University of Amsterdam researchers have made a counterintuitive discovery about the physics of active matter: motion can actually prevent order rather than create it. In a study published recently in Physical Review Letters, Twan Hooijschuur and his team demonstrated that when self-propelled filaments are packed densely together, they become significantly harder to organize compared to their passive counterparts—a finding that challenges how scientists understand phase transitions in living systems.
The research speaks to a fundamental question in physics: how do disordered systems spontaneously organize into ordered states? For decades, scientists have understood this process well in passive systems—imagine strings lying randomly on a table, then gradually aligning as you pack more and more of them together. This transition from disorder to order, known in physics as the shift from an isotropic phase to a nematic phase, happens quite abruptly once you reach a critical density. But biological systems—from cytoskeletal filaments inside living cells to bacterial chains and clusters of worms—operate very differently. These active systems constantly consume energy to generate their own movement, and Hooijschuur's team wanted to know: how does this ceaseless motion change the fundamental nature of organization?
Through large-scale computer simulations of active semiflexible polymers—elongated filaments that propel themselves along their own contours while remaining flexible enough to bend and deform—the researchers uncovered something striking. As activity levels increase, the onset of alignment gets pushed to progressively higher densities. More surprisingly, what should be an abrupt transition in a passive system becomes increasingly smooth and gradual in an active one. "We often think that activity helps systems explore new configurations," Hooijschuur explains. "What surprised us is that activity can actually prevent a system from settling into an ordered state."
At sufficiently high activity levels, the researchers found that a fully ordered state may never form at all. Rather than aligning globally like passive filaments do, the active polymers continuously bend, twist and fluctuate. Instead of settling into a predictable pattern, the system creates a dynamic state where ordered and disordered regions coexist throughout the material—neither completely random nor fully organized. This happens because as active filaments push and move, they constantly deform their neighbors, creating large-scale bending motions that destabilize long-range alignment. While individual regions may become locally ordered, the perpetual fluctuations prevent the entire system from coordinating into a single aligned state.
The implications extend far beyond the laboratory. These results suggest that living systems may harness activity not merely to generate motion, but as a deliberate mechanism to regulate their own degree of organization. By controlling activity levels, biological systems might maintain adaptability and responsiveness rather than becoming locked into rigid, inflexible ordered states. A cell's ability to stay flexible and responsive—rather than settling into a permanent, unchangeable configuration—could be essential for survival in dynamic environments.
This work represents part of a broader effort within physics to understand active matter: systems whose individual components continuously consume energy and collectively generate complex behavior. Over the past decade, researchers have discovered many examples where activity creates entirely new forms of organization. Hooijschuur's findings add a crucial dimension: activity can fundamentally alter one of the most basic concepts in physics—the nature of phase transitions themselves.