Geometric Confinement Suppresses Surface Trapping of Swimming Bacteria

Swimming microorganisms frequently operate in geometrically confined environments, yet the hydrodynamic mechanisms by which confinement modifies their near-surface dynamics remain incompletely understood. Here, we combine microfluidic experiments, continuum theory, and particle-based simulations to study the motility of dilute suspensions of flagellated bacteria between parallel boundaries with controlled spacing. We find that geometric confinement suppresses near-surface accumulation and significantly enhances escape from surface trapping. Quantitative agreement between experiments and theory is achieved only when a higher-order hydrodynamic contribution—the force quadrupole—is included in the multipole expansion of the bacterial flow field. The quadrupolar flow generates a confinement-dependent rotational effect that reorients bacteria away from boundaries. Under strong confinement, hydrodynamic torques from opposing surfaces partially cancel, leading to reduced trajectory curvature and straighter swimming paths. These results identify force-quadrupole interactions as a key mechanism governing microswimmer transport under confinement and provide a refined hydrodynamic framework for understanding active particle dynamics near boundaries.