Imagine that you are pulling on the long ends of a rectangular piece of rubber. It should become narrower and thinner. But what if instead it gets wider and thicker? Now press on the same ends. What if the rubber became narrower and thinner?
Such materials that contradict common sense exist. They are called auxetics and have a number of unique properties that make them suitable for making shoe insoles, bombproof buildings, car bumpers and clothing.
Despite this great potential, auxetic products have been slow to reach the market. Researchers from the National Institute of Standards and Technology (NIST) and the University of Chicago hope to change this situation.
In a new study published in the journal NPJ Computational Materials, they announced that they have developed a new tool that makes it easier and faster to design materials with assistive properties. The algorithm of this tool allows for accurate three-dimensional design of auxetics.
Advanced tool for designing auxiliary materials
"This is a huge step forward for auxetics,", — said NIST materials research engineer Edwin Chan, co-author of the study. "We can actually optimize the material to have whatever specific mechanical properties and behavior you want.
The behavior of elastic materials is described in part by Poisson's ratio, which explains how the material changes shape when you stretch or compress it in one direction. Most materials have a positive Poisson's ratio, meaning that compressing them in one direction makes them wider and/or thicker in other directions. Stretching makes them narrower and/or thinner.
Auxetics have a negative value of Poisson's ratio and act exactly the opposite.
When you punch through a non-selective material, it becomes thinner and expands laterally. When you pierce the auxetic, the material bunches up and narrows in width. Under the right circumstances, this provides greater impact resistance. For example, if you hit a bag filled with water (as in a hiking trip), the water in it will flow out from the impact. If the bag were filled with auxiliary foam, the material would become denser and stiffer upon impact.
Potential applications in the field of safety and comfort
This is one of reasons why auxetics are being considered for use in buildings and automobiles. They can provide greater protection against explosions and collisions. In the insoles of sneakers, auxetic gel or rubber foam can better cushion the foot when it hits the ground.
In clothing, auxiliary nylons, fibers, and other synthetic materials can be more comfortable than traditional materials. Because they expand when stretched, they distribute pressure more effectively throughout the body, potentially reducing stress on the back, joints, neck and shoulders. One study of the use of support materials in bra straps found that "support structures of polyester and nylon demonstrate excellent pressure distribution ability".
The design tool, developed by scientists at NIST and the University of Chicago, is a “reverse design” algorithm, meaning users can input the desired value of Poisson's ratio for their support material. The algorithm then suggests an optimized material structure.
Another way of expressing Poisson's ratio is that it describes the relationship between shape and volume when one changes. The new algorithm allows fine-tuning of this relationship to create support materials that behave in ways not found in nature.
«Our research is a great example of how theoretical, experimental and computational sciences work together to implement something new», — said NIST materials research engineer Marcos Reyes-Martinez. "Having a way to make auxetics better will allow them to become more common in our everyday lives".
The researchers have patented the algorithm, as well as the methodology behind it and its implementation using 3D printing.
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