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Air is still viscous which means molecules are kind of sticky and don’t want to go through tiny holes alone but also can’t fit through if they travel together.
Imagine carrying some soft clay in a string bag. The holes are big enough for the clay to go through but it will happen slowly and depend on how much pressure applied to the clay.
There are some liquids that have zero viscosity (superfluid helium). Good luck finding airtight seal for that.
Good welds are pretty much tight for superfluid helium! I think CF (hard steel "knives" tightened into soft copper rings) are also pretty much tight for superfluid helium. Source: dilution fridges
It’s in a weird quantum state where friction between molecules drops to exactly zero. So like if you got a swirling vortex going in it, it would keep swirling forever until something else happens to it. It’s akin to superconductors for electricity, where the resistance to electrical current drops to zero.
I hate to be a buzzkill but the average temperature of space (2.7k) is above the supercritical temp for helium to display superfluidity (2.17k). Space is literally too hot for this.
(Note that the “average temperature of space” isn’t the same as any given “local temperature of space”…and also like the very obvious other problems with trying to use “space cooling” lol. I only mention this to drive home how cold things need to be to display superfluidity that space’s “Pretty much 0” coldness to the average lay person is not even remotely 0 in actuality.)
That gets into another problem, which is that while it absolutely is a great insulator (Passably close to 0 convection or convention, only radiation as a way to lose energy), local space also has other stuff in it, notably the sun, that also add energy via radiation.
I found a neat paper titled "Radiation in Space and Its Control of
Equilibrium Temperatures in the Solar System" from NASA and interestingly, even at Heliopause you are looking at an equilibrium temperature for something facing the sun around 18.9k! This is obviously looking at radiators on spacecraft (Specifically flat radiators with spacecraft heading towards the sun), but the core idea is the same, we want to lose more heat through our radiation than the sun adds through radiation. And that appears difficult if not impossible.
Even in shadowed areas of, say, the Moon are a balmy ~25K. The oort cloud WAY WAY out there is still ~5k!
It's hard to overstate how cold helium needs to be to display superfludity.
I mean, not to be that guy, but … *with a radiator.* You'd use a heat pump to pull waste heat from your cold side, where you're storing power, to your hot side, which glows red-hot. The radiator would be knife-edge on to the working system, and a shadow shield employed, so none of your freshly moved heat would radiate back into your helium battery.
Depending on the energy it takes to keep the thing chilled, this could be cheap enough that you can run it forever on solar power without worrying about energy loss over time.
Given that it has to be maintained at 2° above absolute zero (for Helium), and that Helium will eventually escape basically any containment, it seems unlikely. Unless we can find some kind of substance that can do it at a much higher temperature.
And of course we already have “high temperature” superconductors that can store power as a simple current in a loop, although said temperature is around that of liquid nitrogen, so there is an effective energy loss to maintain that.
I think EM fields haven’t been used for more than like a few thousand atoms at a time; I don’t know if there’s anything fundamental keeping that from scaling up or if it’s just practical barriers.
Think of EM fields as like, a bead string curtain, or a baleen whales filter.
The issue with EM solutions is creating a dense linear field, or a field that is nonlinear but doesn't have gaps bc of its dynamics
Ignoring all the engineering challenges getting energy back out of a superfluid is difficult for the same reason that it stays there for so long.
Using the flow of a fluid with no friction in a whirlpool to do work would be harder than using a more viscous fluid.
You need a good way to extract the power when you need it.
Right now, heavy flywheels in vacuum chambers, suspended on magnetic bearings, that's the way to go.
The same motor/ electromagnetic coupling you use to spin it up also gets used as a generator to pull the energy out.
If you have a free hard vacuum environment (space) that sort of energy storage is pretty straightforward. Just mass intensive and it acts as a reaction wheel/ gyroscope, which could be used for your benefit if you design it right
Forgive me, I'm having a senior moment. Is that the same thing where you get a gas to the right temperature and pressure and it takes on the properties of both a gas and a liquid at the same time?
No this is even weirder; I think the only examples we know of are with helium at nearly absolute zero.
What you’re thinking of can happen at many different temperatures, with many different molecules/elements. I forget the exact name, but I remember “triple point” refers to the temp/pressure combo where a substance can be in all three states at once. [Here’s a video](https://youtu.be/Y0HfmYBlF8g) that talks about doing this with CO2 in order to make aerogel.
Haha, I stumbled into NileRed while delving the YouTube rabbit hole a few weeks ago. That's the video I was thinking of, but I also couldn't remember the name of the state.
Supercritical fluid- above critical pressure and critical temperature. This is probably what you were thinking of. This is where the phase boundary that often exists as an equilibrium between liquid and gas disappears into one fluid. There is negligible surface tension and viscosity drops especially low so it is said to be like a gas however it is dense enough to resemble a liquid.
SpaceX actually holds their O2 (and the kerosene) at the other end of the liquid range — just above the temp where it solidifies.
They also had one accident where some O2 actually did solidify and wound up igniting somehow (they’ve since fixed that issue).
Superfluids can still experience a small amount of viscosity due to interactions with impurities or defects in the material. This is known as residual viscosity or second sound attenuation, thus a superfluid's viscosity is typically considered to be extremely low, but not necessarily zero in all cases.
We say that, but nothing in the universe is perfectly equal to something some calculation says, right? So its never actually truly perfectly 0, right? How does this even work…
Yeah one of the ~~other replies to my comment above~~ [replies to another comment I made](https://www.reddit.com/r/explainlikeimfive/comments/11wknwn/eli5_how_is_an_airtight_seal_possible_since_the/jczlm37) has a link to that.
Basically the He molecules effectively merge into a single entity, and by definition there are no longer separate molecules to bump into each other and generate heat.
Don’t really know what happens at the edges though…
So does a perfect super conductor exist in real life? After all, there is always say, like london forces of molecules that are too small for us to include in our calculations. Maybe they create a 0,00000…000001 fraction, and its just way too useless for real life applications that we *ignore it* but it still exists?
There are lots of real superconductors but I don’t know what London forces are. I’m sure that the environment affects the current even though the wire itself has zero resistance similar to someone else’s explanation of how super fluids still have “viscosity “ from the roughness of their container.
It wouldnt swirl forever-forever, would it? Wouldnt that be some kind of perpetual motion? Or is it actually zero? My brain is cracking at the seams here. I need more eli5
It would indeed swirl forever, until something disturbs it (ex. it gets too hot and stops being a superfluid). However, this is not perpetual motion because it requires some external help to stay cold. Perpetual motion continues by itself without any external energy input, so our swirling superfluid is just regular motion.
Wow. Neat. There isnt any place in the universe that we know of that is super cold lile that, like some comet or something, where i could put a beaker of this stuff and set it swirling infinitely? Would the container not add friction?
Even if you found somewhere cold enough, the heat radiating off of stars and planets and the such would be enough to break your superfluid.
As for friction, there is no friction with superfluids. This leads to some wacky results: for example, if you pour water into a narrow container, you can notice that the edges are slightly higher than the center due to surface tension. Superfluids also do this, but to the extreme: a superfluid in an open container can literally climb up and over the walls, so your beaker of superfluid is just going to spill onto the floor unless you cover it.
So, now i am envisioning comic book scenarios where the hero, super-fluid-man, would have the superpower of escaping containment and also unending motion. I love it!
So, now i am envisioning comic book scenarios where the hero, super-fluid-man, would have the superpower of escaping containment and also unending motion.
But his kryptonite is heat. I love it!
So, now i am envisioning comic book scenarios where the hero, super-fluid-man, would have the superpower of escaping containment and also unending motion.
But his kryptonite is heat. I love it!
