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ABSTRACT Bacterial motility plays a crucial role in biofilm development, yet the underlying mechanism remains not fully understood. Here, we demonstrate that the flagellum-driven motility ofPseudomonas aeruginosaenhances biofilm formation by altering the orientation of bacterial cells, an effect controlled by shear stress rather than shear rate. By tracking wild-typeP. aeruginosaand its non-motile mutants in a microfluidic channel, we demonstrate that while non-motile cells align with the flow, many motile cells can orient toward the channel sidewalls, enhancing cell surface attachment and increasing biofilm cell density by up to 10-fold. Experiments with varying fluid viscosities further demonstrate that bacterial swimming speed decreases with increasing fluid viscosity, and the cell orientation scales with the shear stress rather than shear rate. Our results provide a quantitative framework to predict the role of motility in the orientation and biofilm development under different flow conditions and viscosities.IMPORTANCEBiofilms are ubiquitous in rivers, water pipes, and medical devices, impacting the environment and human health. While bacterial motility plays a crucial role in biofilm development, a mechanistic understanding remains limited, hindering our ability to predict and control biofilms. Here, we reveal how the motility ofPseudomonas aeruginosa, a common pathogen, influences biofilm formation through systematically controlled microfluidic experiments with confocal and high-speed microscopy. We demonstrate that the orientation of bacterial cells is controlled by shear stress. While non-motile cells primarily align with the flow, many motile cells overcome the fluid shear forces and reorient toward the channel sidewalls, increasing biofilm cell density by up to 10-fold. Our findings provide insights into how bacterial transition from free-swimming to surface-attached states under varying flow conditions, emphasizing the role of cell orientation in biofilm establishment. These results enhance our understanding of bacterial behavior in flow environments, informing strategies for biofilm management and control. 

Abstract Sand‐clay mixtures are common in both freshwater and saltwater environments, yet how they behave under different levels of salinity remains poorly understood. Here, we demonstrate the impact of salinity on the rheological properties and erosion threshold of sand‐clay mixtures through systematically controlled flume experiments and rheological measurements. Mixtures with a representative bentonite‐to‐sand ratio typical of natural estuarine and coastal sediments were prepared at salinities ranging from 0 to 35 parts per thousand (ppt), spanning freshwater to seawater conditions. We measured viscosity, flow‐point stress, and yield stress of the mixtures using a rheometer and determined the critical bed shear stress in a water‐recirculating flume. Our results indicate that as salinity increases from 0 to 35 ppt, the critical bed shear stress decreases by about two orders of magnitude, from about 60 Pa at 0 ppt to less than 1 Pa at 35 ppt. Similarly, both the flow‐point stress and yield stress decreased by over two orders of magnitude with increasing salinity. These changes correspond to a salinity‐induced transition of the sand‐bentonite mixture from a cohesive, strong‐gel state in freshwater (0 ppt), to a weak‐gel state between 3 and 10 ppt, and finally to a fluid‐like state above 10 ppt. Our research highlights the important role of salt in controlling the rheological properties and erosion threshold of fresh, non‐consolidated deposits of sand‐clay mixtures, with implications for predicting coastal landscape evolution and designing erosion‐control strategies. 

Abstract Clay is the main component that contributes to sediment cohesiveness. Salinity impacts its transport, which controls the electrochemical force among the sediment grains. Here, we quantify the impacts of salinity on the erosion threshold, yield stress, and the microstructures of a fluorescently labeled smectite clay, laponite, by combining flume experiments, rheometer measurements, and macro‐ and microscopic imaging. We show that the critical shear stress for clay erosion,τ_b,crit, increases by one order of magnitude with increasing salinity when salinity <1.5 ppt and slightly decreases when salinity >1.5 ppt showing a weaker dependency upon salinity. We further show that the yield stress,τ_y, of the clay remains roughly a constant at salinity less than 1.5 ppt and decreases by over one order of magnitude at salinity larger than 1.5 ppt. This change in the dependency ofτ_b,critand yield stress on salinity corresponds to a change in the gelatinous state of clay, from gel‐like structures to phase‐separated structures as salinity increases. Our results provide a quantitative characterization of the dependency of clay erosion threshold and yield stress on salinity and highlight the importance of the clay gelatinous state in controlling clay transport. 

The erosion and transport of cohesive sediment are more difficult to study than non-cohesive sediment, largely because these processes vary with the salt in the water. Clay minerals are the major components that contribute to the cohesiveness of cohesive sediment because they have significantly larger surface charges and surface area-to-volume ratio than non-cohesive sediment. The electrochemically active clay surfaces can adsorb ions on their surfaces, form an electrical double layer, and cause clay particles to aggregate or form a gel. In this chapter, we first discuss the properties of clay minerals, including the structure of clay primary particles, their surface charge and area, and their interaction with ions in water. The surface charges and surface areas of clay are several orders of magnitude larger than non-cohesive sand, thus predisposing it to interactions with salt in aqueous environments. Second, we summarize studies that reveal the role of salts, specifically salinity and sodium absorption ratio (SAR), on sediment aggregation, stability, and settling speed. An increase in salinity from 0.15 to 1.5 ppt has been shown to increase the erosion threshold of smectite clay by more than 10 times. These findings underscore the crucial role of salt in shaping cohesive sediment transport.