How to Make a Metamaterial that Expands Under Pressure and Contracts In Tension | Hacker News Reader

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

technologyreview.com

35 pointsgruseom23 comments

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!"

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".

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

> 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.

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

Thinking about it more, yea, fair point. When water loses energy and freezes, it expands, that expansion could lift a weight. Is this not a similar change in molecular structure doing a similar thing? Where is the input of energy lifting this weight.

No. When you apply pressure on ice it becomes liquid. In other words: water doesn't lift things by freezing if that requires an amount of energy larger than what is needed to change state.

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.

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.

> 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.

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.

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

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!?!?

Controlled current source Ic = (-R)*var

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

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

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.

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.

Taking up more space with the same mass is the definition of lower density.

Eh, my bad, don't know what I was thinking.

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

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.

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.

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