According to Wired’s Phett Allain, you’ve seen the N95 face masks several times. You might even be putting on one right now. But are you aware of how they work to block a dangerous virus-carrying respiratory blob?
It’s the year 2022, and by now we’ve all been putting on face masks for nearly two years. And unless you are either a surgeon or a construction worker who was already wearing them daily, in those two years you’ve probably learned a lot about them. The ones you like best, where to get them, and whether you have any excesses stashed in a coat pocket or somewhere in your car or not.
But are you aware of what makes the prized N95 mask so special? Let’s find out.
Electric Charges
The fibers in typical cloth or paper face masks filter out particles by physically obstructing them—but the fibers in an N95 mask also use an incredible physics trick. They are electrically charged.
The electric charge is one of the basic properties of all particles. Almost everything around you is made of three particles: the proton, the electron, and the neutron. (Apart from muons and neutrinos—both basic particles that actually exist—as well as other particles that are theoretically possible.)
Just as every particle has a mass, it also has a charge. The proton has a positive electric charge with a value of 1.6 x 10-19 coulombs, the unit for measuring electric charge. The electron has the exact opposite charge. That leaves the neutron with zero charges (thus the “neut” part of “neutron”).
The electric charge is a main part of the electrostatic interaction, that’s the force between electric charges. The magnitude of this force depends on the magnitudes of the two charges and the distance between them. You can calculate this force with Coulomb’s law. Which looks like this:
f=k×q1×q2 over r squared
In the above expression, k is a constant with a value of 9 x 109 N×m2/C2. Whereas the charges are q1 and q2 and the distance between them is r. This will result in a force in newtons.
In case the two charges are of the same sign (either both positive or both negative) then this will be a repulsive force. If the two charges are different signs, then the force will be attractive.
If everything is made of electrons and protons, then shouldn’t there be electric forces between everything? Well, sort of. Electrons and protons are super tiny. That means that even a small drop of water will have something like 1022 protons in it. That drop will probably have the same number of electrons. (And no one cares about the neutrons—at least for now.)
That renders the overall charge of this drop of water equal to zero coulombs. Even if you have more electrons in your water, the total charge is going to be small, since the electron charge is puny. Virtually, most of the stuff you can see is electrically neutral without electric force.
So how do you charge something?
Remember that one time you took a sock out of the clothes dryer and it stuck to your shirt? If that’s a static electricity interaction, how did the sock get charged?
To make a sock negatively charged, there’s only one way to do it—ensure the sock has more electrons than protons. You’ll need a lot of electrons, maybe something on the order of 1013 extra electrons. (To give you an idea of how large this number is, it would be the total number of bills you’d need to give everyone on earth $1,000 in singles.) The extra electrons would give the sock an overall negative charge of around 1 microcoulomb (1 x 10-6 C).
If you prefer to make that same sock positively charged, instead of adding electrons you would remove them. Leaving the sock with more protons than electrons for an overall positive charge. But you cannot just remove protons from most objects willy-nilly. Well, you can, but it might be terrible.
Think back to the periodic table of elements. Let’s assume you start with an object that’s made of carbon, which has six protons in the nucleus. If you got rid of one of these protons, it would no longer be carbon. It would be boron, which has five protons—and you would have created a nuclear reaction.
On the other hand, if you remove an electron from carbon, it’s just a carbon ion. It doesn’t transform into a different element.
OK, but how do you add or remove electrons? You can ask. You only have two options to do so. The most common method is to transfer electrons from one surface to another by rubbing them. I know that appears silly, but it’s true. If you get a plastic pen and rub it on your wool sweater, both the pen and sweater will become charged.
But which one will get the electrons? The answer relies on the two types of materials—and you can figure it out with the help of something called triboelectric series. Using that, you would find that the wool is positively charged and the pen is negative.