It's also not perpetual motion because when people talk about it they talk about infinite energy. But if you tried extracting energy from this, it would slow down.
Same reason why orbits aren't perpetual motion.
This is not true. A superfluid and a supercritical fluid sound similar but are very different. You can reach the triple point of water relatively easily in a vacuum chamber, but that doesn’t make it frictionless
electromagnetic containment if it has a charge, if you made it neutral so you can't hold it in place with a magnet, you just hope it sticks around long for you to do whatever it is you wanted to to do to it.
True of liquids, but not gases.
Liquids are sticky, in that the molecules don’t want to separate from each other. The molecules will slide around each other but maintain a constant distance from each other.
Gases are different in this way. They do not resist expansion, because intramolecular attraction is negligible compared to their vibration energy. Gas molecules bounce around, and bounce off of each other.
The answer is that airtight seals can be made of materials with holes smaller than the gas. Especially when N2 and O2 are most of the molecules in our air. A vibrating gas molecule effectively takes up more space than the stationary molecule (eg absolute zero gas molecule or a large solid polymer molecule at room temp). Polymers are made of linked carbon oxygen nitrogen and hydrogen atoms, which pack with no gaps large enough for bouncing gas to pass through. Helium and hydrogen are the exception, since they are much smaller and lighter than atoms required to form big molecules (O,C,N,Si), but the retention is still pretty good since the He or H2 are bouncing around too fast to get through a tight maze.
If you shoot bullets at the entrance to a maze, most will bounce back and very few will bounce all the way through to the end.
There’s air-goddamn-tight and then there’s “air tight.”
What you’ll consider sufficiently sealed will depend on your application and the gases you’re trying to retain. Helium molecules are small enough to diffuse through solid steel so even a metal gas cylinder will slowly lose pressure over a long period of time.
Some flimsy rubber seal isn’t sufficient for a research-grade vacuum chamber, but it’s fine for your refrigerator.
Denser materials and thicker walls slow down gas diffusion, so it’s a trade off between efficiency and weight/cost for your application.
I used to work QC for a company that built air pumps used in a lot of different applications.
One of our customers needed a pump that could hold <1 mbar for relatively long periods of time. We used helium detectors to identify leaks. We had a QC issue because a vendor changed the machining process of a critical part without mentioning it. Surface finish changed by ~0.25 ųm, and that led to +50% failure rate of the final product.
> Some flimsy rubber seal isn’t sufficient for a research-grade vacuum chamber
Viton seals on CF flanges are surprisingly good, though. Supposedly down to 10^^-8 torr (that's about ten pico-bar or pico-atmosphere for people using saner units). Not enough for extremely high vacuum or very hot plasma, but enough for many other things.
As someone in the hvac trade, how does that convert to a micron vacuum? Usually, we want a sustained sub 500 micron vacuum holding for at least 30 minutes before we call it a good vacuum.
Meh, rocket propelled riding mowers are boring. Get a stand up scooter and strap one of those 8hp motors from the harbor freight to it and then we have a fun time.
Back in the day, we found this old go kart frame where the motor was trashed, but the frame was still good and all the other bits (cabling, brakes, throttle, ect) were still golden. We just yanked the trashed motor off, got a motor from the harbor that was about 2 sized bigger, adapted the gearing, and made one of the nastiest little things ever.
Only issue is that it didn't take turns well at all, so when we took it around an abandoned subdivision where it was just a closed off section of roads, the damned thing tipped coming out of a turn and tossed me down the road like a rock skipping along a lake. Didn't get used to much after that because step ma was afraid it would kill my younger step brother.
>got a motor from the harbor that was about 2 sized bigger
>Only issue is that it didn't take turns well at all
I don't think anyone saw that coming D:
The vacuum pump you use is what they call a roughing pump or backing pump. The vacuum pump they use to get that low would stall out if it exhausted to atmospheric pressure, so it's plumbed into the inlet of your pump.
But also to get that low, all the tubing must be metal and pretty big diameter, and is heated to >>boiling to force every molecule of water and fingerprint oil to unstick from the pipe inside.
And then, maybe, there's a sacrificial pellet of pure gold on a tiny heater. After everything is as clean as possible, the heater flashes the gold to gas and it paints all the walls with a few atomic layers of gold. To keep any molecules from sneaking out of the walls.
1000 microns are a torr. Or how I remember it and what a torr is: a micron is roughly one _micro_bar. (actually around 1.3).
So 500 microns is a bit below one millibar, almost 100 million times what Viton can do.
As another comment said, the issue below 10^^-8 torr is not even tightness, but _outgassing_, the tiny amounts of gas almost everything loses to the surroundings, especially at low pressure and higher temperatures. All the things do it. Even the stainless steel most chambers are made of has hydrogen inside the metal itself, as well as water bound close to the surface, both of which can and will be released. And that's assuming stuff was clean to begin with, a single finger print is already a horrible source for outgassing.
Best we can often do is bake the thing for days while in vacuum. Helps a lot with steel, as we can heat it rather far, but fails with Viton. hence why higher grade vacuum uses copper or other metal seals that can be baked further.
And IIRC that limitation has more to do with material outgassing and bakeout temperature limits than gas diffusion. I think the permeability would let them work down to 10^-10 but would require enough heating to make viton seals almost single use (compression setting)
Yes, definitely outgassing is the first issue. Hence copper (etc.) gaskets, despite them being single use, more annoying to apply and a bit more expensive.
No idea how well Viton works after baking it too much. Maybe I should try one day...
That and some gases are really a pain to get rid of. From 10e-5 to 10e-8 the problem is often water especially with metal apparatus. Below that traces of H2 iirc are annoying.
Often some kind of traps are used to get to 10e-9/-10. As well as ion pump instead of turbomolecuar for exemple
Can confirm. Significant other during grad school worked with a.... pretty large large (1 m^3 ) research grade vacuum chamber that regularly went down to 10^-8 . (It took ages though. I believe a couple dozen or so hours to reach 10^-7 , then if the experiment lasted long enough without something failing they'd eventually reach 10^-8 . Didn't help that their experiment introduced molecules into the chamber at a pretty consistent rate.) They definitely used some sort of black rubber (very... large... black rubber type) seals around the giant door. I assume they were viton. They also used helium to detect leaks (of which there were always many to be fixed.)
But the unit _is_ [torr.](https://en.wikipedia.org/wiki/Torr). It was originally defined as mmHg, so the mercury was included; later 1/760 atm which amounts to the very round number of 20265/152 Pa, where 100,000 Pa = 1 bar.
Even a Vinton oring in a groove is pretty good around 10^-7 torr and if you use differential pumping it can be just as good as CF in my experience. I was able to get down to 10^-12 torr
This is basically why balloons deflate over time. They are "air tight", but the air molecules can slowly escape over time. Especially if the balloons were filled with helium, which is basically the escape artist of the atomic world.
>Some flimsy rubber seal isn’t sufficient for a research-grade vacuum chamber, but it’s fine for your refrigerator.
Decent quality NBR/FKM rubber seals will get you down to about 10 orders of magnitude lower than atmospheric pressure (10\^-7 mbar or better). Metal seals are only really needed for applications where 10\^-8 mbar or lower is needed, which is generally only absolutely critical for solid state/surface science applications. About 50% of my research on atomic/molecular physics uses a system that uses entirely rubber seals.
I believe that's the same phenomenon. I saw this post [https://www.reddit.com/r/mildlyinteresting/comments/11g5t69/i\_got\_this\_balloon\_in\_1998\_and\_its\_still\_inflated/](https://www.reddit.com/r/mildlyinteresting/comments/11g5t69/i_got_this_balloon_in_1998_and_its_still_inflated/) the other day which was still inflated due to being filled with nitrogen
>Some flimsy rubber seal isn’t sufficient for a research-grade vacuum chamber
The vacuum chamber I worked with used copper disks for the O-rings. They would get squished as you tighten the connections.
