# What bubbles insulate best?

Which bubble shapes (spheres, tetrahedra, cubes, etc. or composite) in a foam most effectively insulates against heat?

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I wonder if any clues can be found from nature.I am thinking of a polar bears fur where each hair is hollow.

Danger
Gold Member
My guess would be spheres, since they contain the highest volume of trapped air versus surface area. There would also be air trapped in the intersection areas between them.

My guess would be spheres, since they contain the highest volume of trapped air versus surface area. There would also be air trapped in the intersection areas between them.
I would guess the same thing.

But spheres must have non-spherical spaces between them. So the question becomes, "do the spheres improve insulation more than those non-spherical shapes hurt it?" if so, then probably a 3D honeycomb ( I know it has a name but I'm no geometer).

Also, i'm not sure spheres are best in a gravity field--convection might be minimized with horizontal cavities.

I disagree. If we could ignore convection, then we would not need any material around the bubbles at all, the good thermal insulation of the gas would be enough. Gas or vacuum generally transport much less heat then solid or liquid matter (for obvious reasons). Bubbles are made to prevent convection. So the idea would be to reduce the length of the free gas paths in the direction where you don't want heat transfer to occur, while at the same time reducing the material links in this direction.
So IMHO the ideal structure is wrapping sheets around the object to be insulated, or very oblate "bubbles".

Redbelly98
Staff Emeritus
Homework Helper
Which bubble shapes (spheres, tetrahedra, cubes, etc. or composite) in a foam most effectively insulates against heat?
I don't think the shape would matter a whole lot. As long as air convection is impeded, as OxDEAD said. So I guess a closed-cell foam would be preferred rather than the open-cell type.

I suspect the answer might also depend on the particular environmental conditions encountered since heat transfers via conduction, convection and radiation. We tend to think in everyday terms of conduction which may be insignificant in some applications.

For everyday circumstances, if you need an authortative answer, you might check on high R value insulators and see what tricks have been devised.

Over the years refrigerators, for example, have used more efficient insulation but exactly how that has been accomplished I'm not sure. A consideration is that whatever material is utilized to contain an insulating medium (say air or argon gas) also has its own R value which must be taken into account. And in extreme conditions, like space shuttle applications, perhaps strength is an additional significant consideration.

Note: Benoit Mandelbrot reminds us that nonequilibrium thermodynamics can beget simple geometry, as displayed by some chemical clocks.

DaveC426913
Gold Member
Surely we are ignoring the important factor that the structural material plays? That's got to factor in.

The shapes of the bubbles determine the thickness and transmissivity of the supporting structural material.