Shear-Induced Oscillations and Hydrodynamic Buffering Stabilize Sperm Surface Navigation

Abstract

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.