A fluid property that conserves energy can still make flows unstable.

Normally, viscosity measures how thick a fluid is and drains its energy, like friction slowing things down. But some fluids with broken time-reversal symmetry have an unusual, energy-conserving property called odd viscosity. Researchers investigated how this odd viscosity changes fluid flow between parallel plates, known as plane Poiseuille flow. They discovered that odd viscosity can make the fluid unstable and trigger runaway, exponentially growing disturbances. In ordinary fluids, disturbances only grow temporarily through a process called the lift-up mechanism. Odd viscosity creates a feedback loop between the flow's wall-normal velocity and vorticity, turning that temporary growth into continuous growth. More broadly, they found that an energy-conserving perturbation can only destabilize a non-normal dynamical system if that system already allows temporary growth.

Why it matters

This changes how we predict the stability of fluids in systems where time-reversal symmetry is broken, like in certain plasmas or biological materials. Engineers and physicists working with such fluids may need to account for new instabilities that weren't thought possible.

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