Playing with active matter.

In the past 20 years, active matter has been a very successful research field, bridging the fundamental physics of nonequilibrium thermodynamics with applications in robotics, biology, and medicine. Active particles, contrary to Brownian particles, can harness energy to generate complex motions and emerging behaviors. Most active-matter experiments are performed with microscopic particles and require advanced microfabrication and microscopy techniques. Here, we propose some macroscopic experiments with active matter employing commercially available toy robots (the Hexbugs). We show how they can be easily modified to perform regular and chiral active Brownian motion and demonstrate through experiments fundamental signatures of active systems such as how energy and momentum are harvested from an active bath, how obstacles can sort active particles by chirality, and how active fluctuations induce attraction between planar objects (a Casimir-like effect). These demonstrations enable hands-on experimentation with active matter and showcase widely used analysis methods. Editor's Note: Active matter consists of particles (e.g., birds, cells, or synthetic objects) that can self-propel. These systems are currently being researched not only because they can display collective behaviors and some level of self-organization but also because of their potential applications such as in drug delivery. Both in the classroom and in research labs, toy robots such as Hexbugs can be used to model active matter. This article shows that, by playing with these toys, undergraduate students can visualize the concepts they're being taught in advanced thermodynamics or soft matter classes: chiral and non-chiral active Brownian motion, interaction of the particles with their environment, and particle sorting. Time to play! [ABSTRACT FROM AUTHOR]