Physicists Create a System Where Action and Reaction Are Not Equal
A system of over 10,000 colloidal particles has demonstrated nonreciprocal interactions under an alternating electric field, opening the door to programmable materials.
A system of over 10,000 colloidal particles has demonstrated nonreciprocal interactions under an alternating electric field, opening the door to programmable materials.
Imagine pushing a wall, and it doesn't push back with equal force. This behavior, defying one of Newton's fundamental laws, has been observed by Japanese physicists in new research. The team at the Faculty of Advanced Engineering at Tokyo University of Science has created a system where interactions between tiny particles are nonreciprocal-action and reaction are not equal.
The study, published in the journal Physical Review Letters, is based on a large-scale system containing more than 10,000 colloidal polystyrene particles. To create the conditions for this unusual behavior, researchers suspended particles of different sizes-with radii of 1 and 1.5 micrometers-in water and confined them between transparent electrodes coated with indium tin oxide. After applying an alternating electric field, the particles' behavior was observed for over an hour.
The key to the entire phenomenon lies in the electrohydrodynamic (EHD) flows that form around the particles under the influence of the electric field. The research team found that the strength of these flows depends greatly on particle size. Larger particles produce stronger EHD flows than smaller ones, meaning their mediated attraction to smaller particles is also stronger. The consequence of this imbalance is nonreciprocal interactions: a larger particle attracts a smaller one more strongly than the smaller one attracts the larger one back.
This directly contradicts Newton's third law of motion, which states that "mutual actions of two bodies upon each other are always equal and directed to contrary parts," meaning for every action, there is an equal and opposite reaction. In typical passive systems, interactions are reciprocal.
Due to this unbalanced attraction, particles spontaneously paired up into asymmetric pairs with a clearly defined front and back. Although no single particle could move on its own, together they behaved as a self-propelled unit. "Simply put, particles that attract each other, like sand, powder, or raindrops, generally continue to gather over time, growing into larger clumps," explained Yutaka Sumino, co-author of the study and physicist at Tokyo University of Science, to Gizmodo.
However, the behavior of these pairs was anything but conventional. As more pairs emerged, they began to assemble into larger clusters, but they didn't simply continue growing into static aggregates. Instead, as Sumino notes, they "gathered and then separated." The clusters repeatedly broke apart, rearranged, and reformed. The self-propelled pairs continuously generated motion within themselves, preventing the formation of giant static structures.
For comparison, in suspensions containing particles of only one size, interactions remained reciprocal, and particles gradually assembled into static crystalline structures. Numerical simulations confirmed the experimental results and showed that the nonreciprocal propulsion of pairs is the minimal mechanism required to sustain this unusual dynamics.
"This research demonstrates that breaking the symmetry of action and reaction is a fundamental principle that generates new collective motions and self-organization of matter," Sumino emphasized. The researchers believe that similar nonreciprocal mechanisms could exist in biological systems, such as cell colonies and animal groups. This discovery could influence the development of entirely new technologies, including programmable materials and externally controlled microrobotic systems.
