How to Make a Metamaterial that Expands Under Pressure and Contracts In Tension

http://www.technologyreview.com/view/428492/how-to-make-a-metamaterial-that-expands-under

by gruseom • 14 years ago

35 23 14 years ago

hristov• 14 years ago

So where does it get the energy? Suppose you get a rope made out of this meta material and tie one end to a nail in a wall, and tie a weight to the other end. The material should contract and lift the weight up. Thus, by lifting the weight, the magic material would be doing work, and expending energy. But where does it get the energy from?

Seems very dubious to me. To quote Homer Simpson "In this house we follow the laws of thermodynamics!"

rdw• 14 years ago

Sounds like the energy is bound into the material during its formation: "there is an internal latent strain between the building blocks that constrains the overall shape".

gruseom• 14 years ago

Does that imply that it wouldn't automatically revert to its original state?

gnaritas4• 14 years ago

> Thus, by lifting the weight, the magic material would be doing work, and expending energy. But where does it get the energy from?

From the pull of gravity on the weight.

caf• 14 years ago

"The pull of gravity" is not a source of energy. When the weight rises in the Earth's gravitational field energy must be supplied.

paulgerhardt• 14 years ago

Serious question: doesn't ice/nearly freezing water have these properties?

cma• 14 years ago

Don't think so. Ice is less dense than water. Ice skating creates a small layer of water due to pressure that allows you to slide around.

gnaritas4• 14 years ago

It may be less dense, but it takes up more space. Water expands when it freezes. Density isn't the issue, I'm not sure why you bring it up.

sp332• 14 years ago

Yup! In order to form crystals, the water molecules actually settle a bit further from each other than when they're a liquid. https://en.wikipedia.org/wiki/Ice#Characteristics

icegreentea• 14 years ago

Yeah more or less. The actual paper is about materials exploiting solid/solid phase transitions to achieve this type of behavior. Obviously, the solid/liquid transition is less useful in many applications, so it'd be cool to get a purely solid substance to do this.

andrewcooke• 14 years ago

i thought cork did this? it have vague memories of learning about negative poisson ratios...

hmmm. not quite the same thing - something with a negative pr will contract sideways when pushed from the top (which would help a cork go into a bottle neck, but i haven't found any confirmation of that yet).

meh. "close to zero" is the best i can find for cork. which at least means it doesn't get harder to push it in as you push.

cstavish• 14 years ago

> Imagine negative resisters [sic], for example.

That seems like a remarkably nonchalant statement. How should one imagine a "negative resistor"?

disclaimer: just got my AP Physics C Elec/Mag score back and it wasn't too pretty.

TheAmazingIdiot• 14 years ago

I can imagine a negative resistor as a piece of wire that is substituted in a circuit of less resistance than the standard wire.

I cannot think what would actually happen if that was to be the case. Perhaps the reflectivity would change in some weird way.

rrmm• 14 years ago

Check out a circuit called a 'negative impedance converter'. Gunn and tunnel diodes also exhibit negative differential resistances at some operating points.

pbhjpbhj• 14 years ago

Electronics is my absolute worst subject but wouldn't it just be a component over which a reducing voltage causes an increasing current flow; presumably that breaks Ohm's law? How to achieve that, vacuum energy!?!?

pliny• 14 years ago

Controlled current source Ic = (-R)*var

colanderman2• 14 years ago

You can simulate this with an op-amp. It needs energy input though.

azundo• 14 years ago

These materials already exist and are called auxetics: http://en.wikipedia.org/wiki/Auxetics.

One notable example is the material that makes up Gore-Tex. My materials prof talked about being in a lab that had a cube of it and how surreal it was to have it act this way.

There is already a company that produces auxetics commercially for some of the applications mentioned in the article: http://en.wikipedia.org/wiki/Zetix.

colanderman2• 14 years ago

The materials described sound nothing at all like auxetics.

Auxetics expand perpendicular to the line of force when stretched. The materials described in the article compress parallel to the line of force when stretched.

How to Make a Metamaterial that Expands Under Pressure and Contracts In Tension

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