Navigation near boundaries under strong flow is central to microswimmer transport in active matter. Using microfluidics, we track rolling bovine sperm near planar walls in Poiseuille flow with near-wall shear rates up to 50 s^-1. As flow increases, we observe a universal dynamical transition: circular surface swimming at zero flow, upstream rheotaxis at weak flow, and a novel near-surface oscillation (NSO) state at high shear, characterized by large-angle oscillations and periodic lifting from the wall. Surprisingly, sperm remain concentrated in a near-wall layer even when downstream advection dominates. A minimal mechanistic model combining hydrodynamic wall interactions, shear-driven Jeffery rotation, and steric flagellar-wall collisions reproduces these transitions and reveals a hydrodynamic buffer zone-a range of shear rates where the mean wall distance remains nearly constant. This buffering arises from a competition between wall-attracted and bulk-oscillatory states, providing a robust physical mechanism for surface navigation in fluctuating environments.
Liu, M., Tao, A., Zhang, R., Yuan, J.
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