Effect of biofilm structural deformation on hydraulic resistance during ultrafiltration: A numerical and experimental study

Yielding behaviour of chemically treated Pseudomonas fluorescens biofilms

The mechanics of biofilms are intrinsically shaped by their physicochemical environment. By understanding the influence of the extracellular matrix composition, pH and elevated levels of cationic species on the biofilm rheology, novel living materials with tuned properties can be formulated. In this study, we examine the role of a chaotropic agent (urea), two divalent cations and distilled deionized water on the nonlinear viscoelasticity of a model biofilm Pseudomonas fluorescens. The structural breakdown of each biofilm is quantified using tools of non-linear rheology. Our findings reveal that urea induced a softening response, and displayed strain overshoots comparable to distilled deionized water, without altering the microstructural packing fraction and macroscale morphology. The absorption of divalent ferrous and calcium cations into the biofilm matrix resulted in stiffening and a reduction in normalized elastic energy dissipation, accompanied by macroscale morphological wrinkling and moderate increases in the packing fraction. Notably, ferrous ions induced a predominance of rate dependent yielding, whereas the calcium ions resulted in equal contribution from both rate and strain dependent yielding and structural breakdown of the biofilms. Together, these results indicate that strain rate increasingly becomes an important factor controlling biofilm fluidity with cation-induced biofilm stiffening. The finding can help inform effective biofilm removal protocols and in development of bio-inks for additive manufacturing of biofilm derived materials.

In-situ measurement of the internal compaction of a soft material caused by permeation flow

HYPOTHESIS

The compaction of hydrogel films under permeation flow can be measured, in-situ, by tracking the internal displacements of their structure, thereby revealing the internal deformation profile. Additionally, monitoring the permeation flow rate and applied pressure over time enables determination of variations in the hydrogel's permeability due to flow-induced compaction. Hydrogels are soft porous materials capable of containing high amounts of water within their polymeric matrix. Flow-induced internal deformation can modify the hydrogel's permeability and selectivity, which are important attributes in separation processes, both industrial (e.g., membrane-based water purification) and natural (mucous filters in suspension feeders and intestinal lining) systems. Measuring the flow-induced compaction in thin hydrogels films can reveal the interplay between flow and permeability. However, the micro-scale internal compaction remains uncharted for due to experimental challenges.

EXPERIMENTS

A technique is demonstrated for analyzing the compaction and stratification of permeable soft materials, in-situ, created by a pressure-driven permeation flow. To this end, the internal deformations within a soft material layer are calculated, based on tracking the positions of fluorescent micro-tracers that are embedded within the soft material. We showcase the capabilities of this technique by examining a hundred-micron-thick calcium-alginate cake deposited on a nanofiltration membrane, emphasizing the achieved micro-scale resolution of the local compaction measurements.

FINDINGS

The results highlight the possibility to examine thin hydrogel films and their internal deformation produced by flow-induced stresses when varying the flow conditions. The method enables the simultaneous calculation of the soft material's permeance, as the pressure-driven flow conditions are continuously monitored. In summary, the proposed method provides a powerful tool for characterizing the behaviour of permeable soft materials under permeation conditions, with potential applications in engineering, biophysics and material science.

In Vivo Microrheology Reveals Local Elastic and Plastic Responses Inside 3D Bacterial Biofilms

Bacterial biofilms are highly abundant 3D living materials capable of performing complex biomechanical and biochemical functions, including programmable growth, self-repair, filtration, and bioproduction. Methods to measure internal mechanical properties of biofilms in vivo with spatial resolution on the cellular scale have been lacking. Here, thousands of cells are tracked inside living 3D biofilms of the bacterium Vibrio cholerae during and after the application of shear stress, for a wide range of stress amplitudes, periods, and biofilm sizes, which revealed anisotropic elastic and plastic responses of both cell displacements and cell reorientations. Using cellular tracking to infer parameters of a general mechanical model, spatially-resolved measurements of the elastic modulus inside the biofilm are obtained, which correlate with the spatial distribution of the polysaccharides within the biofilm matrix. The noninvasive microrheology and force-inference approach introduced here provides a general framework for studying mechanical properties with high spatial resolution in living materials.

