Activity keeps systems flexible, preventing the trap of perfect order
At the University of Amsterdam, researchers have discovered that when matter moves under its own power — as biological filaments, bacteria, and cellular proteins do — the ancient tendency of packed objects to align and self-organize is not merely slowed, but fundamentally rewritten. Where passive systems snap into ordered states at predictable thresholds, active systems resist that settling, held in perpetual tension between order and chaos by the very energy that animates them. This finding suggests that life may have long understood something physics is only now formalizing: that motion itself can be a form of governance, keeping living systems flexible rather than crystallized.
- Decades of physics assumed that dense filaments would reliably snap into alignment — active matter breaks that assumption entirely.
- Computer simulations of self-propelling polymers revealed that stronger activity doesn't accelerate order; it continuously tears alignment apart as fast as it forms.
- At high enough activity levels, no globally ordered state emerges at all — the material hovers in a restless coexistence of organized patches and turbulent disorder.
- Biological systems from nerve cell cytoskeletons to bacterial colonies may be deliberately exploiting this disruption to stay adaptable rather than rigidly locked in place.
- The findings point toward a new generation of engineered materials that could toggle between order and disorder not by changing their chemistry, but by tuning their internal activity.
Pile enough string into a container and something quietly remarkable occurs: without any external instruction, the strings begin to point the same direction. Physicists have mapped this self-organization — called a phase transition to a nematic state — for decades. But the University of Amsterdam's active matter researchers asked a harder question: what happens when the strings are alive?
Living filaments — the protein scaffolds inside cells, chains of bacteria, clusters of worms — don't simply respond to thermal jostling. They burn energy to push, flex, and deform their neighbors. In a study published in Physical Review Letters, the Amsterdam team showed that this activity doesn't merely add noise to the system. It rewrites the rules of phase transition entirely.
Using large-scale computer simulations of self-propelling semiflexible polymers, first author Twan Hooijschuur and his colleagues found that passive filaments transition sharply into alignment once a critical density is crossed — a clean, abrupt flip. Active filaments behave differently. As activity increases, the density required for alignment rises, and the transition itself softens from a sharp threshold into a slow, gradual drift. At sufficiently high activity, global alignment never arrives at all. Ordered and disordered regions coexist in a restless, fluctuating landscape that is neither random nor organized.
The mechanism is almost paradoxical: the same energy driving filaments to move also continuously deforms their neighbors, generating large-scale bending waves that prevent the whole system from ever coordinating. The system keeps trying to find order, and the effort itself keeps pulling it apart.
Study leader Sara Jabbari-Farouji sees the implications extending well beyond physics. Biological systems may use activity not just to generate motion, but as a regulatory dial — staying responsive and adaptable by resisting the trap of perfect order. The findings also suggest a new class of engineered materials: systems that switch between ordered and disordered states not by changing composition, but by adjusting internal activity, behaving less like inert matter and more like something alive.
Imagine a pile of string. Add enough of it, pack it densely enough, and something remarkable happens: the strings begin to point the same direction. They organize themselves without anyone arranging them. Physicists have understood this for decades. But what happens when those strings are alive? When they are worms in your gut, or bacteria, or the protein filaments inside your cells—all of them burning energy to move, to flex, to push against their neighbors? The answer, according to researchers at the University of Amsterdam, is that everything changes.
The phenomenon the strings demonstrate is called a phase transition. In physics, phase transitions are the moments when a system flips from one state to another—like water freezing into ice. For string-like objects called filaments, the transition happens as density increases. At low density, the filaments point randomly in all directions, a state physicists call isotropic. It's like a crowd of people wandering aimlessly through a plaza. But as you pack more filaments together, they begin to align. Eventually, most of them point roughly the same way, creating what's known as a nematic phase—order emerging from chaos.
This transition has been well mapped in passive systems, where filaments simply sit there responding to random thermal jiggling. But living systems are different. The filaments inside cells, the chains of bacteria, the clusters of worms—they all consume energy to generate their own movement. They belong to a rapidly growing field called active matter. And in a study published in Physical Review Letters, the Amsterdam team demonstrated that activity does far more than just add extra motion to the mix. It fundamentally rewrites the rules.
