A toolkit for transformable materials

How to design materials with reprogrammable shape and function

Harvard researchers have developed a general framework to design reconfigurable metamaterials that is scale independent, meaning it can be applied to everything from meter-scale architectures to reconfigurable nano-scale systems (Image courtesy of Johannes Overvelde/Harvard SEAS).

Metamaterials — materials whose function is determined by structure, not composition — have been designed to

• bend light and sound, 
• transform from soft to stiff, and 
• even dampen seismic waves from earthquakes. 

But each of these functions requires a unique mechanical structure, making these materials great for specific tasks, but difficult to implement broadly.
But what if a material could contain within its structure, multiple functions and easily and autonomously switch between them?

Researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and the Wyss Institute of Biologically Inspired Engineering at Harvard University have developed a general framework to design reconfigurable metamaterials. The design strategy is scale independent, meaning it can be applied to everything from meter-scale architectures to reconfigurable nano-scale systems such as photonic crystals, waveguides and metamaterials to guide heat.

The research is published in Nature.

“In terms of reconfigurable metamaterials, the design space is incredibly large and so the challenge is to come up with smart strategies to explore it,” said Katia Bertoldi, John L. Loeb Associate Professor of the Natural Sciences at SEAS and senior author of the paper. “Through a collaboration with designers and mathematicians, we found a way to generalize these rules and quickly generate a lot of interesting designs.”

Bertoldi and former graduate student Johannes Overvelde, who is the first author of the paper, collaborated with Chuck Hoberman, of the Harvard Graduate School of Design (GSD) and associate faculty at the Wyss and James Weaver, a senior research scientist at the Wyss, to design the metamaterial.

The research began in 2014, when Hoberman showed Bertoldi his original designs for a family of foldable structures, including a prototype of an extruded cube. “We were amazed by how easily it could fold and change shape,” said Bertoldi. “We realized that these simple geometries could be used as building blocks to form a new class of reconfigurable metamaterials but it took us a long time to identify a robust design strategy to achieve this.”

The interdisciplinary team realized that assemblies of polyhedra can be used as a template to design extruded reconfigurable thin-walled structures, dramatically simplifying the design process.

“By combining design and computational modeling, we were able to identify a wide range of different rearrangements and create a blueprint or DNA for building these materials in the future, ” said Overvelde, now scientific group leader of the Soft Robotic Matter group at FOM Institute AMOLF in the Netherlands.

The same computational models can also be used to quantify all the different ways in which the material could bend and how that affected effective material properties like stiffness. This way they could quickly scan close to a million different designs, and select those with the preferred response.

Once a specific design was selected, the team constructed working prototypes of each 3D metamaterial both using laser-cut cardboard and double-sided tape, and multimaterial 3D printing. Like origami, the resulting structure can be folded along their edges to change shape.

“Now that we’ve solved the problem of formalizing the design, we can start to think about new ways to fabricate and reconfigure these metamaterials at smaller scales, for example through the development of 3D-printed self actuating environmentally responsive prototypes,” said Weaver.

This formalized design framework could be useful for

• structural and aerospace engineers, 
• material scientists, 
• physicists, 
• robotic engineers, 
• biomedical engineers, 
• designers and 
• architects.

“This framework is like a toolkit to build reconfigurable materials,” said Hoberman. “These building blocks and design space are incredibly rich and we’ve only begun to explore all the things you can build with them.”

This work was supported by the Materials Research Science and Engineering Center and the National Science Foundation.

ORIGINAL: Harvard SEAS
By Leah Burrows
January 18, 2017

Lightning Strikes Can Change Rocks' Atomic Structure

New research suggests that rock crystals melt under the intense force and heat of lightning

Lightning strikes near the U.S. Capitol building (Tech. Sgt. Cherie A. Thurlby/U.S. Air Force)

When lightning strikes sand in the desert, it instantly melts the grains, creating complex structures that geologists call fulgurites. These structures of partially melted minerals can also be found in lightning-struck rocks. But that's just the beginning: While investigating this phenomenon on an outcropping in southern France, researchers discovered that the changes can go even further — down to the atomic level. 

