  Monolayers of growing bacteria, confined within channel geometries, exhibit
self-organization into a highly aligned laminar state along the axis of the
channel. Although this phenomenon has been observed in experiments and
simulations under various boundary conditions, the underlying physical
mechanism driving this alignment remains unclear. In this study, we conduct
simulations of growing bacteria in 2D channel geometries perturbed by fixed
obstacles, either circular or arc-shaped, placed at the channel's center. Our
findings reveal that even sizable obstacles cause only short-ranged disruptions
to the baseline laminar state. These disruptions arise from a competition
between local planar anchoring and bulk laminar alignment. At smaller obstacle
sizes, bulk alignment fully dominates, while at larger sizes, planar anchoring
induces increasing local disruptions. Furthermore, our analysis indicates that
the resulting configurations of the bacterial system display a striking
resemblance to the arrangement of hard-rod smectic liquid crystals around
circular obstacles. This suggests that modeling hard-rod bacterial monolayers
as smectic, rather than nematic, liquid crystals may yield successful outcomes.
The insights gained from our study contribute to the expanding body of research
on bacterial growth in channels. Our work provides new perspectives on the
stability of the laminar state and extends our understanding to encompass more
intricate confinement schemes.
