Pack enough string-like objects together, and they will begin to align with one another. But replace the strings with worms or bacteria living in your gut, and this self-organisation becomes much more difficult. A team of University of Amsterdam (UvA) researchers have demonstrated that activity can fundamentally alter one of the most important phase transitions in soft matter physics.
Many systems in nature spontaneously organise themselves: bird flocks align their flight directions; schools of fish move collectively; snakes and worms protect themselves by forming tight entangled clusters; and even molecules can coordinate their orientation to form ordered phases.
For string-like objects, or filaments, a key transition happens when you increase how densely they are packed together. If the density is low, they point in random directions, much like a crowd of people walking aimlessly through a city square. Physicists call this the isotropic phase. As more filaments are added, however, they begin to align with one another. Eventually, most filaments point roughly in the same direction, creating an ordered state known as a nematic phase.
This transition from disorder to order is well understood for passive systems, where the filaments simply respond to thermal fluctuations. But most biological systems are not passive at all. Cytoskeletal filaments inside living cells, bacterial chains, and clusters of worms continuously consume energy to generate their own movement. Such systems belong to the rapidly growing field of active matter.
In a new study published recently in Physical Review Letters, the UvA research team shows that activity does much more than merely introduce additional motion. Twan Hooijschuur, PhD candidate and first author of the paper, says: “We often think that activity helps systems explore new configurations. What surprised us is that activity can actually prevent a system from settling into an ordered state.”
The new results suggest that living systems may use activity not merely to generate motion, but also as a way to regulate their own degree of organisation. By controlling activity levels, biological systems might remain adaptable and responsive instead of becoming trapped in rigid ordered states.
A different kind of phase transition
To investigate the problem, the researchers performed large-scale computer simulations of active semiflexible polymers. These elongated filaments propel themselves along their own contours while remaining flexible enough to bend and deform.
The simulations revealed a striking result. In passive systems, the transition from disorder to order occurs rather abruptly once a critical density is reached. The filaments suddenly align and form a nematic phase.
For active filaments, however, this process changes dramatically. As activity increases, the onset of alignment is pushed to progressively higher densities. At the same time, the transition becomes increasingly smooth and gradual.
Hooijschuur explains: “Instead of suddenly becoming ordered, the active polymers organise much more slowly. The stronger the activity, the harder it becomes for the system to establish collective alignment. The system is continuously trying to align, while the active fluctuations continuously disrupt that alignment.”
At sufficiently high activity levels, the researchers found that a fully ordered state never forms at all. Rather than aligning globally, the polymers continuously bend, twist, and fluctuate. Ordered and disordered regions coexist throughout the material, creating a dynamic state that is neither completely random nor fully organised.
As active filaments push and move, they constantly deform their neighbours. These interactions create large-scale bending motions that destabilise alignment over long distances. Although individual regions may become locally ordered, the fluctuations prevent the entire system from coordinating into a single aligned state.
A growing field
The study forms part of a broader effort to understand active matter: systems whose individual components continuously consume energy and collectively generate complex behaviour.
Over the past decade, researchers have discovered many examples where activity leads to entirely new forms of organisation. The present work shows that activity can also modify one of the most fundamental concepts in physics: the nature of a phase transition itself.
The researchers plan to continue investigating how activity influences collective ordering in more realistic biological and synthetic systems. While the present study identifies the physical mechanism responsible for the delayed transition, many open questions remain about how active fluctuations affect other types of phase transitions found in nature.
The findings may also help researchers design new classes of adaptive materials. Such materials could switch between ordered and disordered configurations without changing their composition, relying instead on internal activity as a control parameter.
Sara Jabbari-Farouji, who led the study, concludes: “Nature often operates far from equilibrium. Understanding how activity changes the fundamental rules of self-organisation is essential if we want to build materials that behave more like living systems.”

Publication
Emergent Isotropic-Nematic Transition in 3D Semiflexible Active Polymers, T. Hooijschuur, E. Irani, A. Deblais and S. Jabbari-Farouji. Physical Review Letters 136 (2026), 228101.