Unveiling the spatiotemporal dynamics of membrane fouling: A focused review on dynamic fouling characterization techniques and future perspectives

Membrane technology has emerged as a crucial method for obtaining clean water from unconventional sources in the face of water scarcity. It finds wide applications in wastewater treatment, advanced treatment, and desalination of seawater and brackish water. However, membrane fouling poses a huge challenge that limits the development of membrane-based water treatment technologies. Characterizing the dynamics of membrane fouling is crucial for understanding its development, mechanisms, and effective mitigation. Instrumental techniques that enable in situ or real-time characterization of the dynamics of membrane fouling provide insights into the temporal and spatial evolution of fouling, which play a crucial role in understanding the fouling mechanism and the formulation of membrane control strategies. This review consolidates existing knowledge about the principal advanced instrumental analysis technologies employed to characterize the dynamics of membrane fouling, in terms of membrane structure, morphology, and intermolecular forces. Working principles, applications, and limitations of each technique are discussed, enabling researchers to select appropriate methods for their specific studies. Furthermore, prospects for the future development of dynamic characterization techniques for membrane fouling are discussed, underscoring the need for continued research and innovation in this field to overcome the challenges posed by membrane fouling.

A comprehensive evaluation of the temporal and spatial fouling characteristics of RO membranes in a full-scale seawater desalination plant

The fouling of seawater reverse osmosis (SWRO) membranes remains a persistent challenge in desalination. Previous research has focused mainly on fouling separately; however, organic, inorganic, and biofouling can coexist and influence each other. Hence, in-depth study of the spatiotemporal changes in actual combined fouling in full-scale seawater desalination will provide more effective information for fouling investigation and control. In this study, we monitored (i) the operational performance of a full-scale desalination plant for 7 years and (ii) the development and characterization of membrane and spacer fouling at different locations of spiral-wound membrane modules sampled after 2.5-, 3.5-, and 7-year operation. The findings showed that (i) operational performance indicators declined with time (normalized flux 40 % reduction, salt rejection 2 % in 7 years), with a limited effect of the 20-day cleaning frequency, (ii) fouling accumulation in the membrane module mainly occurred at the feed side of the lead module and the microbial community in these area exhibited the highest diversity, (iii) the dominant microbial OTUs belonged mainly to Proteobacteria (43-70 %), followed by Bacteroidetes (10-11 %), (iv) Phylogenetic molecular ecological networks and Spearman correlation analysis revealed that Chloroflexi (Anaerolineae) and Planctomycetes were keystone species in maintaining the community structure and biofilm maturation and significantly impacted the foulant content on the SWRO membrane, even with low abundance, and that (v) fouling accumulation was composed of polysaccharides, soluble microbial products, marine humic acid-like substances, and inorganic Ca/Fe/Mg/Si dominate the fouling layer of both the membrane and spacer. Overall, variation partitioning analysis quantitatively describes the increasing contribution of biofouling over time. Ultimately, the organic‒inorganic-biofouling interaction (70 %) significantly contributed to the overall fouling of the membrane after 7 years of operation. These results can be used to develop more targeted fouling control strategies to optimize SWRO desalination plant design and operation.

