A team from the University of Massachusetts Amherst is creating a material that can both absorb and release energy.
The two main ingredients of the material: magnets and rubber.
With the ability to both drive high-power motion and to quickly dampen impact-loading events, the material has a number of promising applications — from giving robots a boost to making helmets and protective equipment that dissipate energy quickly.
The engineers, led by UMass Professor Alfred Crosby and researchers Xudong Liang and Honbo Fu, used laser cutters to modify the rubber and make snug spots for the 3-mm-wide store-bought magnets.
When you stretch the material, you change a physical property known as the “phase.”
When a phase transition is reached, you get a lot of potential energy — the kind that can power a vehicle, according to Crosby.
“That phase transition actually stores additional energy beyond what I'm putting in mechanically,” Prof. Crosby told Tech Briefs. “So, a drone can recover that additional energy when the material releases, and it can actually give a little bit of an extra boost.”
The magnets control the phase shift, which greatly amplifies the amount of energy the material releases or absorbs. The team at UMass has found a way to fine-tune the phase shift.
The particular placement of the magnets is determined by the elastic properties of the rubber — and the geometry of the holes in the rubber strip. Together, the hole geometry, the elastic properties, and the magnetic strength determine where and how they should be placed to get a specific response.
The engineers at UMass developed the mathematical model that allows the materials to be programmed with expected responses.
“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,” said Xudong Liang , the paper’s lead author, currently a professor at Harbin Institute of Technology, Shenzhen (HITSZ), China.
In a short Q&A below, Prof. Crosby tells Tech Briefs about promising applications for a predictable material.
Tech Briefs: What inspired the creation of this material?
Prof. Alfred Crosby: We're part of a larger team. My group has a project that's funded by the Army Research Office. The project involves several roboticists at different universities, as well as biologists, and we're working to understand how to create very-high-speed, high-acceleration movement in a repeatable way.
This is all taking inspiration from some of the fastest moving organisms in nature, like the mantis shrimp and trapjaw ant. And one of the things that nature does is combine many different “fields” that influence the way energy is stored mechanically in an animal. This may be a chemical field as well as an elastic field.
To understand that concept, our group began to combine magnetic fields with elastic fields to show how you could do this in synthetic devices that may be used in robots or drones.
Tech Briefs: What other applications do you envision with this kind of material?
Prof. Crosby: Our team works with roboticists who are making jumping robots. Jumping robots are a quick way to transverse different obstacles. You can really tune this material in an energy efficient way.
If I stretch the material a little bit, it'll just act like a normal rubber band or a normal spring.
If I stretch it a lot, I start to go through what we call a “phase transition.” That phase transition stores additional energy beyond what I'm putting in mechanically. A drone can recover that additional energy when the material releases, and it can actually give a little bit of an extra boost.
It’s a way of allowing a device like a robot to choose: “Oh, do I need that little boost here? Should I store it a little bit more or should I not?”
We’re discovering that animals do this in in different ways too.
Tech Briefs: For, say, a robotic application, what would initiate and control that kind of tuning?
Prof. Crosby: You would have an onboard processor. An actuator would be connected to the spring to determine: Should I stretch [the material] super far and get this extra stored energy, or should I not? We have done some work across the team for understanding the control theory of how you interact with the processor and the materials like this — to put that down, for engineers to be able to design the robot in a programmed way.
Tech Briefs: Have you considered how this material could be produced on a commercial scale?
Prof. Crosby: To some degree, yes. We have not started producing it on large scales. It's actually fairly scalable. These elastomers or rubbers can be produced on a roll scale or a molded scale. Then, you can use laser cutting or die punching to create the holes and insert the magnets.
A lot of that kind of conversion process already exists in other materials fabrication, so that what we've provided here is the design element: Where do you want the holes, and where do you want the magnets placed, rather than just scattering them anywhere in the in the material?
In our paper, we've told people where you can put holes into rubber materials and place in magnets to get this extra benefit.
In the above video, the free-falling of a weight hung by the metamaterials is triggered by a customized platform at time t = 0 ms. The vibration decays rapidly, with a decay time of about 0.2 s. Elasto-magnetic metamaterials absorbed most of the impact energy via the phase transitions, only returning a small part to the mass for vibrations.
Tech Briefs: So, can you control this materials area and the thickness of this material as well?
Prof. Crosby: Yes, right now through either just cutting and laser cutting and shaping, or from the supplier in terms of how they produce it, in the molds that they're producing it in. To a large extent, we can't control that.
Tech Briefs: What do you mean by phases, with your material?
Prof. Crosby: In our material, we go through what is called an open phase and a closed phase. The open phase is literally where the holes are all open, and you can see through them, and you can pass air or gas through them. In the closed phase all the holes actually make a closed contact. It's a very defined line where you will have these two different kinds of states of the material — they'll look like different shapes or different geometries.
And the magnets, because of where they are, will react in different ways.
What we've identified in this paper is the phase transition line. Starting from a single phase and going into what we call a double phase material will be phase-absorbing. Coming from a double phase back into a single phase will be energy-enhancing.
In the above video, a local configuration transitioning from closed to open phase emerges at the top and bottom end. It propagates toward the sample’s center — the configuration changes to a homogeneous open phase as the deformation increases in the loading cycle. The sample maintains the open phase in the unloading path, ending with a fast transition to the closed phase as the magnets snap to each other within a small distance.
Tech Briefs: What is most exciting to you about this this achievement?
Prof. Crosby: We can now design this phase transition in millimeter- or near-millimeter-sized scales, where you can manipulate it with machines in production lines and make whatever kind of phase-transitioning material that you want, to be able to recover or store energy in a different way.
So, phase transitions, just like the water to ice transition, have been known for a long time. Engineering those transitions, in a precise way however, is really difficult at an atomistic level. We've opened a new way of engineering phase transitions on scales that our machines can deal with right now.
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Read the report from the Proceedings of the National Academy of Sciences .