What type of boundary attracts more bacteria?
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What type of boundary attracts more bacteria?

Micro-organisms encounter solid and fluid boundaries that trigger important microbial processes in both environmental and industrial settings, such as biofilm formation. For the first time, this study presents insights on equilibrium bacteria distribution near various complex interfaces, which can impact a wide range of applications from bioremediation, e.g., near marine oil spills, to micro-robot designs.

We study the accumulation of non-tumbling flagellated bacteria, Escherichia coli, near gas and liquid interfaces through experimental observation and theoretical modeling. The motion of micro-organisms in liquids is governed by low Reynolds number hydrodynamics. However, near a fluid interface that is often enriched with bacteria-generated surfactants and macromolecules, the analysis of bacteria swimming is complex.

We present the first experimental study of bacteria attractions to various gas and liquid interfaces, through which we observe a higher cell concentration near the gas interface compared to that near liquid and solid surfaces. We develop a model based on the Brownian dynamics, including interfacial hydrodynamics, to understand the underlying physics. We show that interfacial viscosity plays the most crucial role in determining the cell distribution near interfaces. For small interfacial viscosities, bacteria accumulation is higher near gas interfaces compared to liquid and rigid surfaces.

(a) Experimental apparatus with a showcase of cell counts in different regions: top interface, bulk region, and near the solid glass. (b) The comparison of the probability density function of finding bacteria of experimental observations (box plots) and numerical results (solid lines) for the CO2 interface (left) and solid interface (right).

(a) The comparison of probability density function between numerical results and experimental observations for various top interfaces (dodecane, mineral, soybean, and solid glasses) and their corresponding solid wall. (b) The comparison of curvature and in-plane migration between experiments and simulation.

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