Inertial and crowding influence of drift reversal in chiral fluids.

Chiral fluids can exhibit anomalous transport associated with an odd mobility tensor, which gives a tracer a characteristic transverse (Hall-like) drift. Interactions with the surrounding fluid can reverse both this Hall drift and the advective drift along the driving direction, yet the microscopic collision-level mechanism behind these reversals remains unclear. Here we introduce a collision-level framework to interpret drift reversal, combining underdamped Brownian dynamics simulations with a reversal-collision effectiveness map. The map identifies the host-tracer interaction-force orientations that favor reversal. A coarse-grained description of the host-particle velocity-direction statistics then connects this particle-level mechanism to the many-particle dynamics. We find that increasing the odd mobility strength shifts the angular probability distribution toward reversal-effective sectors, promoting drift reversal. Particle inertia and packing fraction modulate the two reversals asymmetrically: larger inertia strengthens the reversed advective drift while suppressing the Hall reversal. Increasing packing fraction produces a nonmonotonic response in which both reversals first intensify and then weaken, with both drift components beginning to recover their normal directions at high densities. Together, these results give a particle-level description of drift reversal in interacting chiral fluids, and clarify how inertia and packing fraction influence nonequilibrium transport in systems with broken time-reversal and mirror symmetries.