The Unknown Phase of Matter

This video is sponsored by Shopify. There are only two of these devices in the whole world. One is at the AHA Science Center in Estonia, and the other one is here in my studio. I can't have caffeine, but I love coffee. It's okay. I'll just get one of those super critical CO2 devices. Yeah, I'll do that. >> This thing on top is a transparent vessel. The internal volume's only about the same as a petri dish, but the glass walls are incredibly thick. about 40 mm. That's because it's designed to withstand 80 times atmospheric pressure, which means I should be able to witness something incredibly rare inside. Something that is like a gas and like a liquid, but it's neither of those things. And no, it's not a plasma either. Also, to be clear, this bit on top, there's more than two of those in the whole world, but it's this thing underneath that makes it really special. I'll get to that in a minute, but first, let me show you what it does. First, you add dry ice. That's solid carbon dioxide. And then you tighten all of these bolts to exactly the same tightness. In this case, 40 Newton meters of torque if you want to be exact. That seals the container. Except I've left this little valve open, which is why you can hear it hissing. That's because, as you probably know, solid carbon dioxide turns very quickly into a gas at room temperature, skipping the liquid phase altogether. That's called sublimation. But let's see what happens when I close that valve. The dry ice continues to sublimate into a gas. But because the gas has nowhere to go, the pressure builds up. And it turns out that at really high pressure, solid carbon dioxide no longer sublimates straight into a gas. Instead, it melts into liquid carbon dioxide. Now, this is pretty rare in itself, but it's not the really rare thing that I want to show you. Interestingly enough, you probably encountered liquid carbon dioxide many times in your life because it's in common dioxide fire extinguishers. It's just that you can't see it with your eyes because the container isn't transparent like it is here. So, eventually all the solid carbon dioxide is gone and you're left with liquid carbon dioxide at the bottom and gas carbon dioxide at the top. But here's the really cool part. What happens when you heat up this pressurized vessel? To find out, I could blast it with a heat gun or something, but we actually have a much better solution. And that's what this mysterious black box is all about. You see these copper pipes that hug the container? Well, they go down into the black box. And inside there are two tanks of water. There's a shower heater and a water cooler. And by pressing these buttons, I can pump hot or cold water through these copper pipes, which is great because it means that I can get an instant even heat. So, let's see what happens. Well, it continues to boil for a while, but instead of just boiling off completely into a gas, at some point that line between liquid and gas goes cloudy and then just kind of fades away. So, what's in there now? Is it a liquid or a gas or something else? And what would happen to a tiny boat that was floating on that boundary between liquid and gas when it started to blur? And most importantly, why do people put coffee beans in there? You know, [clears throat] this is a video that I've wanted to make for ages and I've finally got the apparatus to do it. What you're looking at is super critical fluid. To understand supercritical fluid, we really need to understand phases in general because they're weirder than you think. Take dry ice for example, that goes straight from a solid to a gas, which seems weird, but actually that's the sensible thing. It's melting that's weird. I mean, think about it. You've got a solid, all the molecules are stuck together, and then as you heat it, the molecules on the outside surface get enough energy to be able to escape the surface, and they fly off as a gas. Like, why would they hang around in this weird state that's half stuck together, half not stuck together? And in fact, everything supplimates in a vacuum of space. So why do we see liquids here on Earth? To figure it out, let's look at a phase diagram. So this phase diagram shows you when carbon dioxide will be a solid, a liquid or a gas depending on pressure and temperature. So here's atmospheric pressure. And look, as temperature increases, you go from a solid to a gas. That's sublimation. But then as you increase the pressure, a new possibility emerges, the liquid phase. In other words, you only get a liquid phase at high pressure, which is why we have it here on Earth, because we're being crushed from all sides by atmospheric pressure. It's enough to hold the molecules together even though they can slide around each other. And it's also why we have liquid inside this chamber because the pressure is really high. But hold on. If I pick any point on this phase diagram, it gives me either a solid, a liquid, or a gas. But inside this chamber, I clearly have both a liquid and a gas at the same time. So, how is that possible? Well, look, at the beginning, we've set the temperature and the pressure to be here, let's say. So, the liquid wants to be a gas. Yes, I'm anthropomorphizing the liquid, but you should know my feelings on that by now. Okay. But as the liquid boils off, the gas has nowhere to go. So, the pressure builds up. It builds up until we reach the boundary between the liquid and the gas phase. And at that point, the pressure is exactly high enough to prevent any further boiling. So we're in equilibrium. In other words, in a sealed vessel, when you reach equilibrium, you end up on this line between a liquid and a gas. And so you can have both. So we've got liquid and gas living in harmony. And this