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The interaction between fluid dynamics and biofilm growth plays a key role in both medical and industrial applications. Biofilms, or bacteria that are embedded in a protective matrix of extracellular polymeric substances, settle on interfaces such as on implanted devices or ship hulls. These biofilms canthen cause infectious diseases or significantly increase drag. In this thesis, we investigate the influence of flow, specifically shear stress, on the development of biofilm. The first paper presents a new facility to investigate biofilm growth in laminar flow cells over extended periods of up to several weeks. Optical coherence tomography is used to obtain three-dimensional scans of the biofilm structure at regular intervals. From these time series, we derive a simple model that relates the growth of an individual microcolony to the growth of the full biofilm depending on the wall shear stress. Additionally, we show that biofilm streamers, thin, flexible filaments that extend into the bulk flow, can form on sharp biofilm structures in laminar flow, even if the substratum is a flat surface. The second contribution is a report detailing preliminary studies on biofilm experiments. We investigate the formation of biofilm in the shear layer behinda backward-facing step. The results indicate a maximum shear stress, beyond which biofilm growth is inhibited. We also provide guidelines for the design of experimental setups for the investigation of the influence of fluid dynamics on biofilm and vice-versa., Samspelet mellan fluiddynamik och biofilmtillväxt spelar en nyckelroll i både medicinska och industriella tillämpningar. Biofilmer, eller bakterier som är inbäddade i en skyddande matris av extracellulära polymera substanser, sätter sig på ytor som på implanterade enheter eller fartygsskrov. Dessa biofilmer kan sedan orsaka infektionssjukdomar eller avsevärt öka vattenmotståndet. I den här avhandlingen undersöker vi hur flöde, speciellt skjuvspänning, påverkar utvecklingen av biofilm. I den första artikeln presenteras en ny uppställning för att undersöka biofilmstillväxt i flödesceller med laminärt flöde under längre perioder på upp till flera veckor. Optisk koherenstomografi används för att få tredimensionella skanningar av biofilmstrukturen vid regelbundna intervall. Från dessa tidsserier härleder vi en enkel modell som relaterar tillväxten av en enskild mikrokoloni till tillväxten av hela biofilmen beroende på väggskjuvspänning. Dessutom visar vi att biofilm filament som sträcker sig in i bulkflödet, kan bildas på skarpa biofilmstrukturer i laminärt flöde, även om substratum är en plan yta. Det andra bidraget är en rapport som beskriver preliminära studier av biofilmsexperiment. Vi undersöker bildandet av biofilm i skjuvskiktet bakom ett bakåtvänt steg. Resultaten indikerar en maximal skjuvspänning, bortom vilken biofilmstillväxt hämmas., QC 240314

QC 20240313

Bacillus subtilis biofilms were grown in laminar channel flow at wall shear stress spanning one order of magnitude (tau_w = 0.068 Pa to tau_w = 0.67 Pa). We monitor, non-invasively, the evolution of the three-dimensional distribution of biofilm over seven days using optical coherence tomography (OCT). The obtained biofilms consist of many microcolonies where the characteristic colony has a base structure in the form of a leaning pillar and a streamer in the form of a thin filament that originates near the tip of the pillar. While the shape, size and distribution of these microcolonies depend on the imposed shear stress, the same structural features appear consistently for all shear stress values. The formation of streamers seems to occur after the development of a base structure, suggesting that the latter induces a curved secondary flow that triggers the formation of the streamers. Moreover, we observe that the biofilm volume grows approximately linearly over seven days for all the shear stress values, with a growth rate that is inversely proportional to the wall shear stress. We develop a simple model of friction-limited growth, which agrees with the experimental observations. The model provides physical insight into growth mechanisms and can be used to develop accurate continuum models of bacterial biofilm growth., QC 20240313