Bubble-Driven Detachment of Bacteria from Confined Microgeometries

Review on the Significant Interactions between Ultrafine Gas Bubbles and Biological Systems

Having sizes comparable with living cells and high abundance, ultrafine bubbles (UBs) are prone to inevitable interactions with different types of cells and facilitate alterations in physiological properties. The interactions of four typical cell types (e.g., bacterial, fungal, plant, and mammalian cells) with UBs have been studied over recent years. For bacterial cells, UBs have been utilized in creating the capillary force to tear down biofilms. The release of high amounts of heat, pressure, and free radicals during bubble rupture is also found to affect bacterial cell growth. Similarly, the bubble gas core identity plays an important role in the development of fungal cells. By the proposed mechanism of attachment of UBs on hydrophobin proteins in the fungal cell wall, oxygen and ozone gas-filled ultrafine bubbles can either promote or hinder the cell growth rate. On the other hand, reactive oxygen species (ROS) formation and mass transfer facilitation are two means of indirect interactions between UBs and plant cells. Likewise, the use of different gas cores in generating bubbles can produce different physical effects on these cells, for example, hydrogen gas for antioxidation against infections and oxygen for oxidation of toxic metal ions. For mammalian cells, the importance of investigating their interactions with UBs lies in the bubbles' action on cell viability as membrane poration for drug delivery can greatly affect cells' survival. UBs have been utilized and tested in forming the pores by different methods, ranging from bubble oscillation and microstream generation through acoustic cavitation to bubble implosion.

Investigating the effects of ultrafine bubbles on bacterial growth

Several previous studies have considered ultrafine bubbles as a potential research target because their properties can be applied in many different research areas. In particular, the interaction between UFBs and microorganisms has always been one of the aspects that receives much attention due to the high difficulty in controlling a living system. The properties of UFBs, as mobile air-water interfaces, are greatly determined by their gas cores which play a critical role in regulating microbial growth. This study aims to investigate the effects of ultrafine bubbles on bacterial growth. Two well-studied organisms were chosen as models - Escherichia coli and Staphylococcus aureus. Their growing behavior was examined based on the growth rate, phenotype and biomass. Three types of Luria-Bertani cultures were tested, including a standard culture containing distilled water, an air ultrafine bubble culture, and a hydrogen ultrafine bubble culture. The UFBs were generated via ultrasonic cavitation and stabilized by 50 μM SDS, which was proven to have negligible effects on bacterial growth. By comparing among the three cultivation conditions, the bacterial growth rates were observed to be the highest in exposure to HUFBs. The results also signified that UFBs had an enhancement on cell proliferation. On the other hand, while proposing an increase in cell density, bacteria cultured in HUFB media have their sizes decreased uniformly and significantly (p-value < 0.05). This study confirmed that bacterial growth was promoted by UFBs; and better effects recorded were due to the HUFB present in the culture media. However, the average morphological size of bacteria was in negative correlation with their population size.

Capillary-driven horseshoe vortex forming around a micro-pillar

HYPOTHESIS

Horseshoe vortices are known to emerge around large-scale obstacles, such as bridge pillars, due to an inertia-driven adverse pressure gradient forming on the upstream-side of the obstacle. We contend that a similar flow structure can arise in thin-film Stokes flow around micro-obstacles, such as used in textured surfaces to improve wettability. This could be exploited to enhance mixing in microfluidic devices, typically limited to creeping-flow regimes.

EXPERIMENTS

Numerical simulations based on the Navier-Stokes equations are carried out to elucidate the flow structure associated with the wetting dynamics of a liquid film spreading around a 50 μm diameter micro-pillar. The employed multiphase solver, which is based on the volume of fluid method, accurately reproduces the wetting dynamics observed in current and previous (Mu et al., Langmuir, 2019) experiments.

FINDINGS

The flow structure within the liquid meniscus forming at the foot of the micro-pillar evinces a horseshoe vortex wrapping around the obstacle, notwithstanding that the Reynolds number in our system is extremely low. Here, the adverse pressure gradient driving flow reversal near the bounding wall is caused by capillarity instead of inertia. The horseshoe vortex is entangled with other vortical structures, leading to an intricate flow system with high-potential mixing capabilities.