meniscus is the boundary between the two. So then when we heat it up by pressing this button, we're traveling up this curve. And this is the really weird part. At some point this line just stops. This is called the critical point. So what happens if you keep increasing the pressure? Well, you end up in this region here called the supercritical region. It's not a liquid. It's not a gas. It's something in between. I was struggling to get my head around what supercritical fluid is. And then I had a bit of a breakthrough when I tried floating something on that liquid CO2. Action Lab did it a little while back with a little bit of polystyrene. Link in the description for that. And when I saw that video, I really wanted to try it for myself and run some of my own tests. And the results are really surprising. You know, sometimes when I show something cool in a video, like a little boat floating in liquid CO2, I think I should massproduce that and sell it. And then I think, ah, that sounds too hard. But very occasionally, I think it's worth it. And Matt Parker had a similar thought, which is why we set up Math's Gear about a thousand years ago. In the beginning, we used a blogging platform with an e-commerce plug-in. And in fact, we tried a few different platforms before eventually trying Shopify. And it was like, oh, okay, this just works. Like, it's a little bit more expensive than, you know, free. But we saved a huge amount of time and the shop looked way better and functioned way better. Basically, if you're thinking of selling something and you're looking at how to do it and you just want someone to tell you which option to pick, well, I've been through all of that and I'd say that for 99.9% of people, the answer is just use Shopify. Like, no one ever said, "I wish I hadn't switched to Shopify." You know, we sold the Assassin's Water bottle through Inquisitive. When I was talking to them about how they do their back end and everything, they literally said, "Oh, we just use Shopify." Like, why would you mess around with anything else? It does most things that you'd want to do out of the box, including selling in person. But for everything else, there's plugins, and you can even get your hands dirty with their templating language if you're that way inclined. I did. It was really fun. So, if you're thinking of selling something or you already do and you suspect there might be a better way, go to shopify.com/stevemold to start your trial. The link is also in the description and there should be a QR code somewhere on the screen as well. So, check it out today. You know, I might mass-produce a super critical device at some point in the future, but for now, you'll have to visit the AHA Science Center in Estonia if you want to experience it for yourself. Right, back to these float tests. What I've got here is a boat. So, the material itself is more dense than liquid CO2, but it floats because the boat disperses the liquid. Now, if liquid sloshes into a boat, it stops working. So, what will happen when the gas inside the boat and the liquid under it become super critical? Will they retain certain properties locally or will it all become homogeneous instantaneously? Making sure the boat didn't tip over or stick to the sides was a real challenge. By the way, I ended up with this design with little fins that nearly touched the glass walls, and it's got a slightly thicker base for stability. I did think about using Benie, of course, but I mean, that thing can't even stay upright. So what we see is that as the chamber heats up, the level of liquid goes down as it evaporates. But see how the boat is getting lower and lower in the liquid. That's because the gas is getting denser and denser, which reduces the buoyancy of the boat. We don't get to see what happens when it turns supercritical because the boat sinks before then. But look how a cloud of gas from inside the boat puffs out as it falls. You can really see how close they are in density. Like it's really different to how steam bubbles through water, for example. Shortly after that, the meniscus starts to fade, and we're super critical in that region. Moving the camera up and down, you can see that even after the meniscus is gone, there's still this region that bends light differently to the top and bottom regions. So, I think we still have gas here and liquid here, but it's super critical in the growing region in between. So, clearly something odd is going on with density. And it turns out that if you can understand what the deal is with density, then you can understand supercriticality. We saw earlier how a liquid and a gas can coexist in terms of the phase diagram. You just sit on that line if you want both. But like why why have two separate densities in the first place? Why doesn't the CO2 just spread out until the density is even everywhere? To answer that, we just need to think about minimizing energies. So, in the same way that a ball will roll down to the point of lowest energy, the molecules in a container will arrange themselves into a state of lowest energy. And just like the valley that a ball rolls into, there will be some kind of energy valley that the molecules roll into. The main thing that shapes this valley is an attraction between the molecules. So, imagine you've got one molecule here, another molecule here. This molecule is attracted to this one. it's going to roll down that energy slope. But when the molecules get very close, there's a repulsion force because of like, you know, ply exclusion principle or whatever, but it means that there's this steep wall on the other side of the valley. Now, we're not just dealing with two molecules. We're dealing with a whole load of molecules. So really, we should be thinking about this in terms