What do submarines and football helmets have in common? Someday they could be built with the highly specialized materials Ramathasan Thevamaran is researching. Joining the Department of Engineering Physics in fall 2017 as an assistant professor, Thevamaran is interested in manipulating the structure and mechanical properties of materials to provide a very specific functionality. Materials like these could, for example, make military submarines undetectable with a wave-absorbing outer surface, or protect athletes from brain injuries with an energy-absorbing foam that makes helmets both lighter and tougher.
Although the potential applications for structured materials are virtually endless, Thevamaran isn’t really thinking about those right now. “I enjoy doing fundamental research that has the potential to influence different application areas in the future,” he says. “Advancing fundamental knowledge is one of my primary purposes right now.”
And there’s a lot of fundamental knowledge under the very big umbrella of structured materials to advance. In particular, he’s experimenting with hierarchical and gradient property changes, as well as nonhermitian and parity-time symmetric acoustics.
Hierarchical property changes can be made at varying length scales—as small as the nanoscale and as big as several millimeters. Take vertically aligned carbon nanotube foams, for example. At millimeter scale, you can see a foam with the naked eye. Through a microscope, you can see bundles of aligned nanotubes. Zooming in further, you’re able to see individual nanotubes and the multiple walls that construct them. Each level, or length scale, provides an opportunity to edit structural features. In this example, the number of walls could be altered, patterns could be redesigned, and even the geometry of a single nanotube could be modified.
The ability to edit every single feature during synthesis allows for more control over the design of materials. “Whatever mechanical properties you’re interested in, you can affect bulk scale properties—strength, stiffness, damping—with changes introduced at different length scales,” Thevamaran says.
It’s also more resource-efficient to design materials this way. “With this kind of control, we don’t have to design a bulk material with a huge amount of material volume,” he says. “Instead, we can choose to put structural features where they are needed.”
Gradient properties, on another hand, are properties constructed to vary over a gradient. Think of a robot trying to pick up an egg without cracking it. Bringing the very stiff mechanical component of the robot into contact with the very soft medium of the egg presents a challenge. But if the mechanical component is constructed to vary in stiffness in such a way that it becomes softer closer to the surface where the contact is made, the robot could successfully handle the egg. “Grading the material’s stiffness provides a specific, predictable, and desirable function,” Thevamaran says.
Gradient properties can be used to create ultra-strong and ductile materials, too. For example, Thevamaran looks for ways to maximize a metal’s strength and ductility at the same time. Strength is how much stress a material can withstand before it breaks, and ductility is the amount of energy that material can absorb before it fractures. Both are crucial to the mechanical functionality and integrity of a structure, but it’s nearly impossible for a single material to have one without compromising the other. For example, a glass has good strength, but because it has very low ductility, it fractures rapidly. Alternatively, metal is deformable without fracturing due to its high ductility, but presents lower strength.
Combining strength and ductility in one optimal material is the challenge—and Thevamaran is researching the solution. It’s called gradient-nano-grained metal. It works by altering the size of the grains that make up a metal using a spatial gradient—from nanoscale to coarse-grain scale. “The nanograins provide high strength while the coarse grains provide ductility, rendering the best of two mutually exclusive properties,” he says.
He’s also researching a newly emerging discipline, nonhermitian and parity-time symmetric acoustics. Basically, he’s taking principles from quantum field theories and ideas from nonlinear dynamics, and applying them to acoustic systems to control sound propagation. “I’m taking energy loss in materials, which is usually considered a disadvantage, and using it to my advantage to create structured materials with periodically varying energy loss,” he says. “Potential uses for materials like these are directional vibration control or nonreciprocal wave transport.”
Nonreciprocal wave transport, for example, allows sound to travel through one way, but not the other. Imagine being able to hear someone on the other side of a wall, but knowing they cannot hear you back. Or, think of that undetectable submarine. A submarine built with a nonhermitian system could completely absorb detection waves—not reflect any waves back, and camouflage the submarine.
While Thevamaran’s areas of research and experimentation are varied and many, so is his background. He’s spent time in multiple disciplines—civil engineering as an undergraduate, mechanical engineering for his master’s and PhD, and materials science and engineering as a postdoctoral researcher—to develop a spectrum of fundamental knowledge that gives him the skills and perspective to solve problems with a multidisciplinary approach.
At UW-Madison, he is excited to collaborate with people in different disciplines and from different backgrounds. “When you walk into a new research area with one particular skill, you bring in a fresh perspective that allows you do something in a very creative fashion,” he says.
Author: Adrienne Nienow