A hidden rhythm brings microscopic particles into unison

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A hidden rhythm brings microscopic particles into unison

Swimming in a shared medium makes particles synchronize without touching

• Link to: Northwestern Now Story

• From blinking fireflies to cells in a beating heart, synchronization occurs across nature
• Researchers found similar behavior emerges in a simple system of microscopic particles
• Suspended in liquid, the particles naturally oscillated together as though they sensed one another's motion
• Using computational modeling, the team found the particles influence each other's motion by stirring their shared medium

EVANSTON, Ill. --- Several years ago, scientists discovered that a single microscopic particle could rock back and forth on its own under a steady electric field. The result was curious - but lonely.

Now, Northwestern University engineers have discovered what happens when many of those particles come together. The answer looks less like ordinary physics and more like mystifying, flawlessly timed choreography.

In the new study, the team found that groups of tiny particles suspended in liquid oscillate together, keeping time as though they somehow sense one another's motion. Nearby particles fall into sync, forming clusters that appear to sway in unison - rocking back and forth with striking coordination.

According to computer simulations, the conductor behind this coordination is the liquid itself. As each particle oscillates, it gently stirs the surrounding fluid. Those tiny ripples flow outward to nudge neighboring particles. Even though the particles do not directly touch one another, they influence each other's motions. The motion of the fluid enables the particles to "feel" one another at a distance.

The findings could help explain how complex, collective behavior emerges without communication or signaling. From fireflies that blink in unison to heart cells that beat together, many living systems rely on coordinated timing without a central leader. By moving through a shared medium, individual components can influence one another's timing. The results suggest that in biological systems, too, the environment itself - whether fluid, tissue or air - may play a crucial role in orchestrating collective rhythms.

The study was published on Friday (Jan. 23) in the journal Nature Communications.

"This project took years to complete," said Northwestern's Monica Olvera de la Cruz, the study's senior author. "The main question was 'why are these particles moving together?' Somehow, they appeared to be influencing each other and eventually synchronize their movements. It's almost like the particles cooperate, and it was impossible for us to understand why. We reproduced the experimental model as a complex simulation, so we could watch the interactions in high detail."

Olvera de la Cruz is the Lawyer Taylor Professor of Materials Science and Engineering in Northwestern's McCormick School of Engineering. She also is the director of the Center for Computation and Theory in Soft Materials and deputy director of the Center for Bio-Inspired Energy Science. Olvera de la Cruz co-led the study with Kyle Bishop, a professor of chemical engineering at Columbia University. The paper's first author is Sergi Leyva, a postdoctoral fellow in Olvera de la Cruz's research group.

Called synchronization, emerging coordination across a group of individuals is common in nature and engineered technologies. But researchers did not expect to see this phenomenon emerge so clearly in a simple physical system. In computer simulations, Leyva studied hundreds of simplified microscopic particles moving through a shallow, water-like fluid. He color-coded each particle, based on its position within its own oscillation cycle. Entire clusters lit up in matching hues, revealing groups of particles behaving like a single, coordinated unit.

Olvera de la Cruz and Leyva initially wondered if the electric field could explain this behavior. But, after studying the computer simulation, they quickly ruled that out.

"In our simulations, we were able to turn off the electrostatics," Leyva said. "You can't do that in an experiment. But in the model, we could isolate the hydrodynamics while keeping the oscillatory dynamics of the particles."

By combining the detailed simulation with experiments and a simplified mathematical model, the team demonstrated that fluid-driven interactions alone could explain why the particles synchronized. The researchers even could predict which color (or oscillation phase) each particle would adopt based on its position within the group.

"When a particle moves in a fluid, it generates a flow," Leyva said. "If there is another particle nearby, it's affected by this flow. So, if you have two particles that initially oscillate at different phases, eventually they end up oscillating together. They synchronize with their closest neighbors."

Now that the underlying mechanism is clear, the researchers want to learn how to control the synchronization. By tuning particle density, geometry and confinement, future work could turn the collective motion on and off - laying groundwork for programmable materials and microscale systems with functions that emerge from coordinated behavior. The findings also offer a new physical framework for understanding how synchronization arises in living systems, where motion through shared fluids plays a central role.

"Sometimes the most complex behavior comes from the simplest ingredients," Olvera de la Cruz said. "In this case, motion through a fluid is enough to bring an entire system into sync."

The study, "Self-oscillating synchronematic colloids," was supported by the U.S. Department of Energy.

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Monica Olvera de la Cruz

Co-corresponding author

Lawyer Taylor Professor of Materials Science and Engineering

• [email protected]