Separate effect of turbulent pulsation on internal mass transfer in porous biofilms

Turbulence is considered to be the result of coupled time-averaged and pulsating velocities, making it difficult to distinguish the respective effects, and the quantitative effect of turbulent pulsation alone on mass transfer within biofilms has not been discussed in the literature. In this study, we constructed a special oscillating grid biofilm reactor combining Particle Image Velocimetry (PIV) measurements and Computational Fluid Dynamics (CFD) simulations to achieve nearly isotropic turbulence in a designed ambient without time-averaged velocity and shear stress. Subsequently, velocity and contaminant concentration distributions were obtained by solving a mass transfer model with a k-ε turbulence model, combined with measurements of biofilm structure parameters. The results showed that the increase in turbulent pulsation intensity led to a significant stratification of the percolation velocity gradient in biofilms, which enhanced convective mass transfer. The changes of biofilm density and porosity under turbulent pulsation were more strongly correlated with convective mass transfer. When the turbulent intensity (q) increased to 2.50 cm s^-1, the removal rate reached the highest value of 96.93%, accelerating the migration of contaminant concentration and the diffusive mass transfer effect was obvious. In addition, the trend of biofilm thickness under turbulent pulsation was consistent with the change of contaminant concentration distribution, and the correlation between them was greater. In summary, at q of 2.50 cm s^-1, there was a positive effect on both convection and diffusion mechanisms in biofilms, and the contaminant removal rate and biofilm thickness reached the maximum, which was the recommended turbulent pulsation conditions.

Gravity-driven membrane filtration with compact second-life modules daily backwashed: An alternative to conventional ultrafiltration for centralized facilities

Gravity-driven membrane (GDM) filtration is a strategic alternative to conventional ultrafiltration (UF) for the resilient production of drinking water via ultrafiltration when resources become scarce, given the low dependency on energy and chemicals, and longer membrane lifetime. Implementation at large scale requires the use of compact and low-cost membrane modules with high biopolymer removal capacity. We therefore evaluated (1) to what extent stable flux can be obtained with compact membrane modules, i.e., inside-out hollow fiber membranes, and frequent gravity-driven backwash, (2) whether we can reduce membrane expenses by effectively utilizing second-life UF modules, i.e., modules that have been discarded by treatment plant operators because they are no longer under warranty, (3) if biopolymer removal could be maintained when applying a frequent backwash and with second-life modules and (4) which GDM filtration scenarios are economically viable compared to conventional UF, when considering the influence of new or second-life modules, membrane lifetime, stable flux value and energy pricing. Our findings showed that it was possible to maintain stable fluxes around 10 L/m²/h with both new and second-life modules for 142 days, but a daily gravity-driven backwash was necessary and sufficient to compensate the continuous flux drop observed with compact modules. In addition, the backwash did not affect the biopolymer removal. Costs calculations revealed two significant findings: (1) using second-life modules made GDM filtration membrane investment less expensive than conventional UF, despite the higher module requirements for GDM filtration and (2) overall costs of GDM filtration with a gravity-driven backwash were unaffected by energy prices rise, while conventional UF costs rose significantly. The later increased the number of economically viable GDM filtration scenarios, including scenarios with new modules. In summary, we proposed an approach that could make GDM filtration in centralized facilities feasible and increase the range of UF operating conditions to better adapt to increasing environmental and societal constraints.

Effects of hydrodynamic conditions on the composition, spatiotemporal distribution of different extracellular polymeric substances and the architecture of biofilms

Microbial biofilms are common on abiotic and biotic surfaces, especially in rivers, which drive crucial ecosystem processes. The microorganisms of biofilms are surrounded by a self-produced extracellular polymeric substance (EPS). In this study, we investigated the effects of different hydrodynamic conditions on the composition, spatiotemporal distribution of different extracellular polymeric substances, and the architecture of biofilms. Multidisciplinary methods offer complementary insights into complex architecture correlations in biofilms. The biofilms formed in turbulent flow with high shear force were thin but dense. However, the biofilms formed under laminar flow conditions were thick but relatively loose. The thickness and compactness of the biofilms formed in the transitional flow were different from those of the other biofilms. The compact structure of the biofilm helped to resist shear forces to minimize detachment. Under the turbulent flow condition, bacteria, exopolysaccharides, and extracellular proteins permeated through the biofilm, and more extracellular polysaccharides enveloped bacteria and extracellular proteins. However, under the transitional flow condition, the extracellular polysaccharides and proteins were fewer than those under the turbulent flow condition; bacteria and algae were seen more prominently in the upper layer of the biofilm. Under the laminar flow condition, the distribution of extracellular polysaccharides, extracellular proteins, and bacteria was relatively uniform throughout the biofilm. The number of extracellular polysaccharides was greater than that of extracellular proteins. The total number of EPS in the biofilm was the largest under turbulent flow condition, followed by that under transitional flow condition and then under laminar flow condition. This study also observed that soluble EPS (S-EPS) were secreted first, followed by loosely bound EPS (LB-EPS) and tightly bound EPS (TB-EPS). In particular, the adhesion of LB-EPS and flocculation capability of TB-EPS play some role in regulating biofilm formation. This study would help to perfect the five-stages theory of biofilm formation.