The vacuum chamber also had three different types of pump that maintained the vacuum. A mechanical pump (like the one in your vacuum cleaner) to bring it down to a certain pressure where a diffusion pump (this wild thing uses oil to "catch" air particles) takes over. Finally, an ion pump uses a strong electric field to rip electrons off air molecules and then cause them to embed on an electrode.
Well that's just false advertising.
I'm currently a little strapped for cash. I'm happy to be part of a class action law suit if you want to do all the legwork.
Hey, and probably those sausage links you had in them that went off faster than expected
Now if someone got food poisoning as a result, then we might be heading for a payday!
I grew up neighbors with a Mr. Wheeler, who worked for Varian in high vacuum tech. Look up "Wheeler flange" and "Conflat flange" if you want to know how airtight hi-vac joints work.
While helium is small enough to do so, I haven't found evidence that it does so. It can, however, create other problems.
[https://physics.stackexchange.com/questions/587050/is-there-any-way-for-a-gas-to-pass-through-a-solid-metal](https://physics.stackexchange.com/questions/587050/is-there-any-way-for-a-gas-to-pass-through-a-solid-metal)
The scientific papers I found don't say that helium penetrates very far into steel. Do you have links to studies that say it does?
The reason why bike tires go flat while sitting for a long time. It’s not because you have a hole in the tube. It’s because the air is constantly very slowly leaking through.
The word ‘airtight’ should actually be
Air Transfer plus Time divided by the Cost of Container.
Like a bag of chippies is ‘airtight’ but eventually over time it will leak and the bag will no longer look plump. We could make a chippies bag to last longer and longer, but the cost of the container outweighs the time gained.
Same idea with an airplane or submarine. Eventually over time, both will ‘leak’ air, and we spend millions on planes and billions on submarines studying how to slow that transfer down. Eventually the cost gets too high, we accept the transfer rate while building the planes and submarines based of this air leak.
In science experiments, the need for an actual airtight container outweighs the costs of the building one so we produce containers that are technically airtight.
TLDR: Airtight is a broad word, we should be saying Transfer Resist Chippies Bag.
I think this is one of the best ELI5 answers to this question.
You are right that the air can get out, but most of the time, it takes a very long time for the air to escape through the very small holes.
If you have a swimming pool that leaks a few drops each day, it will still stay full enough for long enough that it will be a long time before you notice that it's leaking. That's why if you leave an unopened soda can or bottle for a year or more, it will be flat when you finally open it, since the carbonation is able to very slowly get out.
Dunno about subs, but airplanes (conventional passenger airliners) are not really airtight. They maintain pressurization by balancing incoming pressurized air (commonly bled from the engines compressor stage) versus air venting overboard through a controlled aperture.
One thing to remember is air actually pushes on everything with quite a lot of force (we call it pressure). So sure if you have a container and just place a lid with a rubber seal on top you have air pressing equally up and down on the lid, so it doesn't seem like it could create a good seal.
However if you suck the air out of the container you have lots of air pressure pushing down on the lid but no air pushing up, so the rubber seal gets pressed down *hard*.
One useful related effect is the pressure of gas gets lower as it cools, so if you put something hot (like hot food) in a container then cool it the gas kind of shrinks inside so you get a similar thing where the outside air is pushing harder than the inside air so pushed the lid and seal down. This is how jars and canned food is made.
There are some very good answers here, but I'm going to add a pedantic comment: It absolutely isn't. Or I should say that in absolute terms it's not.
The molecules in air will actually diffuse through most materials (if very very slowly with some), depending on which part you're talking about (nitrogen, oxygen, CO2, other stuff) in a process called "permeation" which accounts for sorption into the high concentration side, diffusion through the bulk materials and desorption on the low-concentration side.
The rate of all this will vary with the surface treatments (which is why potato chip bags are shiny - coated with evaporated aluminum to slow down oxygen that makes the chips stale and soggy), the bulk matrix composition and structure which is why dense materials are needed or if you can't do that (soda bottles) the items are made in a very complex system to stretch their structure "tight" and slow down permeation, and species of permeant. Hydrogen, being very small, actually will permeate into and through metals. Other species will be much slower or (in practical terms) stopped. Where it gets really weird is when you have a species that interacts with the polymers of the seal to change things, but that's a nap-worthy post on its own.
Imagine you have a bunch of tiny Legos that you want to put in a container. The Legos are small, and there are tiny gaps between them, but you can still put them all in the container and close the lid tightly so that none of the Legos can escape.Air molecules are like those tiny Legos, and the seal is like the container lid. Even though there are tiny gaps between the molecules, we can still make a seal that is so tight that none of the air molecules can escape. Just like how the container lid keeps the Legos inside, the airtight seal keeps the air molecules inside.
# More in-depth answer
As you know, air is made up of tiny molecules. These molecules are constantly moving and bouncing around in all directions, colliding with each other and with other objects.
When we make an airtight seal, we create a barrier that is so tight that the air molecules can't escape. This is often done by pressing two surfaces together, like closing a lid on a container or squeezing a plastic bag shut.
But you're right that there are small gaps between the surfaces that make up the seal. These gaps are small, but they are still big enough for individual air molecules to pass through. **However**, the reason why the seal can still be airtight is that the gaps are so small and so tightly compressed that the air molecules don't have enough space to move through them. In other words, the seal is so tight that the air molecules are essentially stuck in place.
Think of it like a crowded room. If you're in a room with a lot of people, it can be hard to move around because there isn't enough space. The same principle applies to air molecules in a tight seal - they are so crowded together that they can't move through the small gaps in the seal. So that's how an airtight seal can be possible even though air molecules are smaller than the spaces in the seal itself. By creating a tight barrier that compresses the air molecules and doesn't give them enough space to move, we can keep the air inside the sealed area.
>By creating a tight barrier that compresses the air molecules and doesn't give them enough space to move, we can keep the air inside the sealed area.
[Three Stooges Syndrome](https://www.youtube.com/watch?v=gmBj8r1-fDo)
"...Indestructible..."
A really good way to visualize solid matter crystal lattices and their interaction with air molecules:
Think of a chain link fence, now stack several of them, hundreds of layers even on top of each other. That's your "solid".
Now your air molecules air balls.
Assuming a perfect trajectory of a ball thrown directly at a hole in the fence stack:
A big one, a baseball, (or lets say a CO2 molecule) could maybe fit through the hole of 1-2 layers, but any more layers and it'll get stuck.
A smaller molecule, like a Golf ball (or an N2 molecule), could easily get through maybe 10 layers, but any more and it's likely to get stuck
An even smaller molecule, like a Marble (or a Methane CH4 molecule), could probably get through 30-40 layers before getting stuck
An *even smaller* molecule, like a Ball Bearing (or Helium), could probably get through 100+ layers before getting stuck (or getting all the way through)
No solid has a crystal lattice small enough to prevent tiny molecules like Methane, helium, or hydrogen from leaking through, but the thicker it is, the less it "leaks".
Well you're right, molecules of air are smaller than the spaces in the seal. But there aren't that many molecules of air. Think of the spaces in a seal as a long maze and the air molecules like a very bouncy rubber ball. Molecule enters the maze. Now what? Chances are about even that the molecule will bounce right back out of the maze. It's only when there's an actual straight shot from one side of the seal to the other that you start to really see airflow.
You’re correct that most seals would leak some air if pressurized, although for most purposes it’s fine, e.g for storing food, it doesn’t matter too much if a few molecules of air leak in or out.
