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Super cooling is a phenomenon where water or some other substance gets colder than its freezing point without freezing. It can result in very cool demonstrations where you smack a bottle of liquid water and it freezes solid before your eyes. But water can also be super cooled in a way that makes it impossible to freeze even by smacking it. One reason to research this kind of thing is for cryogenics since the ice crystals are often what causes damage to cell walls. If you can keep the ice crystals from forming, you can have much better luck resuscitating your
frozen space traveler. The basic reason super cooling is possible in the first place is that the formation of ice crystals in water has two competing factions. The interior and the surface. The interior of ice has less energy than liquid water. So the interior wants to freeze. But the surface of ice, the boundary between the ice and the liquid water, the surface actually takes more energy to form than if it just stayed liquid. So it wants to stay liquid. Who wins? Well, volume is a multiple of the radius cubed, but surface area is only a multiple of the radius squared.
Which means big things tend to have proportionally large interiors, while small things tend to have proportionally large surfaces. So a really tiny ice crystal will have proportionally way more surface than interior. And the surface's desire to melt will win over the interior's desire to freeze. In super cooled water, any really tiny ice crystals that form melt back into liquid. Only once you get a large enough ice crystal, and the exact size depends on the temperature. Only with a large enough ice crystal is the interior proportionally big enough that its desire to
freeze wins out over the surface and the crystal can grow bigger and bigger. At4°C, the tipping point is when crystals get larger than around 20 nanome in size, aka about 70 water molecules across. At colder temperatures, the tipping point is smaller. For example, at -10°C, you only need a crystal larger than around 10 nanome in size, about 30 molecules across. This battle between interior and exterior is why pure water cooled below the freezing point won't necessarily freeze right away. It remains as a super cooled liquid and has to wait until a
crystal forms that's bigger than the tipping point and can keep growing and freeze the whole bottle, which can happen by waiting or by cooling the water more so the tipping point crystal size gets smaller. Or if you have other stuff like minerals or dust particles in the water that can jump start the ice crystal formation by being bigger than the tipping point. or if you provide a jolt of energy, like by smacking the bottle, then a crystal can get above the tipping point size and freeze, which is all well and good, but you can actually stop super cooled water from freezing at all. I
made a previous video about how since water expands when it freezes, if you try to freeze water inside a rigid container, any of it that becomes ice will expand and pressurize the container, eventually stopping the rest of the water from freezing. So, if you have super cooled water inside a rigid container, there aren't just the two competing factions of interior wanting to freeze and surface wanting to melt. There's a third player, pressure. Pressure makes it harder for ice crystals to grow. So pressure, like the surface, wants ice crystals to melt. Obviously,
if you have a large amount of water, a tiny amount of it freezing into ice and expanding doesn't pressurize the remaining liquid very much. So, a lot of ice can form before there's enough pressure to matter. Pressure simply doesn't come into the picture until way beyond the tipping point size for runaway ice crystal formation. But if you have a tiny amount of water, the pressure can be substantially affected by the formation of even a single tiny ice crystal. For containers that are smaller than a few ten of a micrometer in size, instead of a tipping point where once an
ice crystal gets big enough, it keeps growing and growing. In this case, the pressure's desire to melt overwhelms the interior's desire to freeze for all crystal sizes. And all ice crystals want to melt regardless of their size. So, super cooled water in a tiny rigid bottle won't freeze no matter how hard you smack it, as long as the bottle is tiny enough. For water 1° C below zero, the bottle needs to be less than 200 nm long. For water four degrees below zero, the bottle needs to be even smaller, around 50 nmters, which maybe isn't surprising since for water
four degrees below zero, the tipping point would normally be a crystal around 20 nmters in size, which would be a pretty big proportion of the container. And if you have super cooled water in a tiny rigid bottle, but you really need it to be ice, just remember the prohibition on freezing relies on the container being rigid. If you open the bottle so pressure can escape, then smack away. This video is entirely supported by patrons on Patreon. If you're already supporting, thank you so much. And if you're not, please consider it at patreon.com/minutphysics.
