Here is a problem: you have a mixture of egg-shaped particles and disc-shaped particles. They're uncharged, similar density, similar size. How do you separate them?
Lier's answer: drop them through nitrogen in a magnetic field.
The mechanism is the Senftleben-Beenakker effect. A diatomic gas like N2 has molecules with magnetic moments. In a magnetic field, these moments align, which changes the gas's transport properties — even though the gas is electrically neutral. Specifically, two “odd viscosities” become nonzero. The key finding: at high magnetic field, these odd viscosities have opposite signs.
Odd viscosity is viscosity that doesn't dissipate energy. It generates forces perpendicular to the flow direction, like a Coriolis force built into the fluid. When a particle sediments through an odd-viscous fluid, it drifts sideways. The direction and magnitude of the drift depend on the particle's shape.
For prolate spheroids (eggs), the drift goes one way. For oblate spheroids (discs), it goes the other. Because the two odd viscosities have opposite signs, the lateral forces discriminate between the two shapes. Drop a mixed population through the gas and they separate laterally as they fall.
No charge. No special surface chemistry. No active manipulation. The geometry of the particle interacts with the geometry of the fluid's stress tensor, and the result is spontaneous sorting by shape alone.
The physics is elegant because it chains three separate phenomena. First: molecular alignment by the magnetic field changes macroscopic transport. Second: the resulting odd viscosity creates shape-dependent lateral forces. Third: the sign difference between the two viscosity components ensures oblate and prolate objects drift in opposite directions.
What I find striking is the economy. Each step is well-understood individually — the Senftleben-Beenakker effect has been known since the 1930s. Odd viscosity has been studied in active matter and quantum fluids. Stokes flow around spheroids is textbook. Lier's contribution is connecting them. The sorting capability was always there in the physics, waiting for someone to notice.