if it would be possible for me to try to create something similar (now I see unfortunately not). This is much better than my "try to blow out a candle" test https://www.facegaiter.org/making/ . How can we use this to design a better homemade mask? My approach has been: step 1 - limit the velocity of air from the source, step 2. limit the amount of air going around the mask (stop the leaks), step 3. reduce the amount of water in your exhaled breath getting through the mask - Try to hold on to the big stuff (say anything 1um or bigger) but let the water vapor pass through. Great initiative Matt - Can you detect anything other than heat with this technique?
This is awesome, Matt! It was great to see how even a bandana helped.
My brother Dave (who works for NIST) mentioned this during our weekly sibling Zoom, and we asked him to post the link. Nice work, and very interesting! It adds some real substance to things we've been hearing. I've seen a few Schlieren results during my own work for DTRA with simulations of explosive events aimed at chemical agent defeat.
Can you show the differences between a N95, valved N95, surgical mask and cloth mask? Many people say valved N95 is dangerous but it should still mitigate spread of droplets. There is great debate about this. Thank you!
I am also curious about this. I think some fo the valve shapes would prevent spray to some degree. If it were to stop spray to the extent of some cloth coverings. Why throw the baby out with the bath water? The only problem can’t find any testing to exclude them. Just duh it has a valve. Is it that straight forward?
I had my concerns about the validity of the use of cloth masks ever since the CDCx first declared them ineffective. However, logic told me that something was better than nothing since natural breathing is a release into the air...inside and outside. The test in this video was well executed and should be shown in all venues...especially in churches (singing), schools and colleges and childcare facilities.
I’ve gotta add my thanks. A picture is worth a thousand words and even then words may be misunderstood but pictures not so much. Good work, the point is crystal clear.
Love this! Great job!
Did you happen to test a vented N95? With all those hating on the vent, I am wondering how it stacks up agaist the cloth coverings.
This is a great visual. Thank You for taking time out to make this for other people to see.
Great mate. Any chance you could give some more detail about "fabrics with very tight and nonporous weaves"? Are these particular types of fabric (cotton, etc.)? Or is there some sort of quantiative meassure of how tight/non-porous a weave is, and a number range you could provide for those you found forced air to leak out by the nose and chin? Thanks!
Face masks with valves have traditionally been designed so that the valve closes upon inhaling forcing the air to go through the fabric of the mask. The valve opens when exhaling making it easier to breath out. This is because the design intent is to protect the one who wears the mask. Wouldn't it be make sense to design a mask such that the valve works in reverse? Of course such a mask would only be worn when your only desire is to protect others. The valve would open when you are inhaling and close when you exhale. And compared to mask with the traditional valve, it would make it easier on the wearer as it much easier to exhale through the fabric of the mask than it is to inhale through it.
Great video. Thanks for doing it and sharing it with us. I am looking forward to seeing the one with the fog droplets. I would like some further confirmation of the 6 foot social distancing recommendation. The current video seems to indicate that even without a mask, 6 foot is excessive, especially for normal breathing. (I can always turn my head away from others or cough into my sleeve when I am near others.) I understand that since the video only shows the flow of gas that is warmer than the surrounding air, it doesn't show the full extent of the aerosol dispersion.
If you do make more visualizations of different coverings I would love to see the flows around a face shield without a cloth mask underneath. Traditionally used as essentially enhanced eye protection, they are increasingly being used in place of cloth face coverings due to comfort and ease of cleaning/reuse. I'm particularly interested in the performance of a face shield as compared to a bandana.
Thank you for your hard work and the excellent demonstrational tools!
I would love to see that as well.
Too many people are recommending shields as primary source control instead of masks.
Thank you for this demo. For more complete comparisons, it would be great to add breathing with no mask and coughing with both mask types with the nose covered.
I have the same request. The video shows talking and coughing with no mask, but not with breathing. With the various coverings, coughing was shown only the loose fitting mask pulled down below the nose. None of the shots with properly worn covering include talking or coughing. Thus, we're left to compare apples and oranges. A good experiment varies one variable at a time, when possible. Please make "type of facial covering" the variable, and then show their relative effect for each of the actions: Breathing, talking, and coughing.
Seems like all of this is aimed at people that are not polite enough to refrain from coughing or sneezing in the faces of others. I already refrain from doing these things. If others want optimal protection, they need to spend some money, get proper certified anti-viral face and eye masks. I don't want any responsibility for saving them other than basic hygiene, not coughing on them, and I'll be glad to give a heimlich if needed.
In your experiment your patterns of fluid movement is being shown in the macroscale. What about the microscale? Wouldn’t nanosize droplets created by the pore of the mask increase float time and contact duration? A good example would be the herbicide dicamba, with temperature inversion, wind, and droplet fines the herbicide can carry for miles damaging non-tolerant dicamba crops. So if the mass population is wearing none N95 mask, wouldn’t this create the same effect? (Nano size water droplets with virus inside floating for miles). So would no mask and social distance be the better option?
This visualization technique shows the movement of air, not droplets. So we don't make any claims about droplets based on schlieren visualization activities. (See the notice at the beginning of the video). Other research studies, however, including one by NIST, have demonstrated that face coverings do help slow the spread of the virus that causes COVID-19.
Can you share links to data set or research? Do you have any research that compares droplet size and distance traveled at the velocity of a sneeze or cough? I am worried that improper mask or use will increase contact distance and time due to water fines drifting.
NIST has not performed any research of the type you describe. You can learn more about NIST's and other's research on the efficacy of face coverings at https://www.nist.gov/news-events/news/2020/06/face-coverings-made-layer….
All I can see is the single sideview photo at the top. No other videos or anything are working. Is there a picture similar to the top one showing the front view instead of he sideview? I would be interested to see how the sides allow our breath to escape. Obviously our breath will take the path of least resistance, ergo the sides will have the largest vapor spread, not straight through the front of the masks.