New micro-lattice material is world's lightest at 0.9mg/cm^3

[zacharyvoase]

http://www.gizmag.com/ultralight-micro-lattice-material/20537/

[tomkinstinch]

Does the 0.9mg/cm^3 number seem strange? The density of air is about 1.2mg/cm^3 at STP[1]. Can it be less dense than air? Wouldn't that make it float?

1. http://pdg.lbl.gov/2007/reviews/atomicrpp.pdf

[redthrowaway → tomkinstinch]

The density likely leaves out the weight of the air filling most of the volume, and just represents the mass of the metal divided by the structure's volume. This is reasonable, given that the air plays no role in the structure's stability or strength. It could just as easily be in a vacuum, or in some other gas. You can't say the same of the frozen smoke linked in the article, as air forms an integral part of its structure.

[ars → redthrowaway]

Not so. You can have frozen smoke with a vacuum in it. There are two types, closed cell and open cell (just like any foam).

According to wikipedia the lowest density frozen smoke has a density of 1 mg/cm3 (not including air!), which is just slightly more than this material.

The 4 mg/cm3 number in the article is incorrect (I wrote to them to let them know).

[yread → tomkinstinch]

I would think it's about the "apparent weight" i.e. including the buoyancy in air. So it's density would be 1.2+0.9= 2.1mg/cm^3. Also they count the whole cm^3 which is 99.99% air and only 0.01% solid...

[ars → yread]

No, the apparent weight would be 0.9 - the air inside the material is "floating" in ambient air, and would not be measured on an ordinary scale.

Technically you also have to account for the slight buoyancy the metal has in air (but it's far below the error margin so it's ignored).

Basically on earth when you weigh things you are measuring weight, even if you say grams (which are supposed to be mass).

If you want true mass you have three choices:

1. Weigh in vacuum.

2. Use a scale that shakes the material on a spring and measure the frequency (i.e. measure inertia).

3. Calculate the volume of the material, calculate the weight of air of that volume and add that weight to your measured weight.

No one ever does this since it doesn't matter. But if you want to be exact....

[dmak → tomkinstinch]

I thought the air density might have been a mistake, but I checked and your numbers are correct. Theoretically, it should float, right?

[ars → dmak]

If you put a vacuum in it and shrink wrapped it it would float.

But I don't know if the material is strong enough to result air pressure if it was shrink wrapped (i.e. would the air pressure squish it?)

[pyrhho → ars]

So, basically, it would be ideal for building a zeppelin. Put a bunch of it into a container, and evacuate it to go up. Fill the container with air again to go down. Nothing flammable to explode.

Awesome.

[Retric → pyrhho]

It's still heavy compared to helium gas. Air is 1.2 kg/m^3 helium is 0.164 kg/m^3 and this stuff is .9kg/m^3

[TylerE → Retric]

Of course, if this could be used for large portions of the structure, that really changes the equations a lot. You don't need nearly as much lift for a given weight of cargo.

The Hindenburg used 200,000m^3 of hydrogen (generating a lifting force of approx. 215,000kg. Net useful load was only 10,000kg.

[Retric → TylerE]

When selecting materials for building aircraft weight is only one of many concerns. Durability, flexibility, strength, cost, etc are all major issues. Considering the design goals for this material seem to primarily just be weight I would be surprised if it's strength to weight ratio was impressive enough to overcome cost issues.

Also compared to a modern blimp the Hindenburg an ancient design that's been far surpassed. A modern Zeppelin is much closer to 20% lift to weight ratio than 5 for the Hindenburg despite using less efficient helium gas. Their maximum permitted takeoff weight is 10,690 kg, with a payload of 1,900 kg.(http://en.wikipedia.org/wiki/Zeppelin_NT)

[redwood → ars]

Strength is the key part they didn't mention. DARPA probably wants that bit for themselves :) However one must assume this has some strength, otherwise what would the point be. Very cool stuff

[caf → redwood]

DARPA might well be funding the research for the possibility of a future alternative to FOGBANK in nuclear weapons, which isn't as dangerous to produce.

[rorrr → tomkinstinch]

Weight vs mass.

Weight of air in air is zero.