When two droplets are placed together, most people would expect them to merge into one larger droplet. Instead, physicists at the Max Planck Institute for Dynamics and Self-Organization (MPI-DS) have discovered something stranger: droplets made of molecular condensates can chase each other in circles, propelled by attraction alone—a phenomenon that resembles nothing so much as a lanternfish following prey that is itself following the lantern.
The finding challenges our intuition about how molecular systems behave. Inside living cells, certain biological functions depend on the local clustering of molecules into dense droplets called condensates. These droplets don't sit still; they dynamically rearrange, and how they interact with each other shapes the entire organization of the cell. Understanding these interactions has profound implications for how cells organize themselves and maintain their vitality.
Jacopo Romano, the first author of the study, led a team from MPI-DS's Department of Living Matter Physics in constructing a minimal model to understand phase separation dynamics. Rather than assuming the system would behave as expected, the team introduced mutual attraction between two droplets and watched what happened. What they observed defied conventional wisdom: instead of collapsing into a single, stationary mass, the droplets exhibited what Romano calls "an unexpected emergent property of chasing dynamics resulting in movement and propulsion."
The breakthrough lies in recognizing that these chasing behaviors—called non-reciprocal interactions—can arise from attraction alone. Previously, scientists had assumed that non-reciprocal dynamics, where one object pursues another in a cycle, required both attractive and repulsive forces working together. The team's work, published in Physical Review Letters, demonstrates that this isn't necessarily true. By varying the size, shape, and chemical activity of the condensates in their model, the researchers observed different behaviors, including the run-and-chase mechanism that had only been documented in systems with mixed forces.
"It's natural to think that a system with only attractive forces would form one large, stationary condensate," Romano explained. "However, instead we observed an unexpected emergent property of chasing dynamics resulting in movement and propulsion." The parallel to the lanternfish—a predator that chases prey that is itself attracted to the predator's light—captures the elegance of what's happening at the molecular scale.
Ramin Golestanian, director of the Department of Living Matter Physics, emphasized the significance of the work: "This is an interesting example of how a nonequilibrium emulsion can be engineered to exhibit nonreciprocal chasing interactions between droplets." The findings open new windows into cellular self-organization and have immediate practical applications beyond biology. Researchers are already considering how these principles could be used to design artificial molecular machines capable of self-propelling behavior—systems that could be engineered to move and perform work without external intervention.
What makes this discovery particularly remarkable is its simplicity. By starting with the most minimal system possible—just two droplets governed by attraction—the team revealed a fundamental principle about how complexity and motion can emerge from seemingly straightforward rules. It's a reminder that even in the most controlled laboratory conditions, nature continues to surprise us with its creativity.