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Elasto-Magnetic Materials

September 26, 2022

Many inquisitive children, my youthful self included, are fascinated by the magical properties of some of their toys. Of course, magnets and their amazing action-at-a-distance are at the top of the list, followed by the gyroscope, temperature-induced color changing toys, and motion-activated LED light-up toys. Sometimes, the mechanical design of toys can inspire some science, an example being the Chinese finger trap.

Figure trap toys (Chinese finger trap)

Finger trap toys are also known as Chinese finger traps, Chinese finger puzzles, and Chinese handcuffs; or, to shift from cultural stereotyping to gender stereotyping, a single-ended version from 1870 was named the "Mädchenfänger" ("girl catcher").

The device traps the victim's fingers in both ends of the specially woven cylinder. Pulling your fingers outwards tightens the trap, and the only way to escape is to push the ends toward the middle. This enlarges the openings and frees the fingers.

(Wikimedia Commons image by Carol Spears.)

The action of the Chinese finger trap shows that particular structural linkages can lead to interesting mechanical properties. Auxetic materials, which I wrote about in an earlier article (Auxetics, May 13, 2011) have linkages that give them a negative Poisson's ratio; that is, when they are stretched, they become thicker perpendicular to the applied force, a property that's opposite to that of nearly all materials, an easily visualized example being a marshmallow.[1-2] The two-dimensional example of the figure demonstrates this auxetic property, but such linkages can be chemical bonds in an actual material.

Example of an auxetic material

Example of an auxetic material.

In this two-dimensional model, pulling it from the top and bottom will cause the sides to expand, which is opposite to a "normal" material's response.

(Created using Inkscape.)

The term, auxetics, is derived from the Greek word, αυξητικος (auxetikos), meaning something that tends to increase. Poisson's ratio is typically in the range of about 0.3 for most metals. Cork is an interesting example of an organic material, since it has a Poisson's ratio that's about zero. This makes it an ideal bottle stopper material, since it can be easily inserted and removed. Rubber has a very large Poisson's ratio about 0.5. Crystalline auxetics are few in number, but one example is α-cristobalite, which shows a Poisson's ratio of -0.5 in some crystal directions, with an averaged value for polycrystalline aggregates of -0.16.[3]

The most useful auxetics are not molecular, they're metamaterials, materials created as connected elements of normal materials, usually in a repeating pattern. Foams approximating the interlocking hourglass structure shown in the figure are auxetic. A team of scientists from the University of Massachusetts (Amherst, Massachusetts) and the Harbin Institute of Technology (Shenzhen, China) published an open access article early this year in the Proceedings of the National Academy of Sciences in which they present a novel metamaterial that combines an elastic, rubber-like material with tiny embedded magnets.[4-5] This elasto-magnetic material uses mechanical phase shift to amplify the storage and release of energy.[4-5]

Venus fly trap animation

Closing action of a Venus fly trap (Dionaea muscipula), as triggered by a pencil touch.

The trapping structure is at the end of each of the plant's leaves, and it's triggered by small hairs at the inner surface.

The trapping action of this rapid plant movement happens in about 100 milliseconds.

(Edited Wikimedia Commons image by Markus Nolf.)

The research team was reportedly inspired by some rapid response mechanisms in nature, such as the snap action capture of insects by the Venus fly trap (Dionaea muscipula), and the action of the trap-jaw ant (Odontomachus).[5] There are numerous examples of phase-transforming materials based on atomic- or molecular-level mechanisms; but, the research team developed an elasto-magnetic metamaterial that displays a phase transformation by a nonlinear interaction between macroscopic scale magnetic domains embedded in an elastic structure.[4]

The resultant mechanical phase transition is a non-monotonic stress–strain relation that can be used to store mechanical energy and then release it during high strain rate events, such as impulsive recoil and impact.[4] Says Xudong Liang, the paper’s lead author, who completed this research while a postdoctoral scientist at the University of Massachusetts, Amherst, and is presently a professor at the Harbin Institute of Technology,
"By embedding tiny magnets into the elastic material, we can control the phase transitions of this metamaterial. And because the phase shift is predictable and repeatable, we can engineer the metamaterial to do exactly what we want it to do: either absorbing the energy from a large impact, or releasing great quantities of energy for explosive movement."[5]
Alfred Crosby, a professor of polymer science and engineering at Amherst and the paper's senior author, explains the principle of their metamaterial.
"Imagine a rubber band... You pull it back, and when you let it go, it flies across the room. Now imagine a super rubber band. When you stretch it past a certain point, you activate extra energy stored in the material. When you let this rubber band go, it flies for a mile."[5]
The action of their metamaterial super rubber band is shown in the following University of Massachusetts, Amherst, video.[5]

This mechanical phase transition can be used as a powerful actuator in which stored energy is suddenly released, and the reversibility of its states would enhance performance of certain devices.[4] This new metamaterial can be used to enable efficient robotic motion, and it can be used as an energy absorber for high-force impacts to protective helmets.[5] This research was supported by the Army Research Office of the United States Army Research Laboratory, and the Harbin Institute of Technology.[5]


  1. Rod Lakes, "Negative Poisson's ratio materials," University of Wisconsin web page.
  2. Roderic Lakes, "Foam structures with a negative Poisson's ratio," Science, vol. 235, no. 4792 (February 27, 1987), pp. 1038-1040, DOI: 10.1126/science.235.4792.1038.
  3. Amir Yeganeh-Haeri, Donald J. Weidner, and John B. Parise, "Elasticity of α-Cristobalite: A Silicon Dioxide with a Negative Poisson's Ratio," Science, vol, 257, no. 5070 (July 31, 1992), pp. 650-652, DOI: 10.1126/science.257.5070.650.
  4. Xudong Liang, Hongbo Fu, and Alfred J. Crosby, "Phase-transforming metamaterial with magnetic interactions," Proc. Natl. Acad. Sci., vol. 119, no. 1 (January 4, 2022), Article no. e2118161119, https://doi.org/10.1073/pnas.2118161119. This open access article with a PDF file here.
  5. Scientists engineer new material that can absorb and release enormous amounts of energy, University of Massachusetts Amherst Press Release, February 2, 2022.

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