Using Hexbugs to model active matter

Active matter is made up of particles or units that are constantly in active motion. We see active motion in everyday life, such as bacteria swimming in a dish, self-propulsion of some particles, or birds flying in a flock.

This type of motion can be hard to visualize, but Volpe et al. showed that commercially available toys called Hexbugs can be used as a macroscopic model to demonstrate complex active motion principles in the classroom.

“Our motivation was to try to propose some experiments with active matter using something that anyone could get ahold of,” Volpe said. “We are doing work with robotics on the microscale, but that’s not something that you can propose for classroom use.”

Hexbugs are plastic robots with six pairs of rubber legs propelled by a vibrational motor. As the bug vibrates, friction between the legs and the surface causes the bug to move forward at a constant speed. The researchers were able to show that the motion of a single Hexbug can model active Brownian motion.

The team also demonstrated that Hexbugs can model chiral Brownian motion and a “Casimir-like” effect, in which two planar objects are forced together due to active fluctuations.

Volpe’s team is currently developing their own robots for more elaborate modeling experiments.

“Hexbugs are a nice model system for active particles, but if you want to have intelligence built on that, you need to have something more complex,” Volpe said.

For educational purposes, however, the Hexbugs are a powerful teaching tool. Let’s play!

Source: “Playing with active matter,” by Angelo Barona Balda, Aykut Argun, Agnese Callegari, and Giovanni Volpe, American Journal of Physics (2024). The article can be accessed at https://doi.org/10.1119/5.0125111 .