Coupled CFD-DEM modelling to predict how EPS affects bacterial biofilm deformation, recovery and detachment under flow conditions

The deformation and detachment of bacterial biofilm are related to the structural and mechanical properties of the biofilm itself. Extracellular polymeric substances (EPS) play an important role on keeping the mechanical stability of biofilms. The understanding of biofilm mechanics and detachment can help to reveal biofilm survival mechanisms under fluid shear and provide insight about what flows might be needed to remove biofilm in a cleaning cycle or for a ship to remove biofilms. However, how the EPS may affect biofilm mechanics and its deformation in flow conditions remains elusive. To address this, a coupled computational fluid dynamic– discrete element method (CFD‐DEM) model was developed. The mechanisms of biofilm detachment, such as erosion and sloughing have been revealed by imposing hydrodynamic fluid flow at different velocities and loading rates. The model, which also allows adjustment of the proportion of different functional groups of microorganisms in the biofilm, enables the study of the contribution of EPS toward biofilm resistance to fluid shear stress. Furthermore, the stress–strain curves during biofilm deformation have been captured by loading and unloading fluid shear stress to study the viscoelastic properties of the biofilm. Our predicted emergent viscoelastic properties of biofilms were consistent with relevant experimental measurements.

Controlling the hydraulic resistance of membrane biofilms by engineering biofilm physical structure

The application of membrane technology for water treatment and reuse is hampered by the development of a microbial biofilm. Biofilm growth in micro-and ultrafiltration (MF/UF) membrane modules, on both the membrane surface and feed spacer, can form a secondary membrane and exert resistance to permeation and crossflow, increasing energy demand and decreasing permeate quantity and quality. In recent years, exhaustive efforts were made to understand the chemical, structural and hydraulic characteristics of membrane biofilms. In this review, we critically assess which specific structural features of membrane biofilms exert resistance to forced water passage in MF/UF membranes systems applied to water and wastewater treatment, and how biofilm physical structure can be engineered by process operation to impose less hydraulic resistance ("below-the-pain threshold"). Counter-intuitively, biofilms with greater thickness do not always cause a higher hydraulic resistance than thinner biofilms. Dense biofilms, however, had consistently higher hydraulic resistances compared to less dense biofilms. The mechanism by which density exerts hydraulic resistance is reported in the literature to be dependant on the biofilms' internal packing structure and EPS chemical composition (e.g., porosity, polymer concentration). Current reports of internal porosity in membrane biofilms are not supported by adequate experimental evidence or by a reliable methodology, limiting a unified understanding of biofilm internal structure. Identifying the dependency of hydraulic resistance on biofilm density invites efforts to control the hydraulic resistance of membrane biofilms by engineering internal biofilm structure. Regulation of biofilm internal structure is possible by alteration of key determinants such as feed water nutrient composition/concentration, hydraulic shear stress and resistance and can engineer biofilm structural development to decrease density and therein hydraulic resistance. Future efforts should seek to determine the extent to which the concept of "biofilm engineering" can be extended to other biofilm parameters such as mechanical stability and the implication for biofilm control/removal in engineered water systems (e.g., pipelines and/or, cooling towers) susceptible to biofouling.

