Drop a water droplet onto a superhydrophobic surface and it bounces straight back up. The impact is symmetric: the droplet spreads radially, recoils, and lifts off along the same axis it arrived. Left and right are indistinguishable.
Replace the water with a fluid that has odd viscosity — a non-dissipative viscosity coefficient found in chiral active matter, where the internal constituents rotate — and the bounce breaks symmetry. The droplet spreads asymmetrically during impact and lifts off at an angle, drifting laterally while spinning. It chooses a direction. Left or right is determined by the sign of the odd viscosity, which encodes the handedness of the internal rotation.
The counterintuitive finding: as odd viscosity increases, the direction of the lateral deflection reverses. At moderate values, the droplet bounces left. At higher values, it bounces right. The relationship between internal chirality and macroscopic trajectory is non-monotonic — you can't predict the bounce direction just from knowing the handedness. You need to know the magnitude too.
The mechanism is in the coupling between the deformable interface and the asymmetric stress tensor. Odd viscosity generates forces perpendicular to velocity gradients — forces that have no analogue in ordinary fluids. During the violent deformation of impact, these transverse forces accumulate asymmetrically across the contact region, biasing the recoil. But the sign of the bias depends on how the deformation dynamics interact with the stress asymmetry, which changes with the strength of the odd viscosity.
A droplet that knows left from right. The same fluid, at different concentrations of chirality, bouncing in opposite directions from the same surface. The internal structure of the fluid determines the external trajectory of the impact — and not in the simple way you'd expect.