Scientists have long sought to invent materials capable of responding to the outside world in predictable and self-regulating ways. Now, new research being conducted at the University of Massachusetts at Amherst and appearing in the Proceedings of the National Academy of Sciences brings us closer to that goal. For their inspiration, scientists turned to nature.
Lampreys swim, horses walk, and insects fly: each of these behaviors is made possible by a network of oscillators – mechanisms that produce repetitive motion, such as wagging the tail, striding, or flapping a wing. In addition, these natural oscillators can respond to their environment in a predictable way. In response to different signals, they can quickly change speed, switch between different modes, or stop changing altogether. “The question,” says Hyunki Kim, co-lead author of the article, as well as Subramanian Sundaram of Boston University, a recent PhD recipient in polymer science and engineering from UMass Amherst, “can we make flexible materials, such as plastics, polymers and nanocomposite structures, which may respond in the same way? “The answer, as the team documents, is a definite yes.
One of the main challenges the team addressed was getting a series of oscillators to work in unison, a prerequisite for coordinated and predictable movement. “We have developed a new platform where we can control the coupling of oscillators with remarkable precision,” says Ryan Hayward, James and Catherine Patten, full professor of chemical and biological engineering at the University of Colorado at Boulder, and the ‘one of the co-authors of the article. This platform relies on another natural force, known as the Marangoni effect, which is a phenomenon that describes the movement of solids along the interface between two fluids driven by changes in surface tension. A classic and real example of the Marangoni effect occurs every time you wash the dishes. When you pour dish soap into a pot filled with water that has your dinner crumbs evenly sprinkled over the surface, you can watch the crumbs leak to the edges of the pot once the soap hits the water. This is because the soap changes the surface tension of the water and the crumbs are removed from areas of low soapy surface tension, towards the edges of the pan where the surface tension remains high.
“It all comes down to understanding the role of interfaces and the profound impact of combining polymeric and metallic materials in composite structures,” says Todd Emrick, co-author and professor of polymer science and engineering at UMass. Instead of soapy water and saucepans, the team used hydrogel nanocomposite disks made up of polymer gels and gold nanoparticles, which were sensitive to changes in light and temperature. The result was that the team was able to design a diverse range of oscillators that could move in unison with each other and respond predictably to changes in light and temperature. “We can now design complex coupled behavior that responds to external stimuli,” says Kim.
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