Water in bacterial biofilms: pores and channels, storage and transport functions

Bacterial biofilms occur in many natural and industrial environments. Besides bacteria, biofilms comprise over 70 wt% water. Water in biofilms occurs as bound- or free-water. Bound-water is adsorbed to bacterial surfaces or biofilm (matrix) structures and possesses different Infra-red and Nuclear-Magnetic-Resonance signatures than free-water. Bound-water is different from intra-cellularly confined-water or water confined within biofilm structures and bacteria are actively involved in building water-filled structures by bacterial swimmers, dispersion or lytic self-sacrifice. Water-filled structures can be transient due to blocking, resulting from bacterial growth, compression or additional matrix formation and are generally referred to as "channels and pores." Channels and pores can be distinguished based on mechanism of formation, function and dimension. Channels allow transport of nutrients, waste-products, signalling molecules and antibiotics through a biofilm provided the cargo does not adsorb to channel walls and channels have a large length/width ratio. Pores serve a storage function for nutrients and dilute waste-products or antimicrobials and thus should have a length/width ratio close to unity. The understanding provided here on the role of water in biofilms, can be employed to artificially engineer by-pass channels or additional pores in industrial and environmental biofilms to increase production yields or enhance antimicrobial penetration in infectious biofilms.

Formation and ripening of alginate-like exopolymer gel layers during and after membrane filtration

The properties of biofilm EPS are determined by the multiple interactions between its constituents and the surrounding environment. Because of the high complexity of biofilm EPS, its constituents' characterisation is still far from thorough, and identification of these interactions cannot be done yet. Therefore, we use gels of bacterial alginate-like exopolysaccharides (ALEs) as a model component for biofilm EPS in this work. These gels have been examined for their cohesive properties as a function of CaCl₂ and KCl concentration. Hereto, ALE gel layers were formed on membranes by dead-end filtration of ALE solutions. Accumulation of the cations Ca²⁺ and K⁺ in the gels could be well predicted from a Donnan equilibrium model based on the fixed negative charges in the ALE. This suggests that there is no specific binding of Ca²⁺ to the ALE and that on the time scale of the experiments, the Ca²⁺ ions can distribute freely over the gel and the surrounding solution. The concentration of fixed negative charges in the ALE was estimated around 1 mmol/g VSS (volatile suspended solids, organic mass) from the Donnan equilibrium. Moreover, an accumulation of H⁺ was predicted. Gels with more CaCl₂ in the supernatant were more compact and bore a higher osmotic pressure than those with less CaCl₂, revealing the role of Ca²⁺ ions in the network crosslinking. It is hypothesised that this mechanism later transitions into a rearrangement of the ALE molecules, which eventually leads to a fibrous network structure with large voids.

Effect of nanobubble application on performance and structural characteristics of microbial aggregates

Herein an investigation on the performance and structural properties with aspects of stability, composition, functional group, and three-dimensional distribution were approached to evaluate the influence of nanobubble aeration to the two most common microbial aggregates, activated sludge and biofilm. This study found that applying nanobubble effectively provided extra oxygen for microbial aggregates and achieved a 10.58% improvement in total nitrogen removal. The structure of microbial aggregates was enhanced, where extracellular protein and polysaccharides respectively increased as maximum as 3.40 and 1.70 times in biofilm and activated sludge, accompanied by the development of activated sludge floc size and the thickness of biofilm. Further investigation on extracellular polymeric substance and surface of microbial aggregates showed the composition of functional substances of microbial aggregates were shifted by the application of nanobubble, especially the oxygen-sensitive ones. Confocal laser scanning microscopy imaging visualized that the nanobubble changed the morphology of biofilm to a more evenly one. However, an adaptive process was more needed for activated sludge rather than biofilm, it suggested application of NB optimized the distribution of functional microorganisms in-depth and the metabolism pathway of them by accelerating the structure development of microbial aggregates, especially for biofilm.