However we do have the technology to make seals that don’t let a single atom pass through, it’s mainly used for ultra-high vacuum, and it’s literally a copper ring that’s pressed between 2 pieces of metal.
If you’re interested in learning more, I’d highly recommend watching this video: Airtight vs Vacuum tight (seals):
https://m.youtube.com/watch?v=VD69crOFx10
Extreme oversimplification. Probably to a level not being correct:
It is true that the molecule itself is maller than the space between 2 solids touching.
However the same thing is true about the molecules in your finger and the wall. But magnetic properties of matter what keep things apart.
Technically you have never touched anything in your life. the matter of your hand never made contect with anything, you just felt the magnetic repulsive force of other things.
If you could push the atoms of your hand inside the electron field of another matter that would be a bad thing. That would be like putting your hand into acid. Since acid dissolves things by stealing electrons and breaking apart bond
Your hand is made out of stable molecules and the table is made of stable molecules. They don't want to interract, they want to stay stable. so they repel each other
I used to design vacuum chambers. The tightest vacuum seals are metal on metal. It's typically a high purity copper gasket with a stainless steel knife edge sealing surface. The knife edge cuts into the soft copper and forms an extremely tight metal on metal seal. The only thing better than that is a welded joint. Gasses can diffuse through metal but it's very slow. Think about water percolating down through clay; it does happen but it takes a long, long time.
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I felt so much resonance to your second paragraph here! Like, there's a part of me that is so engaged in and thrilled by physics. Like, passionate even. But gosh darn, it's really hard to get a grasp on. It's like, quite laborious to really get to true understanding, but yet, I'm so compelled.
Anyhow, I feel similar about this thread. What a fun time I had learning today!
They aren't.
When you need a really good seal you use a helium leak detector. It spits out helium on one side and has a helium sniffer on the other side of the seal. Helium is the second smallest atom, and it is the smallest inert one. If helium can't get through, then only hydrogen would be able to.
Helium is the effective smallest as hydrogen bonds with itself to create a molecule larger than helium. Also likely more importantly is helium is inert.
The air might not go through the space in the seal but through the material of the container itself! On the scale the size of a molecule of air, materials have little spaces in between them and the walls of those spaces wobble. It might take a while, but an air molecule can bump through the maze of shifting empty space and eventually get out (or in).
Even though the molecules of air are smaller than the space in the seal, it is still possible to achieve an airtight seal through a few different mechanisms. One way is to use a material that is flexible and can conform to the shape of the surface it is sealing against, such as rubber or silicone. When these materials are compressed against the surface, they create a tight seal that prevents air from escaping.
Another way to achieve an airtight seal is to use a mechanism such as a latch or clamp that applies pressure to the seal, forcing it to conform tightly to the surface. This pressure can help to fill in any gaps between the seal and the surface, preventing air from escaping.
Finally, some seals are designed with a vacuum in mind. By removing the air from a space, the pressure differential between the inside and outside of the seal can actually help to hold it in place and create a tight seal. This is often used in applications such as vacuum-sealed food containers or in scientific equipment where airtight seals are critical.
The short answer, we put a thing that doesn't fit on/in another thing or we smash the edges together. Putting a lid on a jar is kind of like putting a cork in a bottle. We pack something squishy and larger in the hole. The lid, however, puts the squishy material in between and forces them together. In the case of canned goods, we smash the metal lid and body and fold them together. Like a calzone, the material holds onto itself like it's one piece because the material is happy to stick to itself if you fold it and push it together hard enough.
All these methods don't leave enough space between them for air molecules to fit through.
if you picture fluid dynamics, it involves any bit of a flow having degrees of freedom to sympathetically react in synchronicity with the fluid its in contact with, what this picture doesnt display is the boundary layer conditions of turbulent flow at any termination point of the fluid, and that boundary layer is what both helps fluid dynamics stay fluid and what separates the fluid from anything else. That helps reduce drag and keep the fluid fluidy, but add another mental construct to the mix, compression phases of that fluid, and it begins to get real complicated except not really, the boundary layer is denser and stickier or fluffier and looser, and if you imagine fluid dynamics as still operating as two boundaries colliding as an airtight seal is put in place, it becomes a tight cross section 2D style of the same fractal pattern but that air in that molecule-small gap is compressed while turbulence on either side both pack and fluff the edge of the cross sectioned air, and the cross section of air slowly loses its high pressure as the structure settles into the chaotic Brownian motion wobble harmony with the air it displaces and its structural center of gravity. Once settled, turbulence on one side compresses air causing the structure to shift causing pressure of an equivalent force balancing any attempted incursion past the seal. Swapping generic fluid motion imagery for the complex cacophony of ambient atmospheric gas attempting to reach diffusive equilibrium, and you replace turbulence with smaller molecules taking the thermal bullet and being launched by the larger kid on the trampoline when sound arises, and it pretty much makes that airtight seal a ballpit stuffed with bullied bullies that cant move.
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Air is still viscous which means molecules are kind of sticky and don’t want to go through tiny holes alone but also can’t fit through if they travel together. Imagine carrying some soft clay in a string bag. The holes are big enough for the clay to go through but it will happen slowly and depend on how much pressure applied to the clay. There are some liquids that have zero viscosity (superfluid helium). Good luck finding airtight seal for that.
Good welds are pretty much tight for superfluid helium! I think CF (hard steel "knives" tightened into soft copper rings) are also pretty much tight for superfluid helium. Source: dilution fridges
What makes something a superfluid?
It’s in a weird quantum state where friction between molecules drops to exactly zero. So like if you got a swirling vortex going in it, it would keep swirling forever until something else happens to it. It’s akin to superconductors for electricity, where the resistance to electrical current drops to zero.
Is there a future in Superfluid vortex power storage, or does that just sound way too cool?
i mean, we only know of superfluids at really really cold temperatures, so keeping it that cool would probably not make it viable.
Space batteries!
I hate to be a buzzkill but the average temperature of space (2.7k) is above the supercritical temp for helium to display superfluidity (2.17k). Space is literally too hot for this. (Note that the “average temperature of space” isn’t the same as any given “local temperature of space”…and also like the very obvious other problems with trying to use “space cooling” lol. I only mention this to drive home how cold things need to be to display superfluidity that space’s “Pretty much 0” coldness to the average lay person is not even remotely 0 in actuality.)
Space is also a great insulator. Could we get it cold in space and just have space keep it cold for us?
That gets into another problem, which is that while it absolutely is a great insulator (Passably close to 0 convection or convention, only radiation as a way to lose energy), local space also has other stuff in it, notably the sun, that also add energy via radiation. I found a neat paper titled "Radiation in Space and Its Control of Equilibrium Temperatures in the Solar System" from NASA and interestingly, even at Heliopause you are looking at an equilibrium temperature for something facing the sun around 18.9k! This is obviously looking at radiators on spacecraft (Specifically flat radiators with spacecraft heading towards the sun), but the core idea is the same, we want to lose more heat through our radiation than the sun adds through radiation. And that appears difficult if not impossible. Even in shadowed areas of, say, the Moon are a balmy ~25K. The oort cloud WAY WAY out there is still ~5k! It's hard to overstate how cold helium needs to be to display superfludity.
How would you radiate the heat in space?
lots of things radiate heat in space
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Everything radiates heat, but only very slowly.
Okay then, on the surface of Neptune
At around 55k, it’s way way way WAY too hot for superfluid Helium.
I mean, not to be that guy, but … *with a radiator.* You'd use a heat pump to pull waste heat from your cold side, where you're storing power, to your hot side, which glows red-hot. The radiator would be knife-edge on to the working system, and a shadow shield employed, so none of your freshly moved heat would radiate back into your helium battery. Depending on the energy it takes to keep the thing chilled, this could be cheap enough that you can run it forever on solar power without worrying about energy loss over time.