Density-modification displacement using colloidal biliquid aphron for entrapped DNAPL contaminated aquifer remediation

Colloidal biliquid aphron (CBLA) is a strong density modifier for dense nonaqueous phase liquids (DNAPLs). However, the underlying mechanisms responsible for density modification and displacement is not yet clear. Here, a series of batch column and sandbox experiments were conducted to achieve substantial removal and irreversible density reduction of tetrachloroethylene (PCE). The mass of PCE retained in the column and sandbox was less than 1% under suitable injection conditions, and the density of PCE in the effluent was less than that of water (fluctuated in the range of 0.74-0.96 g/cm³). The displacement process was controlled by the high viscosity ratio of CBLA to PCE (52.3). The emulsified and dissolved phase of PCE formed after reaction with CBLA, and the light nonaqueous phase liquid (LNAPL) phase formed after injecting demulsifier solution. The phase analysis played a significant role in monitoring the changes in concentration and density of PCE. The density-modification displacement technique using CBLA reduced the mass of residual PCE by a factor of 165 compared to surfactant flushing, and there was no risk of downward migration of PCE. This study contributes to a better remediation of entrapped DNAPL in contaminated aquifer.

Microfluidics as an Emerging Platform for Exploring Soil Environmental Processes: A Critical Review

Investigating environmental processes, especially those occurring in soils, calls for innovative and multidisciplinary technologies that can provide insights at the microscale. The heterogeneity, opacity, and dynamics make the soil a "black box" where interactions and processes are elusive. Recently, microfluidics has emerged as a powerful research platform and experimental tool which can create artificial soil micromodels, enabling exploring soil processes on a chip. Micro/nanofabricated microfluidic devices can mimic some of the key features of soil with highly controlled physical and chemical microenvironments at the scale of pores, aggregates, and microbes. The combination of various techniques makes microfluidics an integrated approach for observation, reaction, analysis, and characterization. In this review, we systematically summarize the emerging applications of microfluidic soil platforms, from investigating soil interfacial processes and soil microbial processes to soil analysis and high-throughput screening. We highlight how innovative microfluidic devices are used to provide new insights into soil processes, mechanisms, and effects at the microscale, which contribute to an integrated interrogation of the soil systems across different scales. Critical discussions of the practical limitations of microfluidic soil platforms and perspectives of future research directions are summarized. We envisage that microfluidics will represent the technological advances toward microscopic, controllable, and in situ soil research.

Film entrainment and microplastic particles retention during gas invasion in suspension-filled microchannels

Understanding of microplastics transport mechanism is highly important for soil contamination and remediation. The transport behaviors of microplastics in soils are complex and influenced by various factors including soil and particle properties, hydrodynamic conditions, and biota activities. Via a microfluidic experiments we study liquid film entrainment and microplastics transport and retention during two-phase displacement in microchannels with one end connected to the air and the other connected to the liquid with suspended particles. We discover three transport patterns of microplastic particles, ranging from no deposition to particle entrapment and to particle layering within liquid films, depending on the suspension withdrawal rates and the particle volume fraction in the suspension. The general behavior of particle motion is effectively captured by the film thickness evolution which is shown to be dependent on a modified capillary number Ca₀ taking into account the effects of flow velocity, particle volume fraction, and channel shape. We also provide a theoretical prediction of the critical capillary number Ca₀* for particle entrapment, consistent with the experimental results. In addition, the probability of microplastics being dragged into the trailing liquid film near the gas invading front is found to be proportional to both particle volume fraction and the capillary number. This work elucidates the microplastics transport mechanism during unsaturated flow, and therefore is of theoretical and practical importance to understand the contaminant migration in many natural and engineered systems spanning from groundwater sources to water treatment facilities.

CO2-Driven diffusiophoresis for maintaining a bacteria-free surface

Dissolution and dissociation of CO2 in an aqueous phase induce diffusiophoretic motion of suspended particles with a nonzero surface charge. We report CO2-driven diffusiophoresis of colloidal particles and bacterial cells in a circular Hele-Shaw geometry. Combining experiments and model calculations, we identify the characteristic length and time scales of CO2-driven diffusiophoresis in relation to system dimensions and CO2 diffusivity. The motion of colloidal particles driven by a CO2 gradient is characterized by measuring the average velocities of particles as a function of distance from the CO2 sources. In the same geometrical configurations, we demonstrate that the directional migration of wild-type V. cholerae and a mutant lacking flagella, as well as S. aureus and P. aeruginosa, near a dissolving CO2 source is diffusiophoresis, not chemotaxis. Such a directional response of the cells to CO2 (or an ion) concentration gradient shows that diffusiophoresis of bacteria is achieved independent of cell shape, motility and the Gram stain (cell surface structure). Long-time experiments suggest potential applications for bacterial diffusiophoresis to cleaning systems or anti-biofouling surfaces, by reducing the population of the cells near CO2 sources.

