Programmable Materials

By Michael Lucibella

APS March Meeting 2015 – Physicists are finding new ways to dramatically alter the mechanical properties of a material by changing its physical form. Researchers at the March Meeting in San Antonio showed how they are developing ways to control the compressibility of elastic materials.

“We can engineer the energy absorption by engineering the structure,” said Katia Bertoldi of Harvard University.

The materials that she’s developing can dissipate the force of a collision better than naturally-structured materials can. Introducing a lattice of regular, round voids into blocks of ordinary rubber fundamentally changes how it reacts to an impact.

“I can deform it very fast or very slow, the response is going to be the same. This is not the case for most of the materials currently used,” Bertoldi said. “Another interesting feature is that it’s scale independent … I can make it very small, or I can make it very big. I can make it meter scale or I can make it nanometer scale.”

When compressed, the block’s normally-round cavities abruptly collapse into horizontally- or vertically-aligned ovals. This rapid switching helps to dissipate the impact, but serves also as a way to design customized materials with different compliances.

Bertoldi showed an example in which a truss-shaped structure collapsed down after a threshold of force was applied. An egg cushioned by the truss was intact after a two-foot drop, a feat that a solid block couldn’t perform.

Bastiaan Florijn of Leiden University added another variable to the mechanical programming of materials. He too is working with blocks of elastomers with circular holes, and he found a way to further customize how they behave. He placed pins on the side of the blocks to control which holes compress and in what direction.

“It’s just a slab of rubber with holes of different sizes,” Florijn said. “We use just these simple clamps on the side of the matter to confine the compression in the horizontal direction.”

Like Bertoldi’s egg demonstration, the material stays rigid until it encounters a threshold level of force and then collapses down abruptly, dissipating much of the impact in the process. The pins along the block’s sides program how much that threshold force is.

“Just by changing the confinement in the horizontal direction, we can change the mechanical response,” Florijn said. “We don’t need to have a lot of different materials, [you] can use just one material and get all of this very exotic behavior out of it.”

Moreover, this process is reversible. Even after a crushing collision, pulling on the elastomer block will pop it back into its original shape, with its original properties unchanged. “Our system is still intact, and our system is still elastic,” Florijn said. “We can, for example, imagine using this material to make a car bumper.”

Using pins to control the flexible structure’s deformation is an early step towards designing truly programmable mechanics into materials. Already Florijn and his team are working on designing a three-dimensional material that can crush down from any direction.