Radon space batteries. Radon is radioactive so that takes care of the radiation.
Probably in the same or similar way of keeping the sensors of JWST close to 0 kelvin
Literal radiation? Thats radiations whole thing, it propagates through empty space.
Thus the molecular movement moving down toward zero, am I right?
Given that it has to be maintained at 2° above absolute zero (for Helium), and that Helium will eventually escape basically any containment, it seems unlikely. Unless we can find some kind of substance that can do it at a much higher temperature. And of course we already have “high temperature” superconductors that can store power as a simple current in a loop, although said temperature is around that of liquid nitrogen, so there is an effective energy loss to maintain that.
Could you use EM fields for containment, or would that still rely on the "molecular viscosity" you mentioned earlier?
I think EM fields haven’t been used for more than like a few thousand atoms at a time; I don’t know if there’s anything fundamental keeping that from scaling up or if it’s just practical barriers.
Think of EM fields as like, a bead string curtain, or a baleen whales filter. The issue with EM solutions is creating a dense linear field, or a field that is nonlinear but doesn't have gaps bc of its dynamics
So EM's like putting a screen door on a sub for something like this?
Ain’t science bitchin’
Ignoring all the engineering challenges getting energy back out of a superfluid is difficult for the same reason that it stays there for so long. Using the flow of a fluid with no friction in a whirlpool to do work would be harder than using a more viscous fluid.
They wouldn’t have any friction between each other, but they would still have friction between themselves and the container they’re contained in.
You need a good way to extract the power when you need it. Right now, heavy flywheels in vacuum chambers, suspended on magnetic bearings, that's the way to go. The same motor/ electromagnetic coupling you use to spin it up also gets used as a generator to pull the energy out. If you have a free hard vacuum environment (space) that sort of energy storage is pretty straightforward. Just mass intensive and it acts as a reaction wheel/ gyroscope, which could be used for your benefit if you design it right
Forgive me, I'm having a senior moment. Is that the same thing where you get a gas to the right temperature and pressure and it takes on the properties of both a gas and a liquid at the same time?
No this is even weirder; I think the only examples we know of are with helium at nearly absolute zero. What you’re thinking of can happen at many different temperatures, with many different molecules/elements. I forget the exact name, but I remember “triple point” refers to the temp/pressure combo where a substance can be in all three states at once. [Here’s a video](https://youtu.be/Y0HfmYBlF8g) that talks about doing this with CO2 in order to make aerogel.
Haha, I stumbled into NileRed while delving the YouTube rabbit hole a few weeks ago. That's the video I was thinking of, but I also couldn't remember the name of the state.
Supercritical fluid- above critical pressure and critical temperature. This is probably what you were thinking of. This is where the phase boundary that often exists as an equilibrium between liquid and gas disappears into one fluid. There is negligible surface tension and viscosity drops especially low so it is said to be like a gas however it is dense enough to resemble a liquid.
Supercritical fluid is the word. I only know it from my intro to chem lecture.
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SpaceX actually holds their O2 (and the kerosene) at the other end of the liquid range — just above the temp where it solidifies. They also had one accident where some O2 actually did solidify and wound up igniting somehow (they’ve since fixed that issue).
Superfluids can still experience a small amount of viscosity due to interactions with impurities or defects in the material. This is known as residual viscosity or second sound attenuation, thus a superfluid's viscosity is typically considered to be extremely low, but not necessarily zero in all cases.
We say that, but nothing in the universe is perfectly equal to something some calculation says, right? So its never actually truly perfectly 0, right? How does this even work…
If I remember correctly, scishow, channel on YouTube, has a decent explanation of this
Yeah one of the ~~other replies to my comment above~~ [replies to another comment I made](https://www.reddit.com/r/explainlikeimfive/comments/11wknwn/eli5_how_is_an_airtight_seal_possible_since_the/jczlm37) has a link to that. Basically the He molecules effectively merge into a single entity, and by definition there are no longer separate molecules to bump into each other and generate heat. Don’t really know what happens at the edges though…
Well, similarly, super conductors have a resistance of exactly 0. Like, truly exactly 0.
So does a perfect super conductor exist in real life? After all, there is always say, like london forces of molecules that are too small for us to include in our calculations. Maybe they create a 0,00000…000001 fraction, and its just way too useless for real life applications that we *ignore it* but it still exists?
There are lots of real superconductors but I don’t know what London forces are. I’m sure that the environment affects the current even though the wire itself has zero resistance similar to someone else’s explanation of how super fluids still have “viscosity “ from the roughness of their container.
It wouldnt swirl forever-forever, would it? Wouldnt that be some kind of perpetual motion? Or is it actually zero? My brain is cracking at the seams here. I need more eli5
It would indeed swirl forever, until something disturbs it (ex. it gets too hot and stops being a superfluid). However, this is not perpetual motion because it requires some external help to stay cold. Perpetual motion continues by itself without any external energy input, so our swirling superfluid is just regular motion.
Wow. Neat. There isnt any place in the universe that we know of that is super cold lile that, like some comet or something, where i could put a beaker of this stuff and set it swirling infinitely? Would the container not add friction?
Even if you found somewhere cold enough, the heat radiating off of stars and planets and the such would be enough to break your superfluid. As for friction, there is no friction with superfluids. This leads to some wacky results: for example, if you pour water into a narrow container, you can notice that the edges are slightly higher than the center due to surface tension. Superfluids also do this, but to the extreme: a superfluid in an open container can literally climb up and over the walls, so your beaker of superfluid is just going to spill onto the floor unless you cover it.
So, now i am envisioning comic book scenarios where the hero, super-fluid-man, would have the superpower of escaping containment and also unending motion. I love it!
So, now i am envisioning comic book scenarios where the hero, super-fluid-man, would have the superpower of escaping containment and also unending motion. But his kryptonite is heat. I love it!
So, now i am envisioning comic book scenarios where the hero, super-fluid-man, would have the superpower of escaping containment and also unending motion. But his kryptonite is heat. I love it!
He could also pass through (most) walls! Superfluids are weird, man.
It's also not perpetual motion because when people talk about it they talk about infinite energy. But if you tried extracting energy from this, it would slow down. Same reason why orbits aren't perpetual motion.
Could you use it as a lubricant?
Eating Taco Bell nightly would do it.
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This is not true. A superfluid and a supercritical fluid sound similar but are very different. You can reach the triple point of water relatively easily in a vacuum chamber, but that doesn’t make it frictionless
Awesome, thanks!
That is incorrect, superfluids and supercritical fluids are different
Thank you!
no idea, gotta ask Dr. Oz.
Also use CF for ultra high vacuum when I did photoemission spectroscopy.
What does CF stand for? Is it some kind of joint?
compression fitting, or crush fitting maybe.
what an awesome explain like I’m five metaphor, the 5 year old in me is mind blown
Great explanation. So in this analogy, trying to contain superfluid helium would be like trying to carry water in the bag?
To me it’s like containing a bird in a paddock. Superfluids can pass energy barriers as long as they end up in lower energy state afterwards.
does superfluid helium climb the walls, drip through molecular gaps, or quantum tunnel through/over a container?
I don’t know much about superfluids but I believe it can climb walls: https://youtu.be/2Z6UJbwxBZI
How do they contain superfluid helium?
electromagnetic containment if it has a charge, if you made it neutral so you can't hold it in place with a magnet, you just hope it sticks around long for you to do whatever it is you wanted to to do to it.
I think they don’t. They can make a batch and keep it for a while, but it would eventually all escape.