of molecules either being all close together or all being spread out. So this direction represents close together. This direction represents spread out. In this simple setup, we expect all the molecules to roll into this valley to get really close together. In other words, to form a liquid or a solid. But there's something else influencing the shape of this valley, and that's entropy. So, if we had a bunch of energetic molecules all piled up together in the corner of a container, they would spread out over time because of entropy. So without the attractive force between the molecules, we would expect them to just spread out and get further away from each other until they reach the limits of the container that they're in. In other words, we expect the landscape to look like this. Remember, this direction represents the molecules being more spread out. So having all the molecules here is more spread out and therefore more like a gas than when they're all bunched up like this. But of course, the molecules are attracted to each other. So, we have to combine these two landscapes. We have to add them together. And something really interesting happens when you do that. Look, we now have two valleys. I'm oversimplifying here a little bit, by the way, but I think this is good for intuition. And we're describing something like Helmholt's free energy, and it means that there are two possible places for our molecules to end up. Some settle down here that represents molecules touching each other. And some settle down here, which represents molecules that are spread out and far apart. In other words, a liquid phase and a gas phase can both be energetically favorable at the same time. But then what happens when I increase the temperature by pushing hot water through these copper pipes? Well, the influence of entropy increases and the pressure goes up and you end up pushing these two valleys together. And when you do that, suddenly there's no distinction between a liquid and a gas. It's all just fluid. How cool is that? And it shows up in a really weird way in the phase diagram. If you take this route around the diagram, you transition smoothly between a liquid-like state and a gas-like state, which is basically what happens when you travel from the top to the bottom of this chamber before it's fully mixed. So when you step back, you realize that there isn't two separate regions on the phase diagram for liquids and gases. It's all the same region. And the only reason we make a distinction is because the region touches itself along this line. So that when you cross it, you get this sudden jump in density. I've also figured out why the boundary goes cloudy to begin with, and it's really cool. This is a heat map of density around the critical region. So see how the liquid and gas phases maintain very different densities right up to the critical point. So just beyond that point you've got this huge variation in density with just tiny changes in temperature. That means that when a substance goes supercritical its density can fluctuate wildly and refractive index depends on density. So when that supercritical region first appears the refractive index is bouncing around all over the place. So light gets scattered in all directions. And you'll know from watching my video about the Buddha board that when light gets scattered, things turn white, which is why the meniscus turns cloudy to begin with. Before I get into why people put coffee beans in there, I just want to show you the amazing things that happen when you cool the cell back down. How beautiful is that? It's like there's different weather phenomena in there. Got clouds, storms, mist, rain, even. So [snorts] cool. The cell has a little valve at the bottom for safely letting out the CO2. Look what happens towards the end. Dry ice suddenly pops into existence. I couldn't resist getting that in slow motion. So cool. What makes supercritical fluid useful is that it retains some properties of a liquid and some properties of a gas. So, it can be used for things like extracting caffeine from coffee beans. That's because it has low viscosity like a gas and so it can penetrate into the beans really easily. but it retains the solvent power of a liquid and it dissolves the caffeine out of the beans. I guess the beans have a range of densities because they're kind of spread out vertically which I wasn't expecting. It's cool to be able to see the density distribution of the fluid like that. Now, I hadn't set the gasket properly on this run and at one point it popped, releasing all the pressure and all the carbon dioxide very suddenly. But they were in there for several hours. So theoretically, I should have some coffee beans here with most of the caffeine removed. Let's find out. Oh, you're supposed to use green beans. See, when you roast the beans, that's when you create all these tasty coffee molecules. Unfortunately, those molecules are also soluble in CO2, so you lose all of that nice coffee flavor. Makes sense. A huge thank you to the AHA Science Center for making this device. They also made the Life-Size Pop Pop Boat, if you remember that video. You know, I emailed them about this when I saw they had an exhibit. I said, "Can you make one for me? I'll pay you." And they said, "No, you can't pay us. We're going to make you one for free." So, I'm just hugely grateful to them. You know, if you're ever anywhere near Estonia, you have to visit the AHA Science Center. They just do the most crazy stuff. It's so cool there. So, there you go. Super critical fluids. I hope you enjoyed this video. If you did, don't forget to [music] hit subscribe. And the algorithm thinks you'll enjoy this video next. [music] [music] >> [music]

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