New 'Mechanical Metamaterial' Expands When You Compress It, Shrinks When You Stretch It

It's 'opposite day' for metamaterials

By Clay Dillow Posted 05.23.2012 at 12:07 am 7 Comments

Mechanical Metamaterials Could Create Cushions That Push Back Johan via Wikimedia

Metamaterials research has generally focused on media with strange or unique electromagnetic properties, like the ability to bend light or sound in an unnatural way, but materials scientists at Northwestern University are experimenting with an entirely new kind of material with unique mechanical properties. A team there has designed materials with “negative compressibility” that in theory will compress when they are pulled and expand when they are compressed.

In other words, sit on a cushion and it pushes back against the force of your weight. This sounds counterintuitive, of course, but their design makes some kind of sense. One would expect that any material that behaves this way would be, by definition, unstable--and therefore would collapse into a stable state on its own, thus nullifying whatever characteristics the unstable state possessed. But the Northwestern team designed around this by creating an internal structure that does indeed fall into a stable state. The stable state is just more compressed or expanded than the original state.

That’s confusing, so we’re going to borrow from New Scientist's explanation (click through for some more thorough visuals). Think of a row of four horizontal particles making up a material that inherently wants to expand. Each is made up of molecules that attract one another to certain and unequal degrees, and that attraction keeps the material from expanding.

The inner two particles share a weak attraction that can be broken when the material is stretched. But when the bond between the inner particles is broken, the outer particles respond by attracting each other even more, creating compression that counters the stretching. But compressing the material brings the two inner particles back together again, re-establishing the weak bond and allowing the material to resume expanding.

It’s confusing, but in theory it could work. Someday such materials could be used to coat the outside of military vehicles or shelters to push back against an incoming blast.

[New Scientist]

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7 Comments

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AGreenhill

05/23/12 at 1:37 pm

Confusing yes. Impossible yes.

You could make really simple perpetual motion machines if you had a material that expands when compressed... aka energy from nothing... aka impossible.

Amazing this guy is wasting time on something that's bound to fail... maybe he's a kook... time to look up some credentials!

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jayphox

05/23/12 at 6:22 pm

Anybody else thinking of the sintered armorgel in Neal Stephenson's Snow Crash?

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3DTOPO

05/23/12 at 8:58 pm

"...focused on media with strange or unique electromagnetic
properties, like the ability to bend light or sound..."

Uhhhh yeah sound is not an electromagnetic property, it is compressed air. Something they teach in what the 3rd grade?

Who writes this stuff?

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pixelstuff

05/23/12 at 11:08 pm

Sounds to me like the wording is inaccurate. Probably intentionally to make it sound more sensational that it is. As it is worded here it appears to be saying the same thing as "speed up to go slower".

I figure it's probably a scenario where if you push against the outer edge of the material, a few cells break loose (like a clicker pen) and then they push against the cells further in. Triggering a chain reaction through the material. However it seems to me there would be an equilibrium reached where the expanding cells wouldn't be strong enough to trigger more cells while simultaneously pulling against the cells to either side. You would have to manually push further in to trigger more cellular release.

Therefore I would guess in reality we would end up with an IF/THEN scenario. If we push on the material, then after we let go it expands.

That's what I gather from the drawings anyway.

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roboninjosh

05/24/12 at 11:43 am

agreenhill, first off all their is no mention of anything considered impossible. additionally I believe that people who say something is impossible should not interrupt while others accomplish it.

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drchuck1

05/24/12 at 3:51 pm

it is just a hypothosis, whether it pans out or not time will tell, you who trash it for fun or whatever, bugger off, negative cheers

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ct451

07/10/12 at 11:05 am

I remember seeing something like these back in 1990. you pulled on a string it got thicker. the drawings were something like this -)(-
it looks the same. what have they been doing for 25 years? pass

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