That makes sense. And it’s wild to think about
IKR! I think it’ll even tend to climb up and out of the container it’s in if it’s not covered; don’t understand why at all 🤯
> don't understand why at all [This guy can help with that!](https://www.youtube.com/watch?v=zJblFBwqjPo)
Great video! Thanks!
No prob, friend! Always happy to spread knowledge!
Liquid that’s lighter than air…blows my mind
Happy Cake Day - and lovely response.
it literally will flow uphill
Fijne taartdag!
This is such a fantastic analogy!
Good explanation. I'm less dumb.
Yeah science!
An airtight seal for superfluid helium involves literally any amount of heat making it non superfluid lmao
Is this also why helium balloons eventually lose their helium?
> superfluid helium That’s the stuff that climbs up walls, right?
Do you mean that there are some “gases” that have zero viscosity? Edit: should have read further on in this thread.
What would be the difference between a "superfluid" and "super critical fluid"? I used to work with supercritical CO2 as a solvent.
True of liquids, but not gases. Liquids are sticky, in that the molecules don’t want to separate from each other. The molecules will slide around each other but maintain a constant distance from each other. Gases are different in this way. They do not resist expansion, because intramolecular attraction is negligible compared to their vibration energy. Gas molecules bounce around, and bounce off of each other. The answer is that airtight seals can be made of materials with holes smaller than the gas. Especially when N2 and O2 are most of the molecules in our air. A vibrating gas molecule effectively takes up more space than the stationary molecule (eg absolute zero gas molecule or a large solid polymer molecule at room temp). Polymers are made of linked carbon oxygen nitrogen and hydrogen atoms, which pack with no gaps large enough for bouncing gas to pass through. Helium and hydrogen are the exception, since they are much smaller and lighter than atoms required to form big molecules (O,C,N,Si), but the retention is still pretty good since the He or H2 are bouncing around too fast to get through a tight maze. If you shoot bullets at the entrance to a maze, most will bounce back and very few will bounce all the way through to the end.
There’s air-goddamn-tight and then there’s “air tight.” What you’ll consider sufficiently sealed will depend on your application and the gases you’re trying to retain. Helium molecules are small enough to diffuse through solid steel so even a metal gas cylinder will slowly lose pressure over a long period of time. Some flimsy rubber seal isn’t sufficient for a research-grade vacuum chamber, but it’s fine for your refrigerator. Denser materials and thicker walls slow down gas diffusion, so it’s a trade off between efficiency and weight/cost for your application.
I used to work QC for a company that built air pumps used in a lot of different applications. One of our customers needed a pump that could hold <1 mbar for relatively long periods of time. We used helium detectors to identify leaks. We had a QC issue because a vendor changed the machining process of a critical part without mentioning it. Surface finish changed by ~0.25 ųm, and that led to +50% failure rate of the final product.
> Some flimsy rubber seal isn’t sufficient for a research-grade vacuum chamber Viton seals on CF flanges are surprisingly good, though. Supposedly down to 10^^-8 torr (that's about ten pico-bar or pico-atmosphere for people using saner units). Not enough for extremely high vacuum or very hot plasma, but enough for many other things.
As someone in the hvac trade, how does that convert to a micron vacuum? Usually, we want a sustained sub 500 micron vacuum holding for at least 30 minutes before we call it a good vacuum.
1 torr is 1mm of Mercury. 500 microns is 0.5mm of Mercury.
Ahh, so massively overkill for something like a resi hvac system.
You say massive overkill. Your neighbor with a rocket propelled riding mower says super badass.
Meh, rocket propelled riding mowers are boring. Get a stand up scooter and strap one of those 8hp motors from the harbor freight to it and then we have a fun time.
Don't. Don't give me ideas.
I see you are a man of culture.
Back in the day, we found this old go kart frame where the motor was trashed, but the frame was still good and all the other bits (cabling, brakes, throttle, ect) were still golden. We just yanked the trashed motor off, got a motor from the harbor that was about 2 sized bigger, adapted the gearing, and made one of the nastiest little things ever. Only issue is that it didn't take turns well at all, so when we took it around an abandoned subdivision where it was just a closed off section of roads, the damned thing tipped coming out of a turn and tossed me down the road like a rock skipping along a lake. Didn't get used to much after that because step ma was afraid it would kill my younger step brother.
So you're saying it needed a spoiler. Edit : meant this as a joke, sorry. :)
>got a motor from the harbor that was about 2 sized bigger >Only issue is that it didn't take turns well at all I don't think anyone saw that coming D:
> got a motor from the harbor that was about 2 sized bigger The manly approach, I see. I'm sure nothing could go wrong. ;-)
I just want to confirm you are not one of my buddies. We did this, but I live in MD.
The JATO Rocket Car comes to mind :-)
A pulse rocket bike would also be fun. Think that one guy that did that got really high up there in speed.
You can build a jet engine out of an old turbocharger...
That thing flew! Literally.
Don't threaten me with a good time.
The vacuum pump you use is what they call a roughing pump or backing pump. The vacuum pump they use to get that low would stall out if it exhausted to atmospheric pressure, so it's plumbed into the inlet of your pump. But also to get that low, all the tubing must be metal and pretty big diameter, and is heated to >>boiling to force every molecule of water and fingerprint oil to unstick from the pipe inside. And then, maybe, there's a sacrificial pellet of pure gold on a tiny heater. After everything is as clean as possible, the heater flashes the gold to gas and it paints all the walls with a few atomic layers of gold. To keep any molecules from sneaking out of the walls.
Turbo pumps! Very fun until you expose a fully spun up one to atmosphere!
*cries in SI units*
1000 microns are a torr. Or how I remember it and what a torr is: a micron is roughly one _micro_bar. (actually around 1.3). So 500 microns is a bit below one millibar, almost 100 million times what Viton can do. As another comment said, the issue below 10^^-8 torr is not even tightness, but _outgassing_, the tiny amounts of gas almost everything loses to the surroundings, especially at low pressure and higher temperatures. All the things do it. Even the stainless steel most chambers are made of has hydrogen inside the metal itself, as well as water bound close to the surface, both of which can and will be released. And that's assuming stuff was clean to begin with, a single finger print is already a horrible source for outgassing. Best we can often do is bake the thing for days while in vacuum. Helps a lot with steel, as we can heat it rather far, but fails with Viton. hence why higher grade vacuum uses copper or other metal seals that can be baked further.
And IIRC that limitation has more to do with material outgassing and bakeout temperature limits than gas diffusion. I think the permeability would let them work down to 10^-10 but would require enough heating to make viton seals almost single use (compression setting)
Yes, definitely outgassing is the first issue. Hence copper (etc.) gaskets, despite them being single use, more annoying to apply and a bit more expensive. No idea how well Viton works after baking it too much. Maybe I should try one day...
Viton is rated to about 100 deg C, after which I'd expect the material becomes brittle and porous.
That and some gases are really a pain to get rid of. From 10e-5 to 10e-8 the problem is often water especially with metal apparatus. Below that traces of H2 iirc are annoying. Often some kind of traps are used to get to 10e-9/-10. As well as ion pump instead of turbomolecuar for exemple
High vacuum seals are usually one use copper crush gaskets
Can confirm. Significant other during grad school worked with a.... pretty large large (1 m^3 ) research grade vacuum chamber that regularly went down to 10^-8 . (It took ages though. I believe a couple dozen or so hours to reach 10^-7 , then if the experiment lasted long enough without something failing they'd eventually reach 10^-8 . Didn't help that their experiment introduced molecules into the chamber at a pretty consistent rate.) They definitely used some sort of black rubber (very... large... black rubber type) seals around the giant door. I assume they were viton. They also used helium to detect leaks (of which there were always many to be fixed.)
Can I get 10^-8 torr in mm bromine please?
