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Abstract Harmful algal blooms (HABs) pose significant threats to aquatic ecosystems and human health, necessitating efficient mitigation strategies. Although clay-algae aggregation has been widely used for controlling HABs, the slow sedimentation of clay-algae aggregates hampers its effectiveness. We examine how turbulence dynamics affect the formation and settling of clay-algae aggregates. Using a custom-designed plankton tower equipped with an oscillating grid and an in-situ imaging system, we investigated how varying dissipation rates of turbulent kinetic energy (ε = 8 × 10⁻⁹to 9 × 10⁻⁵m²/s³) affected the removal efficiency ofMicrocystis aeruginosaby laponite clay. In addition, we directly measured the settling velocity and size of clay-algae aggregates over time. The results demonstrate that turbulent mixing, on a time scale typical of the diurnal mixed layer of lakes, can enhance the removal efficiency of HABs by up to threefold. Higher turbulence dissipation rate,ε, leads to an increase in the settling velocity and size of clay-algae aggregates. We demonstrate that the maximum removal efficiency ofMicrocystis aeruginosais achieved when the ratio of the diameter of clay-algae aggregates is half the Kolmogorov length scale. Our findings highlight the importance of turbulence in enhancing clay-based HAB mitigation and provide actionable insights for field applications, such as leveraging natural wind-driven mixing or implementing mechanical agitation in the lakes’ surface mixed layer. This study bridges the gap between well-controlled laboratory experiments and real-world HAB implementation. 

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 Biofilms play critical roles in wastewater treatment, bioremediation, and medical-device-related infections. Understanding the dynamics of biofilm formation and growth is essential for controlling and exploiting their properties. However, the majority of current studies have focused on the impact of steady flows on biofilm growth, while flow fluctuations are common in natural and engineered systems such as water pipes and blood vessels. Here, we reveal the effects of flow fluctuations on the development ofPseudomonas putidabiofilms through systematic microfluidic experiments and the development of a theoretical model. Our experimental results showed that biofilm growth under fluctuating flow conditions followed three phases: lag, exponential, and fluctuation phases. In contrast, biofilm growth under steady-flow conditions followed four phases: lag, exponential, stationary, and decline phases. Furthermore, we demonstrated that low-frequency flow fluctuations promoted biofilm growth, while high-frequency fluctuations inhibited its development. We attributed the contradictory impacts of flow fluctuations on biofilm growth to the adjustment time (T₀) needed for biofilm to grow after the shear stress changed from high to low. Furthermore, we developed a theoretical model that explains the observed biofilm growth under fluctuating flow conditions. Our insights into the mechanisms underlying biofilm development under fluctuating flows can inform the design of strategies to control biofilm formation in diverse natural and engineered systems. 

Abstract Biofilms can increase pathogenic contamination of drinking water, cause biofilm‐related diseases, alter the sediment erosion rate, and degrade contaminants in wastewater. Compared with mature biofilms, biofilms in the early‐stage have been shown to be more susceptible to antimicrobials and easier to remove. Mechanistic understanding of physical factors controlling early‐stage biofilm growth is critical to predict and control biofilm development, yet such understanding is currently incomplete. Here, we reveal the impacts of hydrodynamic conditions and microscale surface roughness on the development of early‐stagePseudomonas putidabiofilm through a combination of microfluidic experiments, numerical simulations, and fluid mechanics theories. We demonstrate that early‐stage biofilm growth is suppressed under high flow conditions and that the local velocity for early‐stageP. putidabiofilms (growth time < 14 h) to develop is about 50 μm/s, which is similar toP. putida's swimming speed. We further illustrate that microscale surface roughness promotes the growth of early‐stage biofilms by increasing the area of the low‐flow region. Furthermore, we show that the critical average shear stress, above which early‐stage biofilms cease to form, is 0.9 Pa for rough surfaces, three times as large as the value for flat or smooth surfaces (0.3 Pa). The important control of flow conditions and microscale surface roughness on early‐stage biofilm development, characterized in this study, will facilitate future predictions and managements of early‐stageP. putidabiofilm development on the surfaces of drinking water pipelines, bioreactors, and sediments in aquatic environments. 

Earth's surface materials constitute the basis for life and natural resources. Most of these materials can be catergorized as soft matter, yet a general physical understanding of the ground beneath our feet is still lacking. Here we provide some perspectives. 

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.