Unveiling The Hidden Turbulence of Living world

 

Today was not just another lecture. It was Julia Yeomans standing in front of us — calm, sharp, and with that typical understated British clarity - unfolding the physics of life in motion. Instead of Professor Sumesh taking the class, she decided to step in herself and take us through the world of active matter.

She began from biology - from the most elegant microscopic choreography nature has evolved. Think of kinesin: a protein walking on microtubule tracks, its tiny mechanical feet grabbing, releasing, and stepping forward in perfect chemical rhythm. Each step powered by ATP. Then bacteria - tiny engines with rotary motors driving their flagella, creating traveling waves that propel them forward. It’s physics hiding inside biology; or perhaps biology is just physics that has learned to organize itself.

But one swimmer is only the beginning. What happens when you put many such swimmers together - dense enough that their orientations begin to align? Julia took us from single swimmers to nematic collectives. These are systems where the particles have long-range orientational order but no positional order — the same symmetry that liquid crystals have. And as she reminded us, that order doesn’t come from external forces; it emerges from the swimmers’ own motion, from their anisotropy and interactions.

Then came the question that still rings in my head: if anisotropy is essential, why do epithelial cells — almost round in shape - show nematic ordering? Clearly, shape is not the full story; activity itself can break symmetry and drive ordering.

In passive nematics, distortions relax - defects annihilate and the system calms down. But active systems don’t rest. They spawn pairs of +½ and –½ defects that chase each other, collide, and are reborn. The +½ defects behave like little arrows - they move, create flow fields around them, and stir the fluid. The –½ ones just sit there, symmetric and inert. This imbalance creates perpetual motion, giving rise to what she called active turbulence.

It’s a strange turbulence -born not of inertia but of life. A chaos that sustains itself in slow, viscous flows.

After the talk, I kept thinking of the questions I didn’t get to ask. For instance: if cilia are swimmers themselves, would the beating of one influence its neighbor? The answer, I later realized, is yes. At such low Reynolds numbers, everything is connected through the fluid. When one cilium beats, it sends a ripple of motion that affects its neighbors, advancing or delaying their phase. This is how carpets of cilia synchronize to form traveling metachronal waves - a pure hydrodynamic coupling without any central control.

Then there’s the turbulence itself. Why does it look so chaotic, without distinct modes or characteristic patterns? In classical systems, instabilities often lead to periodic waves or stable patterns. But in active nematics, the energy injection happens continuously at small scales through the active stress term $\sigma_{ij}^{\text{active}} = -\zeta Q_{ij}$. When the activity $\zeta$ is large enough, small distortions grow instead of relaxing -and because the system constantly creates new defects, there’s no single mode that can dominate. It’s a self-sustaining, nonlinear feedback loop - the system forever creating and destroying its own order.

And finally, I wondered: could there exist a natural frequency, a global rhythm where every swimmer synchronizes? It feels like there should be -after all, living systems love coherence. But active turbulence refuses to be tamed. The hydrodynamic coupling is local, and the defects keep breaking global phase relationships. What we get instead are patches - regions where swimmers move coherently for a few body lengths before chaos takes over again. A mosaic of local synchrony and global disorder.

That’s what struck me most about Yeomans’ talk: how much of this turbulence is not noise, but a new kind of order. The kind that emerges not from equilibrium, but from energy constantly being consumed and released - life’s signature in the language of hydrodynamics.

It was one of those rare lectures where you walk out realizing that physics isn’t about dead matter anymore. It’s about things that move, consume, interact - things that live.