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  • Prior to March 2020, there's a good chance you didn't know what an N95 mask was, or

  • at least didn't think about them unless you were doing a home repair project with

  • lots of dust, or live in a part of the world with crazy pollution or wildfire smoke.

  • And upon learning about them, you might think (like I did) that an N95 mask is basically

  • a really really fine strainer: a mesh of fibers with gaps too small for dust and other airborne

  • particles to get through.

  • A strainer filters out particles larger than its openings, and not particles smaller than

  • its openings.

  • So with a mask you'd expect that after a certain point, small enough particles will

  • sneak through.

  • But this isn't how N95 masks work: the particles they filter are generally much smaller than

  • the gaps between fibers in the mask!

  • What's more, an N95 mask is actually really really good at filtering both the largest

  • and smallest small particles -- it's medium-sized small particles that are hardest for it to

  • block.

  • This isn't at all like a strainerbecause N95s are much cleverer than strainers.

  • The overarching goal of an N95 mask is instead to get an airborne particle to touch a fiber

  • in the mask.

  • Regardless of how big an airborne particle is, once it touches a fiber, it stays stuck

  • to it and doesn't become airborne again.

  • This isn't anything special about the fibers, but about the size of the particles.

  • At a microscopic scale everything is sticky, because the weakly attractive force between

  • molecules IS strong enough to hold small things in place.

  • So you shouldn't think of N95 masks like a fine window screen that keeps insects of

  • a certain size out; you should think of them more like a sticky spider web that can catch

  • an insect of any size, as long as it touches a strand.

  • And so N95 masks use a bunch of different clever physics and mechanical tricks to get

  • particles to touch their fibers.

  • First, many spiderwebs are better than one.

  • Unlike strainers, where stacking many identical ones doesn't improve the filtering at all,

  • more layers of sticky fibers means more chances for particles to get stuck.

  • And how likely particles are to hit or miss a fiber depends in large part on their size.

  • Particles larger than a thousandth of a millimeter basically travel in straight lines, because

  • of their inertia.

  • And because there are so many layers of fibers, their straight line paths are essentially

  • guaranteed to hit a fiber and stick.

  • Airborne particles that are really really small are so light that collisions with air

  • molecules literally bounce them around, so they move in a random zig-zag pattern known

  • as Brownian motion.

  • This zig-zagging also makes it super likely that a particle will bump into a fiber and

  • get stuck.

  • Particles of in-between sizes are the hardest to filter.

  • That's because they don't travel in straight lines, and they also don't bounce around

  • randomly.

  • Instead, they're carried along with the air as it flows around fibers, meaning they're

  • likely to get carried past fibers and sneak through even a mask with many layers.

  • But N95 masks have a final trick up their sleeve.

  • They can attract particles of all sizes to them using an electric field.

  • In the presence of an electric field even neutral particles develop an internal electrical

  • imbalance which attracts them to the source of the field.

  • This is why neutrally-charged styrofoam sticks to an abused cat - I mean, a cat whose fur

  • has been charged with static electricity.

  • And how static electricity helps N95 mask fibers attract all particles.

  • But unlike a cat's fur, an N95 mask's electric field isn't just ordinary static

  • electricity.

  • Their fibers are like permanent magnets, but for electricity: electrets!

  • Just like you can permanently magnetize a piece of iron by putting it in a strong enough

  • magnetic field, you can 'electretize' a piece of plastic to give it a permanent

  • electric field.

  • By electretizing the fibers in an N95 mask, they gain a long-lasting ability to attract

  • particles, which means they capture about 10 times as many particles as regular fibers.

  • And this is, after all, the point of an N95 mask: filter out particles from the air.

  • By taking advantage of the molecular scale stickiness of matter, using many layers of

  • fibers that catch straight-moving large particles as well as zig-zagging small particles, and

  • having an electric field that attracts all particles, you get a mask - not a strainer

  • - that's really good at trapping both small and large particles, and does a reasonably

  • good job at filtering out middle sized particles.

  • Precisely what fraction of those sneaky medium-sized particles get blocked gives you the number

  • of the mask - if at least 95% of those particles are filtered out, then the mask is rated N95.

  • Ok, so N95 masks can be very effective.

  • But if you're a healthcare worker wearing one of them, here are a few important things

  • to look out for.

  • The biggest influence on the performance of an N95 mask isn't actually the mask - it's

  • whether you wear it properly.

  • If a mask isn't fully sealed on your face, air (& particles you're trying to filter)

  • can just bypass the filter entirely.

  • Dust, smoke, pollen, bacteria, and viruses all have different sizes, and so are filtered

  • by N95 masks to different extents.

  • However, germs for airborne illnesses don't usually travel on their ownwe breathe

  • or cough them out in droplets which have a wide range of sizes.

  • So the size of the virus or bacteria itself isn't particularly relevant.

  • N95 masks are intended to be disposable, but the demand from COVID-19 has led to a global

  • shortage of N95 masks and the reality is that health workers have to reuse them - and thus

  • decontaminate them.

  • It's important to be aware that certain kinds of decontamination (for example, using

  • alcohol or liquids) can damage the electrostatic properties of a mask and destroy their filtering

  • ability, even if the mask appears visually unaffected.

  • N95decon is a volunteer team of scientists developing and sharing research-based decontamination

  • methods so that masks can be reused during this crisis.

  • A big thanks to Brilliant for supporting this video - in this time of social isolation and

  • pandemic, the value of being versed in math and science is particularly evident, and Brilliant

  • continues to be a place you can go to learn and play with mathematical and scientific

  • ideas.

  • Just today I exchanged emails with a viewer who had a question about some special relativity

  • calculations, and I recommended that they check out Brilliant's introductory course

  • on special relativity - there's no better way to learn than by directly engaging with

  • questions & problems!

  • Brilliant also has fun daily challenges on various math, logic, science & engineering

  • topics.

  • To learn more about Brilliant, go to brilliant.org/MinutePhysics and sign up for free; the first 200 people

  • will get 20% off an annual Premium subscription to all of Brilliant's courses, quizzes,

  • and puzzles.

  • Thanks to Brilliant for supporting MinutePhysics.

Prior to March 2020, there's a good chance you didn't know what an N95 mask was, or

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The Astounding Physics of N95 Masks

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    Summer posted on 2020/07/30
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