Results published in 2016 in Nature Materials
The researchers from Laboratoire 3SR and their colleagues from ILM (Lyon) have designed a new material consisting in a single long coiled elastic or super elastic wire which showed unprecedented mechanical behaviour.
When squeezing a saturated wet sponge at low strain rates, its volume usually decreases causing the water to spill out. In the opposite direction, stretching the sponge will increase its volume inducing the sucking up of water. Most porous or dense materials actually behave this way, and a few rare types show the opposite behaviour, i.e. increase their volume when squeezed and decrease their volume when stretched.
Breaking the symmetry
However, for all these materials, the volume change is always symmetric, occurring in opposite directions under compression and tensile loadings. For the first time, researchers from the 3SR Lab and their colleagues from ILM Lyon have designed a mesoscale organisation of the matter able to break this “ancestral” symmetry, opening new material design possibilities for biomedical, mechanical or civil engineering applications.
Their porous material, made from a single entangled coiled wire (see image), actually expands whether being compressed or stretched. Moreover, the use of an elastic or a superelastic coiled wire confers the material the ability to bear repeated cycles of compression and tension without losing its amazing properties.
Thanks to discrete element simulations and mechanical experiments with 3D in situ observations using X-ray microtomography, they captured the complex mesomechanics of this material, which turn out to be a combination of the coiled wire's initial curvature, aspect ratio and level of entanglement as well as strain-induced coils deformation, all of which lead to an overall increase of the material porosity both in tension and compression.
Quite a weird sponge indeed, that could find many applications!
X-Ray tomographic image of a NiTi entangled wire (sample height = 35 mm) showing the internal homogeneity of the material (Credit: 2015 Macmillan Publishers Limited)
David Rodney, Benjamin Gadot, Oriol Riu Martinez, Sabine Rolland du Roscoat and Laurent Orgéas. Reversible dilatancy in entangled single wire materials. Nature Materials 15, 72–77 (2016).