Year: 2026
Type: preprint
Abstract: Bacterial swarming is the collective expansion of dense populations of motile cells across hydrated soft surfaces, yet gravity is usually treated as a fixed background condition in standard horizontal assays. We used surface inclination to examine how agar-surface orientation relative to gravity affects the expansion of Enterobacter sp. SM3 swarms. At macroscopic scales, we found that colony expansion depended non-monotonically on inclination, with the strongest spreading at intermediate inclinations and weaker expansion under shallow or near-inverted conditions. Paired up-facing acute and down-facing obtuse configurations with the same tilt magnitude showed different expansion responses, suggesting that swarm expansion depended on both tilt magnitude and which side of the agar surface faced upward relative to gravity. Inclination also produced downhill-biased colony morphologies. For microscopic scales, particle image velocimetry (PIV) analysis showed generally lower mean activity under inclination than in the flat references, with a non-monotonic angular response. The downhill tip showed slower motion, shorter spatial correlations and smaller vortex cores than the upper region. To interpret the macroscopic trends, we extended a two-phase thin-film model by introducing an effective gravity coupling projected normal and tangential to the agar surface. Simulations suggest that the surface-tangential component mainly drives lateral redistribution, whereas the surface-normal component modifies film pressure and helps distinguish up-facing from down-facing configurations.
Source: arXiv (Cornell University)
Language: English
Topic: Micro and Nano Robotics
Subfield: Condensed Matter Physics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
Award: G2259761574
OpenAlex ID: W7216215623