Rod shape bacterial motion in 2D confinement to decouple hydrodynamic and steric wall effects

As we know, bacterial motion navigates complex environments in natural settings, providing a basis for scientific study to understand their dynamics. Hydrodynamics drive wall alignment and accumulation, but research remains unclear about the extent to which confinement alone, without hydrodynamics, modulates bacterial dynamics. Therefore, we develop a 2D model to study the effects of isolated steric and hydrodynamic forces on bacterial motion in 10- and 50-m microchannels. We used bacteria to model self-propelled rigid rods with run-and-tumble motion, and then compared dry systems (purely steric wall effects) with wet systems (hydrodynamic effects). We found that bacterial speeds and their orientation are independent of channel dimensions in dry systems. In wet systems, we observed strong wall-hugging and alignment due to wall-induced hydrodynamic interactions, enhanced residence times, and a slight increase in the observable effective speed in 10 m microchannels compared to 50 m channels (where bacteria-maintained bulk-like dynamics). Our study thus emphasises when confinement affects bacterial motion solely due to pure dry geometry, and when hydrodynamics play an essential role. This study provides a template for microfluidics-based experimental prediction of bacterial dynamics and could be applied to an analysis of antibiotic resistance. Significance StatementMicrobes rely on self-propelled motion to navigate complex environmental systems and survive and persist. In such an environment, physical interactions with surrounding boundaries play a key role in how bacteria move, orient, or settle in complex spaces. Although many studies show that hydrodynamic forces near walls influence how bacteria swim in liquid, there is still confusion about whether confinement, with or without hydrodynamic effects, can change the complete bacterial pattern. Our 2D model shows that confinement (purely steric wall effects, dry limit) alone does not alter bacterial motion patterns. It is the hydrodynamics (wet limit) that drives bacteria to orient toward walls, remain near boundaries, and direct motion in narrow channels. This work clarifies when confinement and hydrodynamics actually affect bacterias motion and provides a practical way to understand bacterial behaviour in biological systems, including in microfluidic applications and studies of antibiotic resistance strategies.