May 5, 2017 | By David

A team of French and German researchers have recently achieved a 3D printing breakthrough that is unique not just for 3D printing technology, but for physical materials in general. They have created a new type of composite ‘meta-material’, whose 3D structure makes it behave in a counter-intuitive way that seems to go against the laws of physics. The material will expand when pressure is put on it, instead of shrinking.

This project was carried out by members of the Karlsruhe Institute of Technology (KIT) in Germany and the French National Center for Scientific Research, and the results of their work were published in Applied Physics Letters. The base material used for the project was one that, like nearly all materials, is compressed under hydrostatic pressure, which is referred to as having positive effective static compressibility. The team wanted to see if 3D printing could be used to make a new, composite material out of it that would demonstrate negative effective static compressibility.

Referred to as a poro-elastic three-dimensional metamaterial, the 3D printed composite they intend to produce will effectively expand when the hydrostatic pressure of a surrounding gas or liquid is increased. Obviously no fundamental universal laws of physics are actually broken, but the measurable behaviour of the material does give the impression of an increase in volume with an increase in pressure, which seems to violate energy conservation and stability rules.

The structure of the material is where the secret to its seemingly counter-intuitive properties lies. The researchers used 3D printing technology to design a hollow, 3-D cross structure, with circular membranes at each end of the cross. Jingyuan Qu, a doctoral student and researcher at KIT's Institute of Applied Physics and Institute of Nanotechnology, explains that "Akin to a drum, these membranes will warp inward if the outside pressure is larger than the pressure in the enclosed volume inside the cross. By properly connecting these membranes via bars, and by using eight such three-dimensional crosses within one unit cell, it's possible to obtain an isotropic effective volume increase upon increasing the pressure—a negative effective compressibility."



The structure means that the visible volume of the material increases under pressure, whereas the total volume enclosed by it decreases, as you would expect it to. The total volume is not actually directly perceivable, which is why the material behaves in the surprising way that it seems to do. Now the meta-material’s behaviour has been successfully tested in a virtual 3D environment, the team are starting the process of actually fabricating the material. Engineering simulation software has proven the material to be 3D printable in theory, but in practice, the specific details of the structure have not previously been achieved.

The meta-material’s concealed volumes are only possible to make using an additive manufacturing process, as opposed to conventional machining techniques which rely on the removal of material to build these kinds of structures. This is why 3D printing has allowed such a groundbreaking material to be designed and potentially fabricated for actual experimental measurements. Laser-based 3D printing techniques, such as SLS, are able to build structures at the nano scale, which is crucial for this type of material. The unprecedented concealed volume, enclosed structure that this meta-material requires means that it will be a significant challenge for 3D printing technology, but hopefully one that it will be up to.

Posted in 3D Printing Materials

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