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Hank describes how non-Newtonian fluids can save lives - and dreams of a bulletproof hoodie.

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[SciShow intro] Hank Green: Isaac Newton, God bless him for all the incredible things that he did, thought that our world was simpler than it actually was. When that apple fell off the tree, he thought that it was gravity that was pulling it [gets hit in the head by apple, apple bounces off]. Anyway, that was a good example of, of, if not gravity, at least uh, equal and opposite reaction stuff. The apple hit me and I moved away because the apple hit me in the head. [Points to dented apple] That's what my head did. Equal and opposite reaction. The energy had to go somewhere, and so it went into the apple. And if we drop this considerably damaged apple down into a bowl of water, the water would all fly up, that's a Newtonian reaction--that's a fluid reacting in a way that Isaac Newton would expect it to. But what if, instead of splashing in the water, the water suddenly solidified and the apple bounced away? That would be a fluid behaving in a non-Newtonian way. And if you went to the class geek's birthday party in elementary school, you may have some experience with non-Newtonian fluid. And if you got really into it, and got some of it on your clothes, you may have been experimenting with your first non-Newtonian bulletproof vest. This non-Newtonian fluid that you may have some experience with was probably mixed up by that geek's mom . It certainly was mixed up by my mom. Thanks, Mom! And she probably called it ‘oobleck', which is a Geiselian term, meaning it was coined by Theodor Geisel, also known as Dr. Seuss. Simply mix together equal parts cornstarch and water, and you have yourself a physical oddity. It behaves like a liquid when you stir it around or dip your finger in it, but give it a good smash with an apple, and it suddenly solidifies. Fill up a big enough vat and you could run across the surface, but stand on it, and you'd sink like a, like a log? No. Logs don't sink! In Physics, we call this a non-Newtonian fluid. And the concepts we're playing with are those of ‘viscosity' and ‘sheer'. Particles in a Newtonian fluid generally stay the same distance from one another, so viscosity always stays the same. In a non-Newtonian fluid, when you apply pressure, the particles actually clump together into patterned, regular forms. It's almost as if they're temporarily turning into a little crystal. In your oobleck, it's the cornstarch bits that bind together to solidify the goo, the harder you hit it, the harder it gets, but just for a millisecond, and then it returns to a liquid again. But here's the question: What if we could make clothes the same way? Clothes that are flexible and easy to wear and comfortable, but become extremely rigid upon being hit by something moving very fast, like a bullet. Non-Newtonian body armor uses some more complicated chemistry, but it's basically the same thing. Kevlar, the fibers that make up a Kevlar vest, are actually strands of a non-Newtonian fluid so viscous that it's practically a solid. Unfortunately, as you've seen, Kevlar vests are a little bit too stiff for comfort, and they certainly can't go down over your arms and your legs. And I mean, that's great, because most of the really important stuff is in here [gestures to torso] and also up here a little bit [gestures to head], but protect this part, you're pretty -- doing a lot better. But, you know, frankly, if my choice is get shot in the chest or arm, I'm gonna take the arm. But if my choice is get shot in the arm or not shot, I would go for the -- the circumstance in which I don't have a bullet inside of me. So what they're doing now is dipping Kevlar fibers into non-Newtonian fluids, the most common one being Silica dissolved in a polyethylene glycol gel. And thanks to science, putting on a bulletproof vest could be as easy [puts on hoodie] as that! Bulletproof hoodie! And then you've got head protection, too, check it out. And if there's like, really something in your way, [tightens strings], give it a good, like, [tightens and fiddles with strings, covering his face], yeah, I'm safe now! No bullets allowed! I'm not actually safe from bullets. Luckily, I don't really need to be. Thank you for joining us, and I hope that you learned something.