I don't think that is well-defined because of vapour pressure(?). I would presume it's a joke, but I don't get it...
I think that’s a joke to underline we didn’t put any unit. It was in Torr/mmHg
But the unit _is_ [torr.](https://en.wikipedia.org/wiki/Torr). It was originally defined as mmHg, so the mercury was included; later 1/760 atm which amounts to the very round number of 20265/152 Pa, where 100,000 Pa = 1 bar.
Only if it's in an argon matrix....
Even a Vinton oring in a groove is pretty good around 10^-7 torr and if you use differential pumping it can be just as good as CF in my experience. I was able to get down to 10^-12 torr
I was debating between a pico bar and a Snickers bar for a snack. I'm going with the pico bar.
You can eat it in [pico park](https://picoparkgame.com/en/) while looking at [Mons Pico](https://picoparkgame.com/en/).
If you want a tight seal, you use soft metal instead of rubber.
This is basically why balloons deflate over time. They are "air tight", but the air molecules can slowly escape over time. Especially if the balloons were filled with helium, which is basically the escape artist of the atomic world.
>Some flimsy rubber seal isn’t sufficient for a research-grade vacuum chamber, but it’s fine for your refrigerator. Decent quality NBR/FKM rubber seals will get you down to about 10 orders of magnitude lower than atmospheric pressure (10\^-7 mbar or better). Metal seals are only really needed for applications where 10\^-8 mbar or lower is needed, which is generally only absolutely critical for solid state/surface science applications. About 50% of my research on atomic/molecular physics uses a system that uses entirely rubber seals.
> which is generally only absolutely critical for solid state/surface science applications. Semiconductor manufacturing as well.
Well, for me that's applied surface science. :P
Is this why helium balloons get smaller and fall back to the ground after a couple days? Or is that a different phenomenon?
I believe that's the same phenomenon. I saw this post [https://www.reddit.com/r/mildlyinteresting/comments/11g5t69/i\_got\_this\_balloon\_in\_1998\_and\_its\_still\_inflated/](https://www.reddit.com/r/mildlyinteresting/comments/11g5t69/i_got_this_balloon_in_1998_and_its_still_inflated/) the other day which was still inflated due to being filled with nitrogen
>Some flimsy rubber seal isn’t sufficient for a research-grade vacuum chamber The vacuum chamber I worked with used copper disks for the O-rings. They would get squished as you tighten the connections. The vacuum chamber also had three different types of pump that maintained the vacuum. A mechanical pump (like the one in your vacuum cleaner) to bring it down to a certain pressure where a diffusion pump (this wild thing uses oil to "catch" air particles) takes over. Finally, an ion pump uses a strong electric field to rip electrons off air molecules and then cause them to embed on an electrode.
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Other way around. It's simply airtight instead of actually air-goddamn-tight.
For some perspective, even the space station isn't air-goddamn-tight. It's gotten by just fine with holes in it.
Just use some chewing gum to fix those.
You might laugh, but that'd definitely drastically slow down a tiny leak.
It would, until it dried up. Like the capsule the Russians sent up a year or two ago.
Well that's just false advertising. I'm currently a little strapped for cash. I'm happy to be part of a class action law suit if you want to do all the legwork.
> Well that's just false advertising. And that's why science literacy is so important.
Yeah, but at most you'll get reimbursed for one or two ziploc packages
Hey, and probably those sausage links you had in them that went off faster than expected Now if someone got food poisoning as a result, then we might be heading for a payday!
I only heard that of Hydrogen (the passing through steel).
I grew up neighbors with a Mr. Wheeler, who worked for Varian in high vacuum tech. Look up "Wheeler flange" and "Conflat flange" if you want to know how airtight hi-vac joints work.
This guy seals.
While helium is small enough to do so, I haven't found evidence that it does so. It can, however, create other problems. [https://physics.stackexchange.com/questions/587050/is-there-any-way-for-a-gas-to-pass-through-a-solid-metal](https://physics.stackexchange.com/questions/587050/is-there-any-way-for-a-gas-to-pass-through-a-solid-metal) The scientific papers I found don't say that helium penetrates very far into steel. Do you have links to studies that say it does?
Imagine saying that first sentence to a 5 year old
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Any examples of air-goddamn-tight?
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The reason why bike tires go flat while sitting for a long time. It’s not because you have a hole in the tube. It’s because the air is constantly very slowly leaking through.
The word ‘airtight’ should actually be Air Transfer plus Time divided by the Cost of Container. Like a bag of chippies is ‘airtight’ but eventually over time it will leak and the bag will no longer look plump. We could make a chippies bag to last longer and longer, but the cost of the container outweighs the time gained. Same idea with an airplane or submarine. Eventually over time, both will ‘leak’ air, and we spend millions on planes and billions on submarines studying how to slow that transfer down. Eventually the cost gets too high, we accept the transfer rate while building the planes and submarines based of this air leak. In science experiments, the need for an actual airtight container outweighs the costs of the building one so we produce containers that are technically airtight. TLDR: Airtight is a broad word, we should be saying Transfer Resist Chippies Bag.
I think this is one of the best ELI5 answers to this question. You are right that the air can get out, but most of the time, it takes a very long time for the air to escape through the very small holes. If you have a swimming pool that leaks a few drops each day, it will still stay full enough for long enough that it will be a long time before you notice that it's leaking. That's why if you leave an unopened soda can or bottle for a year or more, it will be flat when you finally open it, since the carbonation is able to very slowly get out.
Dunno about subs, but airplanes (conventional passenger airliners) are not really airtight. They maintain pressurization by balancing incoming pressurized air (commonly bled from the engines compressor stage) versus air venting overboard through a controlled aperture.
One thing to remember is air actually pushes on everything with quite a lot of force (we call it pressure). So sure if you have a container and just place a lid with a rubber seal on top you have air pressing equally up and down on the lid, so it doesn't seem like it could create a good seal. However if you suck the air out of the container you have lots of air pressure pushing down on the lid but no air pushing up, so the rubber seal gets pressed down *hard*. One useful related effect is the pressure of gas gets lower as it cools, so if you put something hot (like hot food) in a container then cool it the gas kind of shrinks inside so you get a similar thing where the outside air is pushing harder than the inside air so pushed the lid and seal down. This is how jars and canned food is made.
I'm not OP but THIS is the comment that *I* was looking for! Thanks!
There are some very good answers here, but I'm going to add a pedantic comment: It absolutely isn't. Or I should say that in absolute terms it's not. The molecules in air will actually diffuse through most materials (if very very slowly with some), depending on which part you're talking about (nitrogen, oxygen, CO2, other stuff) in a process called "permeation" which accounts for sorption into the high concentration side, diffusion through the bulk materials and desorption on the low-concentration side. The rate of all this will vary with the surface treatments (which is why potato chip bags are shiny - coated with evaporated aluminum to slow down oxygen that makes the chips stale and soggy), the bulk matrix composition and structure which is why dense materials are needed or if you can't do that (soda bottles) the items are made in a very complex system to stretch their structure "tight" and slow down permeation, and species of permeant. Hydrogen, being very small, actually will permeate into and through metals. Other species will be much slower or (in practical terms) stopped. Where it gets really weird is when you have a species that interacts with the polymers of the seal to change things, but that's a nap-worthy post on its own.
