Walking on (non-Newtonian) water
October 22, 2015 5:10 PM Subscribe
Maybe Jesus could walk on water, but anybody can walk on this non-Newtonian fluid (SLYT)
There was this inventors show on ABC TV (here in Aus) and a kid came up with the idea of making speed humps out of bags filled with a non-newtonian fluid, such that a car proceeding at a moderate pace would find it soft and giving, easy on the suspension, but a car going over a certain speed would find it unyielding as per your usual speed hump. Neat idea, haven't seen it commercialised.
posted by wilful at 5:54 PM on October 22, 2015 [9 favorites]
posted by wilful at 5:54 PM on October 22, 2015 [9 favorites]
Helt Texas
posted by gwint at 5:59 PM on October 22, 2015 [5 favorites]
posted by gwint at 5:59 PM on October 22, 2015 [5 favorites]
what about helmets? soft and gooey till you get hit hard?
posted by greenhornet at 6:00 PM on October 22, 2015
posted by greenhornet at 6:00 PM on October 22, 2015
Metafilter: ...
posted by zippy at 6:19 PM on October 22, 2015 [2 favorites]
posted by zippy at 6:19 PM on October 22, 2015 [2 favorites]
"hard strike to the head" is probably not when you want your helmet to transmit absolutely all of the energy of impact to your skull.
posted by indubitable at 6:22 PM on October 22, 2015 [8 favorites]
posted by indubitable at 6:22 PM on October 22, 2015 [8 favorites]
Oobleck! I was at a hotel party once in the 80s where someone made that in the bathroom sink, and people were pushing at it and such, and someone slapped it so hard that the bottom of the sink just fell to the floor.
That's interesting, xingcat; as I understood what they were saying in the video, if you had a piston resting on the surface of a cylinder full of a Newtonian fluid (pure water, approximately) and gave it a sharp blow, the force would be transmitted more or less evenly to all the surfaces covered by the water, but if it were a non-Newtonian fluid, the force would be felt almost exclusively at the bottom of the cylinder.
posted by jamjam at 6:47 PM on October 22, 2015 [1 favorite]
That's interesting, xingcat; as I understood what they were saying in the video, if you had a piston resting on the surface of a cylinder full of a Newtonian fluid (pure water, approximately) and gave it a sharp blow, the force would be transmitted more or less evenly to all the surfaces covered by the water, but if it were a non-Newtonian fluid, the force would be felt almost exclusively at the bottom of the cylinder.
posted by jamjam at 6:47 PM on October 22, 2015 [1 favorite]
what about helmets? soft and gooey till you get hit hard?
In fact, I believe that's how Kevlar works: it's a non-Newtonian liquid, so it's soft and flexible to allow for body movements, but gets very hard when struck by something moving at high speed (e.g., bullet).
posted by tickingclock at 6:57 PM on October 22, 2015
In fact, I believe that's how Kevlar works: it's a non-Newtonian liquid, so it's soft and flexible to allow for body movements, but gets very hard when struck by something moving at high speed (e.g., bullet).
posted by tickingclock at 6:57 PM on October 22, 2015
i can't tell if you're serious
posted by indubitable at 7:00 PM on October 22, 2015
posted by indubitable at 7:00 PM on October 22, 2015
Isn't kevlar a fabric?
posted by any portmanteau in a storm at 7:10 PM on October 22, 2015
posted by any portmanteau in a storm at 7:10 PM on October 22, 2015
"hard strike to the head" is probably not when you want your helmet to transmit absolutely all of the energy of impact to your skull.
Works for eggs.
posted by alex_skazat at 7:50 PM on October 22, 2015
Works for eggs.
posted by alex_skazat at 7:50 PM on October 22, 2015
In fact, I believe that's how Kevlar works: it's a non-Newtonian liquid, so it's soft and flexible to allow for body movements, but gets very hard when struck by something moving at high speed (e.g., bullet).
