从空气中自制液氮：科学实验与星巴克氮气冷萃的奥秘

Nitrogen is everywhere. It's a fundamental building block of life.

It makes up 78% of the atmosphere, AND it's in these tiny micro bubbles, in my Starbucks 'Nitro-Cold Brew.'

And yes, this episode is sponsored by Starbucks.

They challenged me to liquify nitrogen out of the air, and that's what I'm gonna be doing in this video.

But first I'm gonna try Nitro Cold Brew for the first time...

It's subtly sweet without having to add sugar, and obviously it's cold and frothy, and I'm gonna explain why nitrogen makes this possible.

But first I'm gonna try to liquify some nitrogen for myself out of this atmosphere. Come on, let's go.

Okay, in order to make liquid nitrogen, we need a way to make the air really cold.

And to do that, I'm going to use this cryo-cooler.

So essentially what's inside here is a cylinder of helium and a couple pistons.

There's a piston that compresses the helium down into this section, and when it does that, it makes that gas really hot.

And so, that heat gets ejected to the surroundings through this heat sink.

Then the helium is expanded down in here, and as that happens, it gets really cold.

So it absorbs the heat from the surroundings.

So this is the part of the cryo-cooler that is going to get down to less than -200°C. So cryogenic temperatures.

This is the place where we can really liquify some gas.

So, I want to put this to the test and first do a sort of simple proof-of-concept and make sure that this is working and just try to liquify air, liquify the air in this room.

This is Allen Pan.

Allen is handier with tools than I am, so uuh, I brought him here to help me make this happen.

How should people know you, Allen? Like what do you get into, what's your thing?

Allen: I'm a failed Mythbuster, and also I have a YouTube channel called "Sufficiently Advanced".

Derek VO: You may know Allen from his creations and, relevant to this project, shooting liquid nitrogen from his hands.

Allen: I'm definitely breaking your drill by the way ><, that plastic is coming right off.

D: I see that :) I see that.<

Did I measure this correctly?

Let's see.

Does it go in and stop right there-

Oh my God!

Look at that!

Perfect!

Derek: So now, let's turn it on, and you can listen, you can hear it turn on.

Ohhhh, the pump is starting.

Allen: Its purring.

This is the moment of truth: been running the cryo-cooler for about 4 hours.

Hopefully getting some liquid air in there.

Allen: I bet you 50 bucks there's at least 50 millilitres of liquid air in there.

Derek: "50 millilitres."

D: Sir?

Allen: That's not even that much.

Derek: It ISN'T even that much,>< but I will be amazed if it happens.

Allen: Oh, that- that's 50 millilitres, right there.

Derek: I bring you a 50 millilitre beaker.

Allen: Do you feel any sloshing??

Derek: [Whouf!] [Slight sloshing] I hear something..! >I hear something!<

Derek: Oh ohoh >< oh oh!

Allen: I HEAR something.

Derek: I can hear liquid

D: We have made liquid air.

Allen: That sounds like...

Derek: How much have we made?

Allen: 50 millilitres..

Derek: Is that how much we've made?

Allen: ..of liquid air!

Derek: OooohH..!

Cameraman: Oh hoho hoh!

Allen: 50! 50!

Allen: OOOhh!

A: Dude, that's 50 ml of liquid air.

Derek VO: I know it doesn't look like much, but to make 50 ml of liquid air, we needed to condense around 35 litres of air from the room.

Derek: What's funny to me is I've done a lot of experiments with liquid nitrogen, but this feels like the most precious... [Allen laughs] liquified gas that I've ever had because we made it ourselves.

There was more than 50 ml.

Allen: I know we didn't shake on it, but you definitely owe me $50.

Derek: I definitely do.

Derek: 'Cause that's impressive from just a few hours! Y'know, running it for the first time.

Derek VO: Liquid oxygen is paramagnetic, which means it's attracted to permanent magnets.

I'm going to use this effect to try to extract some liquid oxygen.

Derek: Something came out.

Allen: Hey, dude, you got it! You got some.