Analyzing microalgal biofilm structures formed under different light conditions by evaluating cell-cell interactions

Biofilm structure plays an important role in microalgae biofilm-based culture. This work aims to understand microalgal biofilm structures formed under different light conditions. Here, Scenedesmus obliquus was biofilm cultured under the light spectra of white, blue, green, and red, and the photoperiods of 5:5 s, 30:30 min, and 12:12 h (light : dark period). Biofilms were observed with confocal laser scanning microscopes and profilometry, then the porosity and roughness of biofilm were determined. We found that cells under white light formed a heterogeneous biofilm with many voids, high porosity, and roughness. While under red and blue lights, cells formed homogeneous biofilms with low porosity. Biofilm structures formed under different photoperiods were different. The mechanism of forming different biofilm structures under different light conditions was interpreted from the aspect of cell-cell interactions. Moreover, the results revealed that biomass accumulation increased with the increasing biofilm porosity due to the high effective diffusion coefficient.

Modeling the impact of evolving biofilms on flow in porous media inside a microfluidic channel

This study integrates microfluidic experiments and mathematical modeling to study the impacts of biofilms on flow in porous media and to explore approaches to simplify modeling permeability with complicated biofilm geometries. E. coli biofilms were grown in a microfluidic channel packed with a single layer of glass beads to reach three biofilm levels: low, intermediate, and high, with biofilm ratios (β_r) of 2.7%, 17.6%, and 55.2%, respectively. Two-dimensional biofilm structures and distributions in the porous medium were modeled by digitizing confocal images and considering broad ranges of biofilm permeability (k_b) (from 10^-15 m² to 10^-7 m²) and biofilm porosity (ε_b) (from 0.2 to 0.8). The overall permeability of the porous medium (k), the flow pathways and the overall/local pressure gradients were found to be highly dependent on β_r and k_b but were moderately impacted by ε_b when the biofilm levels were high and intermediate with k_b>10^-11 m². When biofilm structures are well developed, simplified biofilm geometries, such as uniform coating and symmetric contact filling, can provide reasonable approximations of k.

Biofilm mechanics: Implications in infection and survival

It has long been recognized that biofilms are viscoelastic materials, however the importance of this attribute to the survival and persistence of these microbial communities is yet to be fully realized. Here we review work, which focuses on understanding biofilm mechanics and put this knowledge in the context of biofilm survival, particularly for biofilm-associated infections. We note that biofilm viscoelasticity may be an evolved property of these communities, and that the production of multiple extracellular polymeric slime components may be a way to ensure the development of biofilms with complex viscoelastic properties. We discuss viscoelasticity facilitating biofilm survival in the context of promoting the formation of larger and stronger biofilms when exposed to shear forces, promoting fluid-like behavior of the biofilm and subsequent biofilm expansion by viscous flow, and enabling resistance to both mechanical and chemical methods of clearance. We conclude that biofilm viscoelasticity contributes to the virulence of chronic biofilm infections.

In-situ monitoring AHL-mediated quorum-sensing regulation of the initial phase of wastewater biofilm formation

Initial attachment plays an important role in biofilm formation in wastewater treatment processes. However, the initial attachment process mediated by N-acyl-homoserine lactones (AHLs) is difficult to be fully understood due to the lack of non-invasive and on-line investigation techniques. In this study, the AHL-regulated wastewater biofilm attachment was quantified using ultrasonic time-domain reflectometry (UTDR) as an in-situ and non-invasive monitoring technique. Results demonstrated that the reversible adhesion time in municipal and industrial wastewaters was significantly decreased in the presence of exogenous AHLs. Biofilm thickness in municipal and industrial wastewaters increased significantly with the addition of exogenous AHLs. Also, the addition of acylase delayed the initial biofilm formation (lengthened reversible adhesion time and decreased biofilm thickness and density). Compared with biofilm behavior in the presence of low concentrations of AHLs (4.92 ± 0.17 μg/L), both reversible adhesion time and biofilm thickness were not significantly increased (p > 0.05) with an increase in AHL concentration (9.75 ± 0.41 μg/L). Furthermore, the addition of exogenous AHLs resulted in significant changes in the attached bacterial community structures, in which both QS and quorum-quenching (QQ) bacteria were stimulated. The current work presents an effective approach to in-situ monitoring of the regulation of AHL-mediated QS in the initial attachment of biofilms, especially in the reversible adhesion process, which may provide a potential strategy to facilitate biofilm establishment in wastewater treatment processes.