Twan Hooijschuur, the study's first author, described the surprise: activity doesn't help systems find new configurations and settle into order. Instead, it can prevent a system from settling into order at all. The researchers ran large-scale computer simulations of active semiflexible polymers—elongated filaments that propel themselves along their own length while remaining flexible enough to bend and twist. What they found was striking. In passive systems, the transition from disorder to order happens abruptly once a critical density is reached. The filaments suddenly align. But for active filaments, the process transforms entirely. As activity increases, the point at which alignment begins shifts to progressively higher densities. More importantly, the transition becomes smooth and gradual instead of sharp.
Hooijschuur explained the mechanism: instead of suddenly becoming ordered, active polymers organize 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. Instead of 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 organized.
The mechanism is elegant: as active filaments push and move, they constantly deform their neighbors. These interactions create large-scale bending motions that destabilize alignment over long distances. Individual regions may become locally ordered, but the fluctuations prevent the entire system from coordinating into a single aligned state. It's as if the system keeps trying to get its act together, but the very energy that drives it keeps pulling it apart.
The implications reach beyond physics. Biological systems may use activity not merely to generate motion, but as a way to regulate their own degree of organization. By controlling activity levels, living systems might remain adaptable and responsive instead of becoming trapped in rigid ordered states. A nerve cell's cytoskeleton, a bacterial colony, a cluster of worms—all of them may be leveraging motion itself as a tool for staying flexible, for resisting the trap of perfect order.
Sara Jabbari-Farouji, who led the study, noted that nature often operates far from equilibrium. Understanding how activity changes the fundamental rules of self-organization is essential if researchers want to build materials that behave more like living systems. The findings suggest a new class of adaptive materials could be designed—materials that switch between ordered and disordered configurations without changing their composition, relying instead on internal activity as a control parameter. The next frontier is investigating how activity influences other types of phase transitions found in nature, and how these principles might be harnessed to create materials that think and adapt the way living things do.
Citas Notables
Activity can actually prevent a system from settling into an ordered state, rather than merely introducing additional motion.— Twan Hooijschuur, Ph.D. candidate and first author, University of Amsterdam
Nature often operates far from equilibrium. Understanding how activity changes the fundamental rules of self-organization is essential if we want to build materials that behave more like living systems.— Sara Jabbari-Farouji, study leader
La Conversación del Hearth Otra perspectiva de la historia
So the basic finding is that motion prevents order from forming. That seems counterintuitive—doesn't motion usually help things mix and explore?
It does, in most contexts. But here's the thing: alignment isn't about mixing. It's about coordination. The filaments want to point the same way, and in a passive system, once they're packed densely enough, they just do. But when they're active, when they're constantly pushing and bending each other, they keep disrupting that coordination. It's like trying to get a group of people to face the same direction while they're all walking around.
And this matters for living systems because?
Because living systems are never passive. Your cells, bacteria, worms—they're all burning energy. If the physics said that activity should naturally lead to order, then everything alive would be locked into rigid, organized states. But the opposite is true. Activity keeps them flexible, adaptable. It's a feature, not a bug.
Can you give me a concrete example of what this means in a real cell?
The cytoskeleton—the protein scaffolding inside cells. Those filaments are constantly being assembled, disassembled, pushed around by molecular motors. If they organized into a perfectly ordered state, the cell would lose the ability to change shape, to move, to respond to signals. The activity keeps them in this dynamic, partially organized state where they can do their job.
So the researchers are saying we could design materials that use this principle?
Exactly. Imagine a material that could switch between rigid and flexible just by turning up or down an internal energy source. No chemical change, no structural modification—just activity. That's what they're pointing toward.
How far away is that from reality?
Still theoretical, but the physics is now solid. The simulations show it works. The next step is building it in the lab, in synthetic systems, and then figuring out how to control it. That's years away, probably. But the principle is sound.