Surprise twist: Chiral material reveals new phenomenon

// Engineering Physics, Materials Science & Engineering

Photo of gyroid lattice

A gyroid lattice created by Professor Roderic Lakes using 3D printing. Image courtesy of Roderic Lakes.

University of Wisconsin-Madison engineers have made a unique, asymmetric material that behaves in a new and unexpected way: When this “chiral” material is squeezed or stretched, it also twists.

“What we found with the squeeze-twist coupling in this material has implications for a wide range of materials,” says Roderic Lakes, a professor of engineering physics and materials science and engineering at the University of Wisconsin-Madison.

For example, it could help advance actuator technology or lead to high-toughness materials that are immune to stress concentration.

Lakes studies unusual materials that behave in unanticipated or extreme ways—in other words, they defy the standard theory of elasticity—to develop a greater understanding of the fundamental physical laws of nature.

And this standard theory of elasticity—which is what engineers use to predict the behavior of most ordinary materials, including steel, aluminum and concrete—doesn’t predict the squeeze-twist phenomenon.

In his latest work, described in a paper published Nov. 13, 2020, in the journal Physical Review Letters, Lakes set out to investigate the elastic behavior of a chiral material.

Chirality describes an object that is non-superimposable with its mirror image, such as our right and left hands. For example, a glove for the right hand will not fit a left hand.

“Materials, called chiral materials, can have right-handed and left-handed forms as well,” Lakes says. “For instance, sugar is chiral at the molecular level. But materials such as aluminum and steel are not chiral and cannot take right or left-handed forms.”

Lakes and former engineering mechanics graduate student Dan R. Reasa (BSEM ’18, MSEM ’19) used 3D printing to make gyroid lattices in chiral and non-chiral form. Gyroids are infinitely connected periodic minimal surfaces containing no straight lines. The researchers found that when a gyroid with chiral asymmetry is squeezed or stretched, it also twists. By analyzing this chiral asymmetry, the researchers developed an understanding of the material’s behavior, including the squeeze-twist coupling.

“The nice thing about the gyroid we made is that it’s structurally very stiff and strong, making it useful for substantive applications,” Lakes says.

In addition, this research is pertinent to gyroid microstructures that occur naturally in butterfly wings and via physical processes in some plastics called copolymers.

“When people make copolymers, this kind of structure can appear on a microscopic scale,” Lakes says. “In this work, we made a gyroid structure on a scale that is big enough to see with the naked eye, and we can study it to see if there are interesting phenomena that aren’t anticipated.”

Author: Adam Malecek