Lightning can warp quartz crystals in rock to form distinct structures typically found in meteorites, reports Elizabeth Goldbaum for LiveScience. So-called shock lamellae are minute parallel lines that run through the quartz and indicate the rock was hit with an intense force. 

"It's like if someone pushes you, you rearrange your body to be comfortable," researcher Reto Gieré of the University of Pennsylvania says in a press statement by Katherine Unger Baillie. "The mineral does the same thing." Lightning's push, however, is equivalent to a force that's 20 million times greater than a boxer's punch, as Baillie writes.

Goldbaum reports for LiveScience:

After looking at very thin, almost transparent, slices of the fulgurites under a powerful microscope, the researchers noticed that the black fulgurite looked glossy, "almost like a ceramic glaze," Gieré said. The fulgurite was also porous, similar to foam; the researchers suspect it got that way when the sizzling lightning vaporized the rock's surface.

Chemical traces of sulfur dioxide and phosphorous pentoxide were all that remained of what the researcher believe was once lichen growing on the rock’s surface. Under the foamy, glassy fulgurite layer, shock lamellae were hidden, only visible with a transmission electron microscope. There, a thin layer of what were once quartz crystals had melted and deformed so much that the crystal structure was destroyed. The team published their findings in American Mineralogist.

Once he knew what to look for, Gieré started seeing a wet, darkened look to rocks that indicated the presence of fulgurites in many places. He suggests that hikers should stay on the lookout for such rocks — especially when trekking across exposed rocky faces or scrambling to the crowns of mountaintops. Fulgurites could warn of an area prone to lightning strikes and are a good reason to keep an eye on the clouds.

The fulgurite on this rock resembles a dark stain. Via PennNews (PennNews)

ORIGINAL: Smithsonian

By Marissa Fessenden SMITHSONIAN.COM 

AUGUST 17, 2015

Think Like a Tree: What We Can Learn From the Oaks That Survived Katrina

Ten years ago this week, Hurricane Katrina ripped through New Orleans and the Gulf Coast, bringing floods and gale-force winds that devastated the region and displaced more than a million people. But New Orleans’ live oaks were surprisingly resilient, as biologist Janine Benyus describes in our first episode of a new video series on biomimicry, Think Like a Tree. 

As the tallest living things on earth, trees have developed strategies to protect themselves against threats to their leaved towers. In the process, they’ve “managed to solve daunting problems of engineering,” says Steven Vogel, a Duke biologist who studies the ways organisms structure themselves in moving fluids. 

Take the beating a tree gets from a hurricane. Gale force winds hammer trees with a dynamic collection of blows, which unleashes “a suite of mechanical problems that would give an engineer nightmares,” Vogel says. Beyond withstanding high wind speeds, trees need to deal with wind acceleration and the air’s “throw weight”—its mass, basically. Calms between gusts can be damaging, too, as the tree rebounds and sways, potentially building up heavy loads on branches and roots. Not to mention the litany of other environmental factors that come into play during a storm: precipitation levels, soil conditions, the state of the surrounding trees. 

So, what’s a tree to do?
Leaves that work great for photosynthesizing become liabilities in high wind, Vogel says, where they act like little sails with a lot of drag. So in strong, 40 mph winds, the leaves of trees like maple, poplar, and holly will reconfigure into more aerodynamic shapes: curling up into little tubes, clumping together into cones, or flattening to reduce drag. And strong root systems serve as a countermeasure to the drag of the leaves and the wind’s sideways force. 

Trees might be silent, brilliant engineers, but Vogel cautions that they may not be the best candidates for biomimicry. Trees operate under certain constraints—they grow all their own material, which takes energy that could be spent on other needs like reproduction. “Nature usually builds to a design criterion of adequate strength,” Vogel says, and that means maximizing whatever will keep the population going. If one tree goes down, that’s okay as long as most of them survive. But we build our cell towers and skyscrapers much more sturdily than they usually need to be, because we want them to work all the time. And we can account for that, Vogel says, thanks to modern engineering. So we’ll stick with our steel beams for now. 

ORIGINAL: Wired