Vibrations are everywhere—from the hum of machinery to the rumble of transport systems. Usually, these random motions are wasted and dissipated without producing any usable work. Recently, scientists have been fascinated by "ratchet systems" which are mechanical systems that rectify chaotic vibrations into directional motion. In biology, molecular motors achieve this feat within living cells to drive the essential processes by converting random molecular collisions into purposeful motions. However, at a large scale, these ratchet systems have always relied on built-in asymmetry, such as gears or uneven surfaces.
Moving beyond this reliance on asymmetry, a team of researchers led by Ms. Miku Hatatani, a Ph.D. student at the Graduate School of Science and Engineering, Doshisha University, Japan, along with Mr. Junpei Oguni, graduate school alumnus at the Graduate School of Science and Engineering, Doshisha University, Japan, Professor Daigo Yamamoto and Professor Akihisa Shioi from the Department of Chemical Engineering and Materials Science at Doshisha University, demonstrate the world's first symmetric ratchet motor. Published in Volume 35, Issue 8 of the journal Chaos on August 01, 2025, the study describes how a simple circular disk, placed on randomly vibrating particles, can spontaneously break symmetry and spin in a single direction.
"We found that you don't need a special structure to create a ratchet motor," explains Ms. Hatatani. "A simple disk, without any asymmetry, can break the symmetry on its own and start spinning in one direction."
To demonstrate the same, the researchers placed a circular acrylic disk on a shallow layer of glass beads which were contained in a vibrating dish. When vibrated, the beads bounced randomly colliding with the circular disk. But instead of wobbling aimlessly, the disk started to tilt slightly and then began to rotate in a single direction. This directed spin persisted for several seconds, despite the underlying randomness of the particle motion.
The underlying principle of this effect lies in spontaneous symmetry breaking. Initially, the beads are evenly distributed beneath the disk, but as the vibration continues, they gradually accumulate on one side causing the disk to tilt. This tilt, in turn, reinforces the uneven distribution of particles, locking the system into a stable spinning state.
To further confirm the physics behind this, the researchers built a mathematical model based on the precession of a spinning top. This model reproduced the same experimental results, confirming that collisions of the randomly moving particles were enough to sustain the spin in one direction.
"Fundamentally, the system organizes itself," says Prof. Shioi. "The randomness of the particles becomes the very source of order, driving the disk's rotation."
Beyond its novelty, the discovery also hints at future innovations. Noisy vibrations are abundant in daily life. Harnessing this random energy for extraction of regulated motion could inspire new energy-harvesting technologies. This could also extend to real-world applications like powering small devices or sensors without the need of external energy sources.
Moreover, the study also has broad implications in physics. By showing that symmetry itself can break spontaneously to produce motion, it deepens our understanding of non-equilibrium systems and active matter. Looking ahead, the researchers believe insights from their study could pave the way for innovative technologies—helping engineers design systems that extract useful work from ubiquitous background noise at different scales.
"Overall, our study highlights a universal scientific principle that even without external control, order can emerge from disorder," concludes Prof. Shioi.
About Ms. Miku Hatatani from Doshisha University, Japan
Ms. Miku Hatatani is currently a Ph.D. student in the Department of Chemical Engineering and Materials Science at the Graduate School of Doshisha University. She is also a member of the Laboratory of Molecular Chemical Engineering at Doshisha University. Her research interests include biological ratchet motors, Brownian ratchets and its engineering as well as a broad interest in nonequilibrium phenomena and nonlinear science.
Funding information
This work was supported by JST SPRING under Grant No. JPMJSP2129 and KAKENHI under Grant No. 22K03560.