Computational and Experimental Investigation of Biofilm Disruption Dynamics Induced by High-Velocity Gas Jet Impingement

Knowledge of mechanisms promoting disruption though mechanical forces is essential in optimizing biofilm control strategies which rely on fluid shear. Our results provide insight into how biofilm disruption dynamics is governed by applied forces and fluid properties, revealing a mechanism for ripple formation and fluid-biofilm mixing. These findings have important implications for the rational design of new biofilm cleaning strategies with fluid jets, such as determining optimal parameters (e.g., jet velocity and position) to remove the biofilm from a certain zone (e.g., in dental hygiene or debridement of surgical site infections) or using antimicrobial agents which could increase the interfacial area available for exchange, as well as causing internal mixing within the biofilm matrix, thus disrupting the localized microenvironment which is associated with antimicrobial tolerance. The developed model also has potential application in predicting drag and pressure drop caused by biofilms on bioreactor, pipeline, and ship hull surfaces.

Experimental data showed that high-speed microsprays can effectively disrupt biofilms on their support substratum, producing a variety of dynamic reactions such as elongation, displacement, ripple formation, and fluidization. However, the mechanics underlying the impact of high-speed turbulent flows on biofilm structure is complex under such extreme conditions, since direct measurements of viscosity at these high shear rates are not possible using dynamic testing instruments. Here, we used computational fluid dynamics simulations to assess the complex fluid interactions of ripple patterning produced by high-speed turbulent air jets impacting perpendicular to the surface of Streptococcus mutans biofilms, a dental pathogen causing caries, captured by high-speed imaging. The numerical model involved a two-phase flow of air over a non-Newtonian biofilm, whose viscosity as a function of shear rate was estimated using the Herschel-Bulkley model. The simulation suggested that inertial, shear, and interfacial tension forces governed biofilm disruption by the air jet. Additionally, the high shear rates generated by the jet impacts coupled with shear-thinning biofilm property resulted in rapid liquefaction (within milliseconds) of the biofilm, followed by surface instability and traveling waves from the impact site. Our findings suggest that rapid shear thinning under very high shear flows causes the biofilm to behave like a fluid and elasticity can be neglected. A parametric sensitivity study confirmed that both applied force intensity (i.e., high jet nozzle air velocity) and biofilm properties (i.e., low viscosity and low air-biofilm surface tension and thickness) intensify biofilm disruption by generating large interfacial instabilities.

Testing Anti-Biofilm Polymeric Surfaces: Where to Start?

Present day awareness of biofilm colonization on polymeric surfaces has prompted the scientific community to develop an ever-increasing number of new materials with anti-biofilm features. However, compared to the large amount of work put into discovering potent biofilm inhibitors, only a small number of papers deal with their validation, a critical step in the translation of research into practical applications. This is due to the lack of standardized testing methods and/or of well-controlled in vivo studies that show biofilm prevention on polymeric surfaces; furthermore, there has been little correlation with the reduced incidence of material deterioration. Here an overview of the most common methods for studying biofilms and for testing the anti-biofilm properties of new surfaces is provided.

