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  • Go to Brilliant.org/SciShow to learn more.

  • [♩INTRO]

  • This weird white stuff looks a bit like a fungus, but it's not.

  • It's ice!

  • It forms because of the quirky physics of how water freezes in wood.

  • It does need the help of fungi, though.

  • And that's likely part of why it's incredibly rare.

  • Hair ice, ice wool, or frost beard is a strange form of ice comprised of strands

  • that really are similar to human hairs in terms of size and shape.

  • Each is about 20 microns in diameter and can grow up to 20 centimeters long

  • that'd be about shoulder length if it was coming from your scalp!

  • But what's really weird is that hair ice only forms on dead, rotting branches

  • of hardwood trees where there's no bark

  • in forests between 45 degrees and 55 degrees north latitude

  • when the temperature is just below zero degrees Celsius

  • and the air is pretty humid.

  • Part of that specificity comes from how the strands form

  • a process known as ice segregation.

  • As the temperature drops below freezing,

  • the water inside a rotting branch starts to freeze.

  • But, thanks to interactions with the molecules of the wood,

  • it doesn't form solid ice.

  • It becomes supercooled instead.

  • Ice does form near the outer edge, though, in small pores in the wood.

  • Then, at the interface between ice and supercooled water

  • the 'root' of the hair, if you will

  • more ice grows.

  • Since the crystalline ice takes up more space than liquid water,

  • and the air outside puts up less of a fight than the water in the wood,

  • the forming ice pushes the already-formed ice out of the branch

  • and a thin strand the size of the wood's pore forms.

  • As each bit of water next to the base of the strand becomes frozen,

  • it pulls more supercooled water out of the interior of the wood.

  • And this process repeats until all the water in the wood is used up.

  • But for any of this to happen, the temperature has to be just right.

  • If it's too cold, the water inside the branch will freeze solid, and if it's too warm,

  • the starter ice won't form on the outer surface.

  • So a temperature just below zero degrees Celsius is ideal.

  • The air has to be humid too, because if conditions are too dry,

  • the ice will sublimate into water vapor faster than the ice strands can grow.

  • And hardwoods are optimal because they have radial structures

  • inside them called medullary rays.

  • These act as channels, allowing water to easily travel from the center

  • of a branch to the outside.

  • So if you have all of this in place, you can end up with ice hairs.

  • And these can stick around for hours, or even days.

  • Which is surprising, too, because normally, such fine ice crystals would

  • melt ever so slightly and then recrystallise into a less attractive shapeless clump.

  • To explain why this doesn't happen, way back in 1918, a German scientist

  • suggested that fungi might be involved.

  • And it turns out he was right.

  • A 2015 study found that the fungus Exidiopsis effusa is also present

  • in the dead wood.

  • And it secretes proteins called recrystallisation inhibitors.

  • For the fungus, these act much like the antifreeze proteins some animals

  • have to stop the water in their cells from freezing at temperatures below zero.

  • And similarly, when they end up in the supercooled water that becomes hair ice,

  • they stop the strands from recrystallising into formless lumps.

  • Researchers actually demonstrated that the same branches

  • will produce ice hair over and over again, until you kill the fungi inside of them.

  • It's not clear if there are multiple fungal species that can facilitate hair ice

  • or it's just this particular fungus.

  • But either way, forests with hardwood trees

  • at latitudes between 45 and 55 degrees north

  • are probably the only places on the planet

  • where physics and biology come together

  • to allow this awesome and unique kind of ice to form.

  • Hair ice is a great example of why understanding

  • the world takes all sorts of science knowledge.

  • But if it's been a little while since your last math or physics class,

  • well, that's something Brilliant.org can help with.

  • Brilliant has hands-on, interactive courses which cover

  • topics in science, engineering, computer science and math.

  • So if you want to, say, understand how snowflakes form,

  • you can brush up on complex numbers, and then dive deep into group theory.

  • And if the winter weather makes your internet spotty,

  • you can continue the courses offline using Brilliant's iOS and Android apps.

  • You can learn more at Brilliant.org/SciShow.

  • Plus, if you're one of the first 200 people to sign up at that link,

  • you'll get 20% off the annual Premium subscription!

  • So check it out and see if Brilliant is right for you.

  • And as always, thanks for watching SciShow!

  • [♩OUTRO]

Thanks to Brilliant for supporting this episode of SciShow.

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B1 中級

冬が枯れ枝の髪を与えるとき (When Winter Gives Dead Branches Hair)

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    林宜悉 に公開 2021 年 01 月 14 日
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