Derek VO: See that droplet at the end of the magnets?

To make sure it really was liquid oxygen, we tested how it affected a flame.

Allen: Yeheheha!

Allen: So that's 'for-sure' oxygen, that is liquid oxygen.

Derek: You can see how the matchstick REIGNITES with the liquid oxygen, and the flame spreads towards the higher concentration of oxygen.

Derek: Ohh!

Allen: Whwch!

Derek: There you go, we have made: liquid air.

Next step: remove the oxygen, and just make liquid nitrogen.

Derek: Okay, it is day two and time to scale up.

We are uh, moving the cryo-cooler to a bigger stand.

Allen: Should I just go?

Derek Behind Cam: Yeah, go for it.

Allen: I'm putting a lot of sawdust into your-into your carpet. I'm sorry.

Derek: [chuckles] That's okay, Allen :)

Derek VO: To make pure nitrogen, we need pressurised air.

So I've got an air compressor here in the garage.

Derek: The pressure of this compressor is about 10 'atmospheres'.

So I'm going to pressurise this line here...

behind cam: Now the first thing we've got on the line is... a filter to remove.. water vapour.

And to get pure nitrogen, this is the key component.

It is a nitrogen membrane, and what it actually contains is a bunch of hollow polymer fibres.

And so we supply compressed air in this end, and all of the gas molecules try to diffuse out of those fibres because of that high pressure.

But: these polymer fibres are made out of a material that is SELECTIVELY PERMEABLE.

It allows oxygen and carbon dioxide and water vapour to diffuse out much faster than nitrogen.

And so, those gases actually come out preferentially through these holes.

So what you're left with at the end is a high concentration of nitrogen.

And to get the highest purity, what you need to do is have a very high pressure and a slow flow rate, because that gives a lot of time for those other gases to escape.

Derek: Whoo!

D: I don't know about this gauge.

Allen: Let's put some money on it. How about 50 bucks says the oxygen is less than 5%. (<5%) Yeah?

Derek: Are you gonna bet that? 'cause I will easily take that bet.

Allen: Yeah. All right

Derek: I don't think this went very well because that was the first filling of the tank, so we weren't high pressure going through the membrane.

So I don't think it's going to be...

Allen: I think we're getting 95% nitrogen.

Derek: 95% nitrogen

So this meter measures oxygen.

So we're looking for a number less than 5%, which I suspect we will not see.

D: Huuh.

D: Can hear-

Allen: Oh, it going down!

D: Whoa!

A: It's going way down.

D: WHOA!

Allen: Come on 5%! Fi-

Ohhhh!

There it is!

Derek: Whoa!

D: Here we go 99.

Allen: Over 99%, 99.2, 99.3

Derek: Whoa, I am shocked.

Allen:There is no oxygen in this air, baby, it's all nitrogen!

99.5.

Derek: I am so shocked right now!

Allen: So we're at 50 bucks for the liquid air bet?

Derek: Yeah

Allen: We're at 50 bucks for the oxygen.

Derek: Yeah, you just have made yourself 100 bucks.

Look at that, 99.7%..!

Allen: ..% nitrogen!

Derek: This is insane, and you can understand why there would be an alarm if, uh [Allen laughs] you were in a room with Both: 0.3% oxygen.

Derek: Yeah.

Allen: I knew it. I told you.

Derek: You knew that you would win.

Allen: Didn't I tell you? Yes!

Derek: Okay. So we've achieved, I guess, the second part of this, which is isolating nitrogen from the atmosphere.

And now we just have to...

Allen: Get it super cold.

What we got to do is we got to put this thing.. here.

Derek VO: For better insulation, we scaled up to a dewar.

Derek: That's impressive.

Allen: Kinda sci-fi, janky garage, sci-fi kind of a look.

Allen: Go ahead and lower that.

You know just squish right on.

Derek: I hope.

Allen: I mean that's sealed, y'know.

Allen: I think, uh, honestly? We could just turn it on and see if it works at this point.

