Structural Defects in Metamaterials can Produce Different Textures, Behavior

A piece of paper is generally flat and floppy, but when the same piece of paper is scrunched into a wad, it becomes round and stiff. This action shows that crumpling alters the behavior and texture of the same precise material, that is, paper.

Image Credit: Tel Aviv University

Recently, a study was carried out at Tel Aviv University which demonstrates how induced flaws in metamaterials also create drastically different behaviors and consistencies. Metamaterials are synthetic materials and their properties are different from those found in nature.

The latest study has far-reaching applications. For example, it can help protect fragile equipment launched in space as well as protect delicate components in systems that experience mechanical traumas such as passengers involved in car accidents. It can also help in seizing and exploiting remote objects using a tiny series of localized manipulations, such as minimally invasive surgery.

We’ve seen non-symmetric effects of a topological imperfection before. But we’ve now found a way to create these imperfections in a controlled way. It’s a new way of looking at mechanical metamaterials, to borrow concepts from condensed-matter physics and mathematics to study the mechanics of materials.

Yair Shokef, Study Co-Author and Professor, School of Mechanical Engineering, Tel Aviv University

The latest study is the outcome of a partnership between Professor Shokef and Dr Erdal Oğuz from Tel Aviv University and Professor Martin van Hecke and Anne Meeussen from Leiden University and AMOLF based in Amsterdam.

The study was published in the Nature Physics journal on January 27th, 2020. “Since we’ve developed general design rules, anyone can use our ideas,” added Professor Shokef.

We were inspired by LCD-screens that produce different colors through tiny, ordered liquid crystals. When you create a defect—when, for example, you press your thumb against a screenyou disrupt the order and get a rainbow of colors. The mechanical imperfection changes how your screen functions. That was our jumping off point.

Yair Shokef, Study Co-Author and Professor, School of Mechanical Engineering, Tel Aviv University

Using three-dimensional (3D) printing, the researchers developed an intricate mechanical metamaterial and then induced defects inside its structure. They eventually demonstrated how a mechanical response is influenced by such localized defects.

The newly developed material was flat and created from triangular puzzle pieces, the sides of which moved by dimpling in and bulging out.

In an imperfect material, one side of this material is soft while the other side is rigid. But in a “perfect” material, one side of this material is soft when squeezed from both sides. This effect changes when the structure is squeezed at one side and expanded at the other, that is, soft parts become stiff, and stiff parts become soft.

That’s what we call a global, topological imperfection. It’s an irregularity that you can’t just remove by locally flipping one puzzle piece. Specifically, we demonstrated how we can use such defects to steer mechanical forces and deformations to desired regions in the system.

Yair Shokef, Study Co-Author and Professor, School of Mechanical Engineering, Tel Aviv University

The latest work provides a better understanding of structural defects and their topological characteristics in condensed-matter physics systems. It also defines a bridge between unorganized mechanical networks, which are usually found in biomaterials, and periodic, crystal-like metamaterials.

The researchers are planning to continue their study on 3D complex metamaterials and analyze the presence of richer geometry of imperfections.


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