Like all researchers, Itai Cohen, Physics, has a lot of questions. But unlike many, his questions make big, topical leaps. From fruit flies to mosh pits, from origami to cartilage—Cohen dreams of preventing stampedes in Mecca, understanding the complex neuromechanics of fruit fly flight, and making self-folding robots from a single sheet of atoms. How can all this happen in one lab? Well, the answer is: it doesn’t.
“Many faculty tend to dig deep into one area, but there are very few people who sit back and see what the links are between different fields,” Cohen says. “That’s what we’ve figured out how to do. But this approach wholly depends on our collaboration with experts.”
Cohen relies on his own colloid science expertise, his students’ passion and knowledge, and the specializations of researchers from far afield and close to home. “If we want to bring techniques from colloid science to cartilage, then we go talk to Larry Bonassar [Meinig School of Biomedical Engineering/Sibley School of Mechanical and Aerospace Engineering], the world’s cartilage expert,” Cohen says. “Or we bring our research on the mechanics of origami to Paul McEuen’s group [Physics], which studies graphene. Together we can do something that neither of us could do on our own. That’s what makes our lab and, in my opinion, Cornell such a fun place.”
Metamaterials in the Making
One line of inquiry for Cohen’s group involves the use of origami techniques to engineer metamaterials—materials whose properties at the macroscale arise from the specific arrangement of small units. Cohen’s lab works to design these materials, patterning the creases in two dimensions and folding the structures up into three-dimensional structures. With this design paradigm, the group takes advantage of a vast infrastructure for two-dimensional patterning already developed for making computer chips, hijacking it to self-assemble three-dimensional structures.
For example, Cohen’s lab works with units of tessellated folding patterns such as the miura-ori tessellation, a common origami fold. If a metamaterial is comprised of a folded sheet of repeating, identical units, changing one of those units and creating a defect, by popping a valley fold into a mountain fold, for example, can affect the overall behavior of the material. Changing two neighboring units may alter the behavior further, or the changes could cancel each other out—the alterations, or defects, interact to impact the whole.
With these alterations, Cohen and his group can ultimately manipulate the material to have an array of mechanical properties. “Popping these defects in and out allows us to make a pixelated version of a programmable, mechanical material,” says Cohen.
These metamaterials could impact a number of industries, with possible applications in biomedical and mechanical engineering. More surprising, Cohen’s lab recently joined forces with designers to use the origami-inspired patterns to contribute to the textile industry. The designs debuted in a line of clothing and accessories at the Vancouver Fashion Week that included a circular, miura-ori skirt and a purse that contracted vertically to hold as much or as little as needed. The purse received a standing ovation.
Origami at the Nanoscale
Taking these techniques a step further, researchers have found that certain materials can be patterned in ways that will induce them to self-fold. One of Cohen’s collaborators, Ryan Hayward (University of Massachusetts), has the world record for smallest self-folded structure, a bird made out of a sheet of gel only a few microns thick, or one-fiftieth the diameter of a human hair. Cohen, with the help of Paul McEuen now aims to make a similar structure using graphene, a single sheet of atoms. “That’s the absolute limit, so we should be able to get a new and permanent world record,” says Cohen.
Cohen and his team have already achieved the initial step: they’ve folded graphene for the first time. “We take the graphene and put it on top of a layer of glass that’s only two nanometers thick,” Cohen says. “Then by heating one area of the graphene, differentially expanding one surface relative to the other, we can bend it.”
With the initial fold accomplished, Cohen foresees graphene origami structures with a wide range of possible functions. “We’re basically on the verge of ushering in an era of two-dimensional devices, sheets that we can pattern for three dimensions,” he says. “Jiwoong Park’s group [Chemistry and Chemical Biology] knows how to make circuits of graphene imbedded in an insulator—now you can think about folding these circuits up into three-dimensional structures. You can think about putting little motors on them and getting them to zoom around and detect things, pH or a concentration of some chemical. Then they could open up, change their conformation, and report back to a macroscale observer what’s going on at the nanoscale. That’s pretty cool. We’re not there yet, but that’s what this vision is about.”
Fruit Fly Flight Control
Another strand of Cohen’s research concerns a different kind of mechanics, with a different set of controls: he wants to know how exactly Drosophila species respond to aerodynamic instability. Similar to balancing a stick on one’s fingertips, flapping flight is a delicate balancing act made possible by ever-present, subtle, and fast corrective actions. Insects are expert correctors—to study how they do it, Cohen glues a tiny magnet to each fly and applies a magnetic pulse to rotate them during free flight. As the magnet aligns in the magnetic field, each fly experiences a brief, rotating torque. Using a custom-built, three-dimensional tracking apparatus, Cohen then measures how flies correct these disturbances. He and his team are currently working on figuring out how flies activate each of the 13 steering muscles at the base of the wing to control their flight.
With the initial fold accomplished, Cohen foresees graphene origami structures with a wide range of possible functions. “We’re basically on the verge of ushering in an era of two-dimensional devices, sheets that we can pattern for three dimensions.”
In collaboration with scientists at the Howard Hughes Medical Institute, Cohen perturbs flies in which different neurons in the flight control circuit have been silenced. By silencing a given neuron, applying the perturbations, and recording changes in the fly’s response, Cohen can tease out each neuron’s role. The idea is to use top-down approaches in concert with bottom-up neuroscience. “If you took apart a car and wanted to know how it functions just by looking at the parts, it’s really hard,” Cohen says. “I can give you the full neural architecture of the fly, but can we tell what it’s doing? So we really have to link the behavior to the neurons. That’s what these experiments are meant to do.”
Gels, Pastes, and Shear Thickening Materials
Cohen’s research also extends into using in situ microscopy techniques to determine how materials including colloidal suspensions, gels, and biological tissues, such as cartilage, respond to deformation. “The microscopy gives us the ability to determine how the microscale structure leads to the macroscale behavior that we measure,” Cohen says.
From Dance to Migration
Cohen’s interests have recently taken another surprising turn: straight into a mosh pit. Using videos publicly available online, Cohen, in collaboration with James P. Sethna, Physics, studies the highly energized, collective motion of attendees at heavy metal concerts. They have found that these extreme social gatherings generate similarly extreme behaviors: the mosh pit, a disordered gas-like state, as well as an ordered vortex-like state called a circle pit.
Both phenomena are reproduced in computer simulations that have shown human collective behavior to be consistent with the predictions of simplified models. Cohen hopes to apply these models to situations where such transitions from disorganized jostling to a stampede often end in tragedy. “If we can figure out how to make it more difficult for the system to make that transition, many lives could be saved,” says Cohen. He also hopes to scale these studies up so that his models can address larger problems such as human migration in response to extreme events.