Complex Porous Surface Modulate Bacterial Turbulence

Bacteria often inhabit environments with soft or porous boundaries, yet the influence of such interfaces on collective bacterial behavior remains poorly understood. Here, we investigate how the mechanical properties of a boundary, such as agar substrates of varying concentrations, modulate bacterial turbulence in dense Escherichia coli suspensions. While planktonic bacterial swimming remains largely unaffected across substrates, collective motion (bacterial turbulence) exhibits pronounced, non-monotonic changes in turbulence energy, enstrophy, and shear rate. These metrics peak at an intermediate agar concentration (~1.0 wt%), suggesting an optimal mechanical environment for sustaining active flow. To interpret these observations, we develop a continuum model that incorporates Darcy's Law and active pressure, linking substrate permeability and stiffness to bacterial density. Our findings reveal that soft boundary mechanics regulate collective dynamics through a feedback loop involving fluid drainage, density redistribution, and stress modulation. This study advances our understanding of active matter near compliant interfaces and provides insights for engineering microbial behavior in complex environments.