Biofilm compressibility in ultrafiltration: A relation between biofilm morphology, mechanics and hydraulic resistance

Poroelastic fluid-structure interaction models were coupled to experimental data to determine the effects of biofilm spatial distribution of mechanical and hydraulic properties on the biofilm hydraulic resistance and compressibility in membrane filtration processes. Biofilms were cultivated on ultrafiltration membranes for 20 and 30 days under high (0.28 bar) and low (0.06 bar) transmembrane pressure (TMP), in dead-end filtration mode. Subsequently, biofilms were subjected to a compression/relaxation cycles by step-wise TMP changes. Structural deformation of biofilms during compression was observed in-situ using optical coherence tomography. Experimental results show that the observed increase in the biofilm hydraulic resistance during compression is not necessarily accompanied by a detectable biofilm thickness reduction. A dual-layer biofilm model with a dense base and porous top layer could explain these observed results. Because porosity controls indirectly the mechanical response of biofilms under compression, results could be described without assuming a gradient in mechanical properties within the biofilm. The biofilm surface roughness did not significantly influence the water flux in this study. However, the fraction of biofilm base layer directly exposed to bulk liquid could be a good indicator in the determination of water flux. The main implications of this study for the design and operation of low-pressure membrane systems (e.g., MF and UF with fouling layer being the main filtration resistance) lays in the selection of favorable operational TMP and biofilm morphology.

Nondestructive characterization of soft materials and biofilms by measurement of guided elastic wave propagation using optical coherence elastography

Biofilms are soft multicomponent biological materials composed of microbial communities attached to surfaces. Despite the crucial relevance of biofilms to diverse industrial, medical, and environmental applications, the mechanical properties of biofilms are understudied. Moreover, most of the available techniques for the characterization of biofilm mechanical properties are destructive. Here, we detail a model-based approach developed to characterize the viscoelastic properties of soft materials and bacterial biofilms based on experimental data obtained using the nondestructive dynamic optical coherence elastography (OCE) technique. The model predicted the frequency- and geometry-dependent propagation velocities of elastic waves in a soft viscoelastic plate supported by a rigid substratum. Our numerical calculations suggest that the dispersion curves of guided waves recorded in thin soft plates by the dynamic OCE technique are dominated by guided waves, whose phase velocities depend on the viscoelastic properties and plate thickness. The numerical model was validated against experimental measurements in agarose phantom samples with different thicknesses and concentrations. The model was then used to interpret guided wave dispersion curves obtained by the OCE technique in bacterial biofilms developed in a rotating annular reactor, which allowed the quantitative characterization of biofilm shear modulus and viscosity. This study is the first to employ measurements of elastic wave propagation to characterize biofilms, and it provides a novel framework combining a theoretical model and an experimental approach for studying the relationship between the biofilm internal physical structure and mechanical properties.

Determination of mechanical properties of biofilms by modelling the deformation measured using optical coherence tomography

The advantage of using non-invasive imaging such as optical coherence tomography (OCT) to asses material properties from deformed biofilm geometries can be compromised by the assumptions made on fluid forces acting on the biofilm. This study developed a method for the determination of elastic properties of biofilms by modelling the biofilm deformation recorded by OCT imaging with poroelastic fluid-structure interaction computations. Two-dimensional biofilm geometries were extracted from OCT scans of non-deformed and deformed structures as a result of hydrodynamic loading. The biofilm geometries were implemented in a model coupling fluid dynamics with elastic solid mechanics and Darcy flow in the biofilm. The simulation results were compared with real deformed geometries and a fitting procedure allowed estimation of the Young's modulus in given flow conditions. The present method considerably improves the estimation of elastic moduli of biofilms grown in mini-fluidic rectangular channels. This superior prediction is based on the relaxation of several simplifying assumptions made in past studies: shear stress is not anymore taken constant over the biofilm surface, total stress including also pressure is accounted for, any biofilm shape can be used in the determinations, and non-linear behavior of mechanical properties can be estimated. Biofilm elastic moduli between 70 and 700 Pa were obtained and biofilm hardening at large applied stress due to increasing flow velocity was quantified. The work performed here opens the way for in-situ determination of other mechanical properties (e.g., viscoelastic properties, relaxation times, plastic yields) and provides data for modelling biofilm deformation and detachment with eventual applications in biofilm control and removal strategies.