There is another way to get extra electrons onto an object—shoot them at it. Yes, there is something known as an “electron gun.” Maybe you have already seen something that’s similar: Old-style cathode ray televisions shot a stream of electrons to hit the screen to generate those pretty pictures. So it is possible to charge something without touching it.
Interaction Between Charged and Uncharged Objects
In case you’re wearing an N95 mask, the objects you want to curb are the tiny wet blobs that come out of a person’s nose and mouth that could possibly carry a virus. These are virtually uncharged.
You might think that an electrically-charged N95 face mask would only be good for stopping electrically charged objects, but you can have an interaction between uncharged and charged objects.
Let’s begin with an easy demonstration you can perform at home. Begin with a plastic pen (or some other small plastic thing) and one of the plastic grocery bags. Now rub the pen with the bag. It should become electrically charged.
If you can’t get the expected results, you might have to change up materials—you could try rubbing the plastic pen against some wool. Now tear up some paper into tiny pieces and lay them on the table. When you bring the charged pen near the paper, you get some magic-looking physics.
This is called polarization. Let’s consider the easiest model of a molecule of paper. This pretend paper molecule is a sphere with just two charged particles, a proton, and an electron. (If you are thinking back to the periodic table, yes, this would make it hydrogen paper. No, it totally doesn’t exist.)
In atoms, the negative electron acts like it is spread over the blue region. We call it an “electron cloud.” I know that seems weird, but weird stuff happens with tiny objects like molecules. The crucial thing is that the center of the negative blue cloud is at a similar location as the positive charge. In this state, it’s unpolarized.
Now let’s assume the positively charged pen is brought near the paper molecule. The electron cloud will get pulled toward the pen (because they are oppositely charged), and the positive proton will get pushed away.
The paper molecule is now polarized. The positive pen interacts with both the negative electron and the positive proton. However, the effective location of the negative electron cloud is closer to the pen than the proton. The magnitude of the electric force between charges decreases as the separation distance increases.
This means that the attractive force between the pen and the electron is bigger than the repulsive force between the pen and the proton. So there is an overall attractive force pulling the paper toward the pen, even though the paper is neutral.
Yes, that’s just one molecule—but if a similar thing happens with every molecule in the piece of paper you can get an attractive force. That’s cool, right?
How the N95 Mask Works
Imagine something similar to the electrically charged pen and the water—but at a smaller scale. So instead of a pen, you have a bunch of plastic fibers. Instead of the water, you have the drops that normally fly out of someone’s mouth. This is basically what happens in an N95 mask.
The fiber in the mask attracts the drops, protecting the wearer from inhaling them. At a very small scale (similar to that of respiratory aerosols and fibers), things tend to stick together, as a result of what’s known as the van der Waals interaction. This is essentially an attractive interaction between two uncharged objects because of very slight charge separations.
With the N95 fiber, you don’t need to rub it with some other material to get it charged. The fibers in the mask are made from an “electret” material; this word comes from combining the words electric and magnet. No, it’s not an electromagnet—it is a permanently electrically charged object, just in the manner that a bar magnet on your fridge is.
There are a couple of ways to create electret materials. One is to bombard the stuff with electrons so that they get stuck in the fiber to make it stay charged.
The other method is to heat the material in an electric field. The temperature increases enabling the molecules in the material to rotate into a polarized state, owing to their interaction with the electric field. Once the material cools off, the molecules stay polarized. This makes a little different electret material, in that it creates an electric effect even though it’s still neutrally charged.
So, the electret fibers in an N95 mask not only block small particles by getting in the way but can also attract them with the electric interaction, so they get stuck to the fibers. This means that the water droplets carrying a virus don’t get inhaled, and the mask wearer won’t be infected. Of course, an N95 also blocks other small particles, like dust, paint, and other toxic stuff that might not be good for a person to inhale into their body system.
So there you have it—the N95 face mask doesn’t just help us get past this terrible pandemic, it can also protect us from the other toxic stuff, and teach us some awesome physics too.
Would you like to read more news in this line? Feel free to say so in the comments section.
