As a situation changes, it is useful for systems to be able to change as well and adapt to the new state. Depending on the system though, this may not be easy or even possible, but there is a chance someone is working to change that. Researchers at the University of Bristol for example have recently found a way to alter a phononic crystal in real time, which could lead to some interesting new devices.
The phononic crystal in this study is actually a collection of colloidal, metamaterial crystals. Metamaterials are materials with properties not possible in Nature, and being colloidal means these crystals are actually in a liquid suspension. This combination gives the larger phononic crystal special properties and the researchers the ability to manipulate it, and even change how those properties are expressed. That manipulation comes by applying the proper acoustic wavelength to the crystal.
Potentially this phononic crystal could be used to create acoustic barriers that can be changed to best respond to whatever sound is striking it. The research could also be applied to other colloids, granting control over their properties cheaply and in a manner that could be easily integrated into other systems.
Source: University of Bristol