(319c) Emergent Activity

Active matter, a form of matter with components that consume energy and convert it to motion, has already spurred the discovery of new materials properties. Researchers have found novel dynamics-driven phase transitions and activity has been proposed as a means of augmenting modern drug delivery methods. Thus far, active matter has been limited to systems with individually driven components. Here, we uncover a new form of activity that emerges as a collective property of the system. While individual particles remain stationary, clusters of particles gain the ability to transduce energy from the environment and use it to propel their motion.

This emergent activity arises in systems of particles immersed in a wave. Scattering of the incident wave off the particles induces an effective force that is not symmetric: the force of one particle on a second particle is not necessarily equal to the force of the second particle back on the first. This apparent violation is allowed because the system is not closed. However, as a result of this asymmetric, or nonreciprocal, force, the particles exhibit promising new modes of activity. To date, generating new modes of activity has relied on tremendous innovations in particle synthesis, limiting the range of motion and control that can be achieved with active matter. In contrast, emergent activity exhibits several modes of motion, including rotation and translation, and the motion can be tuned via readily accessible experimental parameters, such as particle configuration, size, and density. As a result, this system is well-suited to inverse design. By combining different modes of motion, this system could be used to engineer novel devices such as micron-scale motors or to design activity that occurs only in response to specific interactions.