Folding wine-rack material expands when squeezed

A-Mir & Co.

Take almost any solid material and squeeze it with an industrial piston. It will compress to a greater or lesser degree, depending on the kind of material and how hard you squeeze. However, certain unusual solids have the opposite response: they push back against compression, actually expanding in the direction they are squeezed. These materials are interesting in their own right, but also have potential applications in very sensitive pressure sensors, compensators in systems affected by humidity, or even artificial muscles.

In studying why some solids behave this way, Andrew B. Cairns and colleagues discovered both the secret to their properties and substance that shows the most extreme example of negative compressibility yet found. This material, a lattice of molecules containing gold and zinc, resembled a folding clothes rack on the microscopic scale. When squeezed, it pushed back more than double the amount normal materials typically compress. The researchers attributed this remarkable ability both to the springy internal structure and to the interactions between the gold atoms.

Most materials aren't very compressible. If you exert 10,000 times atmospheric pressure—roughly ten times the pressure at the bottom of the Mariana Trench—a typical metal or ceramic will only compress about 0.5 percent. (Kids! Try this at home!) While more squishable materials exist, these usually aren't very valuable for engineering purposes.