Derek: Do it.

Just letting some nitrogen in there, so that when the nitrogen goes in there, it'll push out any... remaining air that's still in the tank.

Derek: Gonna switch on the 'variac' which powers up the cryo-cooler.

The other thing I need is the fan...

A: Yes.

D:This is the fan.

I'm going to plug THAT in.

That's a cooling fan of the cryo-cooler.

We're up to 8 Watts.

Allen behind cam: OoOooOh!

Derek: Okay, I have been running the nitrogen setup for about 3 hours, and my concern - because the temperature on the thermo-couple never got very low - was that I wasn't actually making any liquid nitrogen.

So I've stopped the test and I just want to see... whether or not there is anything in here.

Doesn't sound like it.

Nothing..!

Huh.

What I think could be part of the problem, I was putting nitrogen possibly at a rate that's too high.

So this is effectively nitrogen coming in at room temperature.

And I wonder if it was just too warm for it to ever reach the liquifying temperature right here at the end of the, uh, the cold finger of the cryocooler.

So I feel like I need a slower flow rate, maybe a better way to introduce the nitrogen into the dewar.

Not- as- easy as I'd hoped.

Okay, my plan is to go with a smaller flask, ..something like this and, we are going to use its lid to make a nice tight seal.

Except we're going to cut a hole through the top.

Allen: I think it's good enough for this coldfinger.

Derek: Alright.

Then we're going to try to drill a hole in the side for the supply of nitrogen.

We need to make nice tight seals around both of those things.

So that is going to be the challenge.

This is the kind of- the new design here that we've got.

Derek: I feel like it's pretty good.

Allen: I think it's going to work.

I think we're going to wake up, and there's going to be liquid nitrogen in there.

Allen: I bet you $160 that there is liquid nitrogen in there.

Derek: I don't know if I'll take that.

I'm gonna turn down the cryo-cooler.

Aaand.. let's see what we got..?

It looks cold..

Allen: Yeah, there's definitely some condensation.

So there's something cold in there.

What do you see?

Both: OHHHH!!!

Allen: THERE'S LIQUID NITROGEN!

Derek: Haha!

Allen: There's like, There's like THIS much.. That's liquid nitrogen!

D: We have: some liquid nitrogen out of the air.

Allen through laughter: That was so much effort for this much...

Derek VO: So, what do you do with homemade liquid nitrogen?

Well, make homemade ice cream.

Just a bit of cream and sugar, and then the liquid nitrogen.

Derek: The most precious liquid nitrogen I've ever had.

Is it enough to make ice cream?

Come have a look, have a look.

That is delicious.

By freezing everything so fast, because it is... -196 Celsius, you get really small ice crystals.

And so that gives the ice cream a really smooth flavour, smooth texture.

In a lot of stores, Starbucks too generates nitrogen just out of the air, because- Why not? It's an available resource.

They purify it and then infuse it inline into the drink, creating millions of those tiny bubbles.

That gives it its unique texture.

So nitrogen bubbles are much smaller than the bubbles you're normally used to: the carbon dioxide bubbles that you find in soda.

The other nice thing about nitrogen is that it's totally inert.

So it doesn't react with the drink.

CO2 reacts with water creating some acid, which you might like in soda.

You might want a little bit of a sour tangy flavour, but you don't want that in coffee.

Hence, the nitrogen.

One thing I found really interesting when they were pouring this Nitro Cold Brew is that you can actually see bubbles coming down on the sides of the glass!

And that's strange, because these bubbles are obviously less dense than the coffee.

They should be rising up to the surface.

So why are they doing that?

Well, the answer is the nitrogen bubbles are rising up to the surface a lot in the center of the drink.

And so that actually creates a current that pushes them down around the outside.

And so that's why you see those bubbles cascading down at the beginning when you're pouring the Nitro Cold Brew.

So there you have it: nitrogen is everywhere and useful for lots of things.

If you want to try out a Nitro Cold Brew, I'll put a link to order it down in the description.

And thanks for watching.