Many materials, both living and engineered, are powered from within. Scientists have thoroughly investigated how such ‘active’ materials operate, but so far, mostly in circumstances where the curvature of the environment does not play a role. In research published in Physical Review Letters this week, a team of physicists proposes a framework to describe how active materials operate in the presence of curvature. The framework explains striking biological observations and may lead to geometry as a design parameter for new materials.
Examples of active materials include tissues made of moving cells and mechanical metamaterials that use embedded motors or actuators. A major difference between such active materials and ordinary materials lies in the way they deform. Push an ordinary material, and it will deform where the force acts. Active materials, on the other hand, can generate further forces that redirect deformations, leading to all kinds of unexpected, but often quite useful, behavior.
A striking example comes from observations in biological experiments, where starfish embryos were found to self-organize into crystal-like structures on a water surface. In this example, an extra factor comes into play: The water surface in the test tubes used in the experiments is not flat but slightly curved—just like the surface of water in an ordinary drinking glass that curves upward where the water touches the glass. One consequence of the curvature is that the starfish embryos don’t fit on the surface in a completely regular pattern: Instead, the pattern has occasional irregularities or defects.