That's not how Kevlar works, but they have added Non Newtonian Liquids to Kevlar and it's amazing.
So, hmm. Yes to this on my helmet. And all my clothes.
In fact, they make mountain bike pads outta this stuff, already.
posted by alex_skazat at 7:56 PM on October 22, 2015 [3 favorites]
That's not how Kevlar works, but they have added Non Newtonian Liquids to Kevlar and it's amazing.
So, hmm. Yes to this on my helmet. And all my clothes.
In fact, they make mountain bike pads outta this stuff, already.
posted by alex_skazat at 7:56 PM on October 22, 2015 [3 favorites]
I remember now, D3O!
It and similar non-Newtonian products are being used to protect bikers, stunt men, soldiers, cell phones, even the elderly.
posted by eye of newt at 11:01 PM on October 22, 2015 [2 favorites]
It and similar non-Newtonian products are being used to protect bikers, stunt men, soldiers, cell phones, even the elderly.
posted by eye of newt at 11:01 PM on October 22, 2015 [2 favorites]
Hey MeFi scientists! Care to explain this in greater detail? For instance, why does corn starch behave this way but not sand? Why can't I do it with flour? Basically, explain this in... Any way at all. As opposed to the video.
posted by shmegegge at 11:27 PM on October 22, 2015 [1 favorite]
posted by shmegegge at 11:27 PM on October 22, 2015 [1 favorite]
When I was an undergrad I worked in a lab where some of the postdocs and grad students were studying this stuff. It seems there are a few parts to the mechanism, but they all come down to the cornstarch particles being unable to move past one another. Basically, as long as you have enough cornstarch in the mixture, the particles are all packed up against each other, but loosely. When you try to push through this packing, the grains have to spread out a little bit to move past one another. If you try to move faster than the grains can respond, then the loosely packed particles get crammed together, the packing density spikes, and the mixture "jams." The surface tension of the water plays a role, too. When the grains try to spread out quickly, the surface tension of the liquid pushes back and causes the packing to jam.
Sand could in principle behave in a similar way--many particle suspensions do to some degree--but in practice the mixture is rarely right, because sand grains settle quickly to the bottom of the water and separate the fluid into two regions: one where there is too much sand and it behaves too much like a solid to begin with, and the other where there is too little sand and it can flow easily. If the mixture is just right, though, you can see some of the mechanisms at work. When you walk across wet sand, look at your feet and you'll see the grains pushing out against the surface: the sand will appear drier near the source of pressure. Cornstarch mixtures do exactly the same thing. This change in surface texture is the grains pushing out against the air-water interface.
Cornstarch is denser than water, but because the particles are very small (10-20 microns is typical) they take hours to settle after mixing. The chemistry between the particles and the fluid matters too--you need the particles to be able to mix well. Try cornstarch in oil and you won't see this, because the cornstarch particles glom onto each other in a small number large clumps to avoid mixing with the oil. But starch is very hydrophilic, so in water it performs well. In fact, many different suspensions can display some amount of this behavior, but for most of them the effect is weak enough that you don't notice it unless you're measuring with sensitive equipment. Cornstarch in water hits some kind of sweet spot of particle size, fluid surface tension, and fluid-particle chemistry that makes for a dramatic contrast between the resting state and the stressed state. (It's also likely that the geometry of cornstarch particles has something to do with it, but that's much harder to assess because they are highly irregular.)
If you want to read a little more (including a quote or two from the scientists in question), here's a writeup in Time that the aforementioned grad student got a few years ago (and a few years after I graduated--I only came across this while writing this comment). And apparently the (then) postdoc is continuing to study this with an eye toward applications in protective equipment.
