Impacts of Flow and Roughness on the Critical Conditions to Develop and Thickness of Biofilms

Biofilm thickness and critical conditions for biofilms to develop are two crucial factors governing biofilm formation and determining the efficacy of biofilm-related remediation projects. Mechanistic understanding of physical factors that control biofilm formation is critical to predicting and controlling biofilm development, yet such understanding is currently incomplete. Through a combination of microfluidic experiments, numerical simulations, and fluid mechanics theories, we reveal the impacts of flow conditions and surface roughness on the thickness of and critical conditions for forming Pseudomonas putida biofilms. Pseudomonas putida is a common bacterium found on the surfaces of sediments, soils, drinking water systems, and bioremediation facilities. Here, through biofilm development experiments under varying flow rates, we demonstrate that biofilm growth is suppressed under high flow conditions. By comparing the distributions of flow velocity and biofilms, we show that the local critical velocity for P. putida biofilms to develop is 50 μm/s. We propose a theoretical model based on fluid mechanics that successfully predict the critical conditions for biofilm development. In addition, through biofilm development experiments on surfaces with varying surface roughness, we show that micron-scale surface roughness promotes biofilm formation by expanding the region of low velocity. We demonstrate that the critical shear stress, above which biofilms cease to form, for biofilms to develop on rough surfaces is 0.9 Pa, three times as high as that for flat surfaces (0.3 Pa). The results of this study highlight the important roles of flow velocity and surface roughness in controlling biofilm formation and will facilitate future predictions and control of biofilm development on the surfaces of drinking water pipelines, vessels, and sediments.