Threads in the Flow: How Noise Creates Order and Chaos express
A thin thread in a water flow is like a hair in a shower stream. If you jerk the water back and forth, the hair either stretches out with the flow or crumples up. The researchers took real protein threads (actin, which makes up muscles) and placed them in a microscopic channel where the fluid oscillated at different frequencies.
At fast oscillations, the thread obediently returned to its original position after each cycle, like a pendulum. But when the oscillations slowed down, everything changed. The thread began to 'get stuck' in one of two states: either stretched out or crumpled. Both states were stable, and the thread could suddenly jump from one to the other due to thermal noise — random kicks from water molecules.
This discovery is like a coin that can land heads or tails, but sometimes flips over on its own. For engineers, it's a way to control soft materials: by changing the oscillation frequency, you can make threads align or, conversely, tumble chaotically.
Original abstract
as written by the authors arXiv:2601.00643
The dynamics of filaments in flow are central to understanding a wide range of biological and soft-matter systems, yet their behavior under time-dependent forcing remains poorly understood. Here, we investigate the long-time dynamics of Brownian inextensible elastic filaments subjected to strong uniform oscillatory shear by combining microfluidic experiments on actin filaments with numerical simulations based on a fluctuating Euler-Bernoulli elastica model in a viscous fluid. As the oscillation period increases, irreversibility emerges from the interplay of flow-induced deformations and thermal noise. This leads to a departure from reversible, deterministic rigid-body dynamics: in this regime, the filaments cycle between nearly straight, flow-aligned conformations at full periods and buckled shapes at half periods. Owing to the time-glide symmetry of the system, two such attracting states in fact coexist with a phase shift of half a period. The system spontaneously selects one, but occasionally switches between them as a result of noise, producing intermittent transitions between apparent order and disorder. This system constitutes an experimentally accessible realization of stochastic symmetry breaking, attractor hopping, and intermittency in a minimal nonequilibrium soft-matter system, with novel implications for the design and control of soft matter systems under time-dependent flows.
Actin filaments are part of the cell's cytoskeleton. They are so thin that about one hundred thousand of them laid side by side would fit in a single millimeter.