Anyway, we made an inflatable pool full of the stuff for a local museum event once and everyone got to run across it and do other stunts. We learned quickly that it's really tough to pull your foot out once you sink a few inches, because it locks up once you start to pull, and then you have to pull hard enough to rip through the hardened material. On a hard drive somewhere there is--or was, maybe it's been lost--a slow-motion video of my hand hitting it mid-cartwheel. Fun stuff.
posted by egregious theorem at 1:21 AM on October 23, 2015 [6 favorites]
Sand could in principle behave in a similar way--many particle suspensions do to some degree--but in practice the mixture is rarely right, because sand grains settle quickly to the bottom of the water and separate the fluid into two regions: one where there is too much sand and it behaves too much like a solid to begin with, and the other where there is too little sand and it can flow easily. If the mixture is just right, though, you can see some of the mechanisms at work. When you walk across wet sand, look at your feet and you'll see the grains pushing out against the surface: the sand will appear drier near the source of pressure. Cornstarch mixtures do exactly the same thing. This change in surface texture is the grains pushing out against the air-water interface.
Cornstarch is denser than water, but because the particles are very small (10-20 microns is typical) they take hours to settle after mixing. The chemistry between the particles and the fluid matters too--you need the particles to be able to mix well. Try cornstarch in oil and you won't see this, because the cornstarch particles glom onto each other in a small number large clumps to avoid mixing with the oil. But starch is very hydrophilic, so in water it performs well. In fact, many different suspensions can display some amount of this behavior, but for most of them the effect is weak enough that you don't notice it unless you're measuring with sensitive equipment. Cornstarch in water hits some kind of sweet spot of particle size, fluid surface tension, and fluid-particle chemistry that makes for a dramatic contrast between the resting state and the stressed state. (It's also likely that the geometry of cornstarch particles has something to do with it, but that's much harder to assess because they are highly irregular.)
If you want to read a little more (including a quote or two from the scientists in question), here's a writeup in Time that the aforementioned grad student got a few years ago (and a few years after I graduated--I only came across this while writing this comment). And apparently the (then) postdoc is continuing to study this with an eye toward applications in protective equipment.
Anyway, we made an inflatable pool full of the stuff for a local museum event once and everyone got to run across it and do other stunts. We learned quickly that it's really tough to pull your foot out once you sink a few inches, because it locks up once you start to pull, and then you have to pull hard enough to rip through the hardened material. On a hard drive somewhere there is--or was, maybe it's been lost--a slow-motion video of my hand hitting it mid-cartwheel. Fun stuff.
posted by egregious theorem at 1:21 AM on October 23, 2015 [6 favorites]
That's great! Now I just need to find out how to walk on sunshine.
posted by fairmettle at 2:37 AM on October 23, 2015 [1 favorite]
posted by fairmettle at 2:37 AM on October 23, 2015 [1 favorite]
It's all fun and games until somebody drowns in non-Newtonian fluid.
posted by usonian at 4:02 AM on October 23, 2015
posted by usonian at 4:02 AM on October 23, 2015
xingcat: I watched the video and got an advertisement for faucets.
posted by jefflowrey at 4:46 AM on October 23, 2015
posted by jefflowrey at 4:46 AM on October 23, 2015
Sand and Water: Doesn't sand behave this way quite often at the beach?
I have noticed when you dig a hole in the sand near the water that is below the "water level" the sand in there will form a solution with the water seeping in and often have this quality. That you can move through it slowly but if you jump into the hole it will "tense up" in the same way that the cornstarch mixture does.
posted by mary8nne at 6:23 AM on October 23, 2015
I have noticed when you dig a hole in the sand near the water that is below the "water level" the sand in there will form a solution with the water seeping in and often have this quality. That you can move through it slowly but if you jump into the hole it will "tense up" in the same way that the cornstarch mixture does.
posted by mary8nne at 6:23 AM on October 23, 2015
Hong Leong Bank ad from 2014 explores non-Newtonian water, including fun goopy "fails"
posted by chavenet at 7:14 AM on October 23, 2015
posted by chavenet at 7:14 AM on October 23, 2015
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That was as close to a rock and roll hotel trashing incident as I've ever gotten.
posted by xingcat at 5:16 PM on October 22, 2015 [18 favorites]