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A drop of thick shampoo falls into water and freezes into a strange rope. A liquid that both flows and springs back is called viscoelastic. In it, tiny particles under ultrasound behave quite differently than in water.

In a microchannel filled with such a liquid, ultrasound is turned on, and two flows arise. One, acoustic, pushes particles into certain zones, like wind driving pollen toward a fence. The second, vortical, from the intense sound, stirs the liquid itself and can blow particles away. In a viscoelastic medium, these forces intertwine differently: the liquid's elasticity acts like an invisible net that sometimes catches a particle, sometimes lets it go. The scientists built a model based on equations for an Oldroyd-B fluid — a mathematical description where the liquid remembers recent deformations, like a rubber band.

The main control knobs turned out to be two numbers. The Deborah number (De) compares the relaxation time of elastic stresses in the liquid to the ultrasound period: the larger it is, the longer the liquid 'remembers' the push. The viscous diffusion number (Dv) shows how quickly vortices decay due to viscosity. By changing them, you can transplant the particle trap from one place to another: from the channel center to the wall, to a line where pressure doesn't oscillate, or to the symmetry axis of the sound.

This gives birth to acoustofluidics for complex liquids — the ability to contactlessly sort particles smaller than a micrometer. It's a direct path to delicate work with living cells, viruses, or drug droplets, where it's crucial not to damage the cargo. And all because the liquid springs back a little.