There are thermo-shrinking plastics, commonly used in electronics, which stay shrunk after cooling down. There are thermally expanding foams. The closest to 'expanding when cooled' I know is water, with ice expanding, but only by a small margin, a scarce couple percent.

Is there a material that would start expanding significantly when cooled? May shrink back when heated, or may stay expanded. It doesn't need to be as forceful as ice (which can explode rocks) but it should have at least 30% expansion ratio, the more the better.

  • Are you sure that water only changes a “couple of percent” when changing from ice to liquid and vice versa? Any source? Steam tables perhaps... – Solar Mike Sep 13 at 15:03
  • Do you understand why materials expand when heated and shrink when cooled? Do you understand why the behavior in the examples you list (heat-shrink and foam and ice) is different? – Jeffrey J Weimer Sep 13 at 15:10
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    @SolarMike: Up to 9% allegedly. In practice this will often be less as air microbubbles compress, water/ice is pushed into pores, some evaporates, and obviously just flows "out of the way" before freezing. – SF. Sep 13 at 15:10
  • @JeffreyJWeimer: Yes, I do. – SF. Sep 13 at 15:11
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    I certainly can't think of any materials with this inherent property, but I think it could be interesting problem to think of how to make a structure that changes conformation in a way that it expands as it cools. I know EWI, somewhat relatedly in concept, has made some cool meta-materials that expand and contract interestingly in response to compression in other axes. – ericksonla Sep 13 at 17:43
up vote 4 down vote accepted

While there's a list of (rare) materials with negative thermal expansion, that 30% expansion ratio excludes most of them and narrows down your search to polymers or structures.

Within the realm of polymers, two-way reversible shape memory polymers might be your option. They can have relatively high reversible strains and can be thermally activated:

enter image description here

WM Huang, Thermo-Moisture Responsive Polyurethane Shape Memory Polymer for Biomedical Devices, The Open Medical Devices Journal, 2010, 2, 11-19

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C. Liu, H. Qinb, P.T. Mather, Review of progress in shape-memory polymers, Journal of Materials Chemistry

You may also want to look for structures which are made of positive thermal expansion materials, but when assembled they have a the opposite effect:

enter image description here

Christopher Spadaccini, Mechanical Metamaterials: Design, Fabrication, and Performance, Frontiers of Engineering: Reports on Leading-Edge Engineering from the 2015 Symposium (2016)

  • A shape memory material that is tightly packed and expands into something that occupies more space (even if actual material volume remains unchanged) would certainly work, but can they be activated by temperature drop? Most shape memory materials I knew were recovering the remembered shape through heating. – SF. Sep 13 at 15:31
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    That's the point of thermally activated two-way reversible shape memory materials, they will change when heated and when cooled. Of course the reversible strain and the operation temperature depends on the material. – user190081 Sep 13 at 15:35
  • I'll wait to see if anyone doesn't come up with something more simple (...and accessible; I don't even begin to start to think where to obtain that stuff) but it certainly fulfills the premise. – SF. Sep 13 at 15:42

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