In a groundbreaking study that pushes the boundaries of active matter physics, researchers have unveiled a novel mechanism by which a collective of self-driven robots can spontaneously come to a complete halt without any external intervention. This phenomenon, termed self-sustained frictional cooling, was reported by physicists from Heinrich Heine University Düsseldorf (HHU) in Germany and La Sapienza University in Rome. The key lies in the intricate interplay between robot collisions and the fundamental principles of static friction, offering exciting prospects for the autonomous control of robotic swarms and active materials.
Active matter systems — collections of particles that consume energy to move — have long fascinated scientists due to their rich and often counterintuitive behaviors. Typically, such systems remain in perpetual motion unless influenced by external forces or programmed controls. However, the team led by Professor Hartmut Löwen at HHU discovered that when numerous miniature robots move and collide within a confined space, their velocity distribution naturally evolves towards a coexistence of “hot” (fast-moving) and “cold” (stationary) clusters. This emergent pattern defies conventional equilibrium dynamics, suggesting new physics governing collective motion under frictional constraints.
At the heart of this discovery is the principle of static friction, also known as Coulomb friction, which prevents two objects in contact from sliding relative to each other below a certain threshold force. When two robots collide, the frictional interaction dissipates their kinetic energy so effectively that they become immobilized relative to the vibrating substrate beneath them. This “arrest” of motion persists until sufficiently strong forces reinitiate movement. Unlike dynamic or sliding friction, static friction enforces a no-motion state even on potentially unstable configurations, effectively “cooling” the system by removing motion energy from some particles.
To probe this phenomenon, the team employed hundreds of 3D-printed mini robots, each moving autonomously atop a vibrating plate that supplied vertical agitation. This experimental setup emulated an active matter system analogous to energized grains or colloidal particles. The robots’ movements were tracked and analyzed meticulously, using sophisticated imaging techniques that mapped their positions, orientations, and velocities over time. Researchers observed that as robot density increased and the driving force neared a critical threshold, sustained collisions triggered frequent arrests through static friction, gradually leading to formation of immobilized, “cold” clusters amidst the otherwise energetic, “hot” background.
The formation of clusters was neither permanent nor static. Instead, these “cold” zones dynamically evolved, emerging and dissolving as local conditions fluctuated. Professor Löwen emphasizes that this coexistence of distinct kinetic “temperatures” within one system is inherently non-equilibrium. In contrast to traditional thermodynamic systems, where temperature gradients tend to equilibrate rapidly, here the frictional mechanism stabilizes these differences, producing a steady-state mosaic of moving and arrested robots.
Complementing the experiments, Dr. Alexander Antonov undertook extensive computer simulations replicating the system’s physics with high fidelity. These simulations incorporated detailed modeling of frictional thresholds and collision dynamics to recreate the self-sustained cooling behavior observed experimentally. The successful computational reproduction validates the theoretical framework and deepens insights into the underpinning physics. According to Antonov, such alignment between theory, experiment, and simulation embodies a physicist’s ideal: unraveling and predicting nature’s complexities.
Beyond fundamental interest, this friction-induced cooling effect may revolutionize how engineered active systems are controlled. Professor Lorenzo Caprini, co-author from La Sapienza University, underscores that the robots require no centralized commands or external inputs to modulate their activity levels. Instead, the intrinsic dynamics and physical constraints autonomously orchestrate the transition between motion and arrest. This self-regulation confers robustness and scalability to swarms of robots, a feature invaluable for future applications ranging from material transport to search-and-rescue missions.
The implications extend even further. Professor Löwen envisions harnessing this passive cooling strategy to govern collective behaviors in soft robotics and bulk granular materials. Current approaches often rely on complex programming or external stimuli to manage large assemblies. Using frictional principles promises simpler, energy-efficient, and more adaptable methods to achieve collective states such as jamming, sorting, or collective locomotion, all driven by internal system mechanics rather than external control.
This work also challenges existing paradigms in soft matter physics and active materials science by revealing how non-linear frictional interactions can fundamentally alter system dynamics. The experimentally observed sharp threshold effects in friction dictate whether robots remain in motion or become immobilized, contrasting with the smooth, continuous models usually employed in active matter theories. These findings open avenues to revisit and extend models of out-of-equilibrium systems incorporating realistic frictional physics.
To summarize, the discovery of frictional cooling in active robot swarms elucidates a fascinating self-organizing principle where static friction quenches kinetic energy locally, enabling coexistence of motion and stasis without external regulation. This emergent behavior illuminates novel physics in active matter and suggests promising pathways for practical, self-controlled robotic collectives and smart material systems.
As this research advances, future studies may explore the influence of varying substrate properties, robot shapes, and excitation parameters on frictional cooling dynamics. The integration of sensing and adaptive algorithms into such passive systems could further empower autonomous swarm intelligence driven by fundamental physics rather than complex software. Such interdisciplinary endeavors could redefine robotics, materials science, and physics alike, marking a pivotal step towards truly self-regulating active systems.
Subject of Research: Physics of active matter, static friction, and collective behavior of robotic swarms
Article Title: Self-sustained frictional cooling in active matter
News Publication Date: 6-Aug-2025
Web References:
https://doi.org/10.1038/s41467-025-62626-9
References:
Antonov, A. P., Musacchio, M., Löwen, H., & Caprini, L. (2025). Self-sustained frictional cooling in active matter. Nature Communications, 16, 7235. https://doi.org/10.1038/s41467-025-62626-9
Image Credits: HHU / Marco Musacchio
Keywords
Friction, Active matter, Soft matter physics, Robotics, Static friction, Coulomb friction, Collective dynamics, Swarm robotics, Out-of-equilibrium systems, Non-equilibrium thermodynamics, Granular materials, Self-organization
