Physicists Make 10,000 Particles Defy Newton’s Third Law for an Hour

Researchers at the Tokyo University of Science have demonstrated a colloidal system where over 10,000 microscopic particles maintain constant motion by breaking Newton’s third law of motion. According to a report by Interesting Engineering, the team used an alternating electric field to create nonreciprocal interactions, allowing particles to propel themselves and reorganize without forming static clumps.

The study, published in the journal Physical Review Letters, was led by professors Yutaka Sumino and Kiwamu Yoshii from the Faculty of Advanced Engineering. The researchers observed these particles for more than an hour, noting that the system defied the standard action-reaction symmetry typically found in passive physical systems.

Newton’s third law states that for every action, there is an equal and opposite reaction. In most passive systems, interactions are reciprocal, meaning if one object exerts a force on another, the second object exerts an equal force back. The Tokyo University of Science team bypassed this balance by introducing size disparities among the particles.

How electric fields create nonreciprocal motion

To achieve this effect, the researchers suspended polystyrene colloidal particles with radii of 1 and 1.5 micrometers in water. They confined these particles between transparent electrodes coated with indium tin oxide, as reported by Interesting Engineering.

When the team applied an alternating electric field, it triggered electrohydrodynamic (EHD) flows around the particles. The strength of these flows varied based on the size of the particle. Larger particles generated stronger flows than smaller ones, which created an imbalance in attraction.

Because larger particles attracted smaller ones more strongly than the reverse, the interaction became nonreciprocal. This imbalance forced the particles to form asymmetric pairs with a distinct front and back. While a single particle could not move on its own, these pairs functioned as self-propelled units.

Cluster dynamics and the prevention of aggregates

The self-propelled pairs began assembling into larger clusters, but they did not behave like conventional attractive particles. In systems where particles are the same size, interactions remain reciprocal, and particles eventually assemble into static crystalline structures, according to Interesting Engineering.

In this nonreciprocal system, the clusters repeatedly broke apart and rearranged. The continuous movement generated within the pairs prevented the particles from forming giant static aggregates. Numerical simulations conducted by the team confirmed that nonreciprocal pair propulsion is the minimal mechanism sustaining this dynamic behavior.

Our system provides an experimentally controllable example of nonreciprocal many-body physics, where broken action–reaction symmetry gives rise to collective phenomena.

Yutaka Sumino

Potential applications in robotics and biology

The ability to control how matter spontaneously forms dynamic order may have implications for several technical and scientific fields. Sumino stated in a press release that the particles exhibited unexpected behavior by gathering and then splitting rather than forming huge clumps.

The research team noted that similar nonreciprocal interactions could occur in biological systems, such as groups of animals or cell colonies. By providing a framework to understand how collective behavior emerges, the findings could influence the development of externally controlled microrobotic systems and programmable materials, according to Interesting Engineering.