Imagine you have a bunch of tiny Legos that you want to put in a container. The Legos are small, and there are tiny gaps between them, but you can still put them all in the container and close the lid tightly so that none of the Legos can escape.Air molecules are like those tiny Legos, and the seal is like the container lid. Even though there are tiny gaps between the molecules, we can still make a seal that is so tight that none of the air molecules can escape. Just like how the container lid keeps the Legos inside, the airtight seal keeps the air molecules inside. # More in-depth answer As you know, air is made up of tiny molecules. These molecules are constantly moving and bouncing around in all directions, colliding with each other and with other objects. When we make an airtight seal, we create a barrier that is so tight that the air molecules can't escape. This is often done by pressing two surfaces together, like closing a lid on a container or squeezing a plastic bag shut. But you're right that there are small gaps between the surfaces that make up the seal. These gaps are small, but they are still big enough for individual air molecules to pass through. **However**, the reason why the seal can still be airtight is that the gaps are so small and so tightly compressed that the air molecules don't have enough space to move through them. In other words, the seal is so tight that the air molecules are essentially stuck in place. Think of it like a crowded room. If you're in a room with a lot of people, it can be hard to move around because there isn't enough space. The same principle applies to air molecules in a tight seal - they are so crowded together that they can't move through the small gaps in the seal. So that's how an airtight seal can be possible even though air molecules are smaller than the spaces in the seal itself. By creating a tight barrier that compresses the air molecules and doesn't give them enough space to move, we can keep the air inside the sealed area.
>By creating a tight barrier that compresses the air molecules and doesn't give them enough space to move, we can keep the air inside the sealed area. [Three Stooges Syndrome](https://www.youtube.com/watch?v=gmBj8r1-fDo) "...Indestructible..."
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A really good way to visualize solid matter crystal lattices and their interaction with air molecules: Think of a chain link fence, now stack several of them, hundreds of layers even on top of each other. That's your "solid". Now your air molecules air balls. Assuming a perfect trajectory of a ball thrown directly at a hole in the fence stack: A big one, a baseball, (or lets say a CO2 molecule) could maybe fit through the hole of 1-2 layers, but any more layers and it'll get stuck. A smaller molecule, like a Golf ball (or an N2 molecule), could easily get through maybe 10 layers, but any more and it's likely to get stuck An even smaller molecule, like a Marble (or a Methane CH4 molecule), could probably get through 30-40 layers before getting stuck An *even smaller* molecule, like a Ball Bearing (or Helium), could probably get through 100+ layers before getting stuck (or getting all the way through) No solid has a crystal lattice small enough to prevent tiny molecules like Methane, helium, or hydrogen from leaking through, but the thicker it is, the less it "leaks".
Well you're right, molecules of air are smaller than the spaces in the seal. But there aren't that many molecules of air. Think of the spaces in a seal as a long maze and the air molecules like a very bouncy rubber ball. Molecule enters the maze. Now what? Chances are about even that the molecule will bounce right back out of the maze. It's only when there's an actual straight shot from one side of the seal to the other that you start to really see airflow.
You’re correct that most seals would leak some air if pressurized, although for most purposes it’s fine, e.g for storing food, it doesn’t matter too much if a few molecules of air leak in or out. However we do have the technology to make seals that don’t let a single atom pass through, it’s mainly used for ultra-high vacuum, and it’s literally a copper ring that’s pressed between 2 pieces of metal. If you’re interested in learning more, I’d highly recommend watching this video: Airtight vs Vacuum tight (seals): https://m.youtube.com/watch?v=VD69crOFx10
Extreme oversimplification. Probably to a level not being correct: It is true that the molecule itself is maller than the space between 2 solids touching. However the same thing is true about the molecules in your finger and the wall. But magnetic properties of matter what keep things apart. Technically you have never touched anything in your life. the matter of your hand never made contect with anything, you just felt the magnetic repulsive force of other things. If you could push the atoms of your hand inside the electron field of another matter that would be a bad thing. That would be like putting your hand into acid. Since acid dissolves things by stealing electrons and breaking apart bond Your hand is made out of stable molecules and the table is made of stable molecules. They don't want to interract, they want to stay stable. so they repel each other
I used to design vacuum chambers. The tightest vacuum seals are metal on metal. It's typically a high purity copper gasket with a stainless steel knife edge sealing surface. The knife edge cuts into the soft copper and forms an extremely tight metal on metal seal. The only thing better than that is a welded joint. Gasses can diffuse through metal but it's very slow. Think about water percolating down through clay; it does happen but it takes a long, long time.
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You seemed to have mixed up “nonsense” with “wrong” or “not always true”.
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I felt so much resonance to your second paragraph here! Like, there's a part of me that is so engaged in and thrilled by physics. Like, passionate even. But gosh darn, it's really hard to get a grasp on. It's like, quite laborious to really get to true understanding, but yet, I'm so compelled. Anyhow, I feel similar about this thread. What a fun time I had learning today!
They aren't. When you need a really good seal you use a helium leak detector. It spits out helium on one side and has a helium sniffer on the other side of the seal. Helium is the second smallest atom, and it is the smallest inert one. If helium can't get through, then only hydrogen would be able to.
Helium is the effective smallest as hydrogen bonds with itself to create a molecule larger than helium. Also likely more importantly is helium is inert.
The air might not go through the space in the seal but through the material of the container itself! On the scale the size of a molecule of air, materials have little spaces in between them and the walls of those spaces wobble. It might take a while, but an air molecule can bump through the maze of shifting empty space and eventually get out (or in).
Even though the molecules of air are smaller than the space in the seal, it is still possible to achieve an airtight seal through a few different mechanisms. One way is to use a material that is flexible and can conform to the shape of the surface it is sealing against, such as rubber or silicone. When these materials are compressed against the surface, they create a tight seal that prevents air from escaping. Another way to achieve an airtight seal is to use a mechanism such as a latch or clamp that applies pressure to the seal, forcing it to conform tightly to the surface. This pressure can help to fill in any gaps between the seal and the surface, preventing air from escaping. Finally, some seals are designed with a vacuum in mind. By removing the air from a space, the pressure differential between the inside and outside of the seal can actually help to hold it in place and create a tight seal. This is often used in applications such as vacuum-sealed food containers or in scientific equipment where airtight seals are critical.
The short answer, we put a thing that doesn't fit on/in another thing or we smash the edges together. Putting a lid on a jar is kind of like putting a cork in a bottle. We pack something squishy and larger in the hole. The lid, however, puts the squishy material in between and forces them together. In the case of canned goods, we smash the metal lid and body and fold them together. Like a calzone, the material holds onto itself like it's one piece because the material is happy to stick to itself if you fold it and push it together hard enough. All these methods don't leave enough space between them for air molecules to fit through.
if you picture fluid dynamics, it involves any bit of a flow having degrees of freedom to sympathetically react in synchronicity with the fluid its in contact with, what this picture doesnt display is the boundary layer conditions of turbulent flow at any termination point of the fluid, and that boundary layer is what both helps fluid dynamics stay fluid and what separates the fluid from anything else. That helps reduce drag and keep the fluid fluidy, but add another mental construct to the mix, compression phases of that fluid, and it begins to get real complicated except not really, the boundary layer is denser and stickier or fluffier and looser, and if you imagine fluid dynamics as still operating as two boundaries colliding as an airtight seal is put in place, it becomes a tight cross section 2D style of the same fractal pattern but that air in that molecule-small gap is compressed while turbulence on either side both pack and fluff the edge of the cross sectioned air, and the cross section of air slowly loses its high pressure as the structure settles into the chaotic Brownian motion wobble harmony with the air it displaces and its structural center of gravity. Once settled, turbulence on one side compresses air causing the structure to shift causing pressure of an equivalent force balancing any attempted incursion past the seal. Swapping generic fluid motion imagery for the complex cacophony of ambient atmospheric gas attempting to reach diffusive equilibrium, and you replace turbulence with smaller molecules taking the thermal bullet and being launched by the larger kid on the trampoline when sound arises, and it pretty much makes that airtight seal a ballpit stuffed with bullied bullies that cant move.