Above microscopic pillars, chaos and orderly waves coexist in the same viscoelastic flow.

In the human body, liquids often flow over tiny hair-like structures, like cilia in airways or villi in the gut. These liquids are viscoelasticviscoelasticA viscoelastic fluid is a liquid that stretches and springs back like a rubber band, rather than flowing as freely as water., meaning they are stretchy and thick, but scientists do not fully understand how they interact with such textured surfaces. To find out, researchers used computer simulations and tiny fluid experiments to study stretchy liquids moving over dense beds of microscopic pillars. They discovered that the liquid above the pillar tips spontaneously turns into a messy, swirling state called elastic turbulenceelastic turbulenceElastic turbulence is the name for chaotic, swirling motion that appears in a stretchy fluid even when the flow is normally smooth and slow.. At the same time, neat, organized elastic waveselastic wavesElastic waves are regular, wave-like motions that travel through the fluid because of its stretchiness. travel through this chaos. This orderly motion forms because the liquid's stretchiness interacts with uneven shear forces from the pillars. Disorder and orderly waves can live side by side in these flows across many different conditions.

Why it matters

Many living surfaces—airway cilia, intestinal villi—are covered in microscopic pillars, and the fluids around them are stretchy, not simple like water. This work shows those fluids can spontaneously organise into both chaotic and wave-like motion above the pillars, which affects how substances are transported, mixed, and absorbed in the body and in lab devices. The finding also means that texturing a surface alone could steer fluid behaviour without moving parts.

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