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Scattering of rod-like swimmers in low Reynolds number environments

In their search for metabolic resources microbes swim through viscous environments that present physical anisotropies, including steric obstacles across a wide range of sizes. Hydrodynamic forces are known to significantly alter swimmer trajectories near flat and low-curvature surfaces. In this work, we imaged hundreds-of-thousands of high-curvature scattering interactions between swimming bacteria and micro-fabricated pillars with radii from ~1 to ~10 cell lengths. As a function of impact parameter, cell-pillar interactions produced distinct chiral distributions for scattering angle -- including unexpected 'counter-rotator' trajectories -- well-described by a sterics-only model. Our data and model suggest that alteration of swimmer trajectories is subject to distinct mechanisms when interacting with objects of different size; primarily steric for objects below ~10 cell lengths and requiring incorporation of hydrodynamics at larger scales. These alterations in trajectory impact swim dynamics and may affect microbial populations in ways that depend on the shape and placement of obstacles within an environment.