Physicists at Heinrich Heine University Düsseldorf and La Sapienza University in Rome have identified a phenomenon in which static friction can bring moving robots to a halt without external control. The research, published in Nature Communications, shows that collisions between robots, combined with the threshold principle of Coulomb (dry) static friction, can dissipate kinetic energy so effectively that the robots are unable to restart motion on their own.
In the study, hundreds of 3D-printed robots were placed on a vertically vibrating plate, moving randomly and colliding with each other. At high densities and low driving forces close to the static friction threshold, collisions caused many robots to stop moving. Over time, clusters of stationary, or “cold,” robots formed alongside moving, or “hot,” robots. According to the researchers, such coexistence of different “temperatures” does not occur in systems at equilibrium, where collisions normally balance out temperature differences.
The experiments, led by Professor Hartmut Löwen, were complemented by computer simulations performed at Heinrich Heine University by Dr Alexander Antonov, which reproduced the observed behavior under the same threshold conditions. The work suggests that the balance between particle activity and static friction can induce self-cooling in active systems.
Professor Lorenzo Caprini of La Sapienza University noted that the cooling occurred without external intervention, as collisions alone were sufficient to slow the robots. The researchers suggest that this mechanism could be applied to control the movement of large groups of robots or manage the behavior of granular materials without active input.
Image: HHU / Anton Ldov and Arnaud Fabian Romain Compagnie