Role of Near-Electrode Solution Chemistry on Bacteria Attachment and Poration at Low Applied Potentials

This research investigated mechanisms for biofouling control at boron-doped diamond (BDD) electrode surfaces polarized at low applied potentials (e.g., -0.2 to 1.0 V vs Ag/AgCl), using Pseudomonas aeruginosa as a model organism. Results indicated that electrostatic interactions between bacteria and ionic electrode functional groups facilitated bacteria attachment at the open-circuit potential (OCP). However, under polarization, the applied potential governed these electrostatic interactions and electrochemical reactions resulted in surface bubble formation and near-surface pH modulation that decreased surface attachment under anodic conditions. The poration of the attached bacteria occurred at OCP conditions and increased with the applied potential. Scanning electrochemical microscopy (SECM) provided near-surface pH and oxidant formation measurements under anodic and cathodic polarizations. The near-surface pH was 3.1 at 1.0 V vs Ag/AgCl and 8.0 at -0.2 V vs Ag/AgCl and was possibly a contributor to bacteria poration. Interpretation of SECM data using a reactive transport model allowed for a better understanding of the near-electrode chemistry. Under cathodic conditions, the primary oxidant formed was H₂O₂, and under anodic conditions, a combination of H₂O₂, Cl^•, HO₂^•, Cl₂^•-, and Cl₂ formations likely contributed to bacteria poration at potentials as low as 0.5 V vs Ag/AgCl.

A surface spectroscopy study of a Pseudomonas fluorescens biofilm in the presence of an immobilized air bubble

A linear spectral mapping technique was applied to monitor the growth of biomolecular absorption bands at the bio-interface of a nascent Pseudomonas fluorescens biofilm during and after interaction with a surface-adhered air bubble. Attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectra were obtained in different locations in a microchannel with adequate spatial and temporal resolution to study the effect of a static bubble on the evolution of protein and lipid signals at the ATR crystal surface. The results reveal that the presence of a bubble during the lag phase modified levels of extracellular lipids and affected a surface restructuring process, many hours after the bubble's disappearance.

Removal of Bacteria from Solids by Bubbles: Effect of Solid Wettability, Interaction Geometry, and Liquid-Vapor Interface Velocity

Air bubbles are a promising means of controlling fouling for a range of applications, particularly delaying fouling in marine environments. This work investigates the mechanism by which the collision of an air bubble with a solid removes adsorbed bacteria. A key feature of the work is that the numbers of bacteria were monitored via video microscopy throughout the collision; so, we were able to explore the mechanism of bacteria removal. When a bubble collides with a solid, an air-liquid interface crosses the solid twice, and we were able to distinguish the effects of the first and second air-liquid interfaces. The bacterium Pseudomonas aeruginosa was allowed to adhere to smooth poly(dimethylsiloxane) and then a collision with a bubble was investigated for one of three different approach geometries: perpendicular, parallel, and oscillating parallel to the solid surface. Other factors examined were the speed of the bubble, the duration of bacterial adhesion on the solid surface, and the wettability of the solid. Surface wettability was identified as the most significant factor. When the solid dewet, almost all bacteria were removed from hydrophobic surfaces upon the passage of the first air-liquid interface. In contrast, when a thin liquid film remained between the solid and the bubble (a hydrophilic solid), variable amounts of bacteria remained. Although almost all bacteria were initially removed from hydrophobic solids, many bacteria were redeposited on hydrophobic surfaces upon the passage of the second air-liquid interface, especially when the first and second air-liquid interfaces moved in opposite directions. As described previously, a lower velocity of the bubble allows more time for the thin liquid film to drain and improved removal efficiency on hydrophilic solids. A rougher solid (8 μm diameter hemispherical protrusions) decreased the detachment efficiency because bacteria and liquid were able to shelter in concavities.

Thermographic characterization of thin liquid film formation and evaporation in microchannels

The science of transport in microchannels has greatly benefited applications ranging from micro-mixing, chemical synthesis and biological analysis to compact and efficient energy devices. One of the most critical and intricate phenomena in this field of science is the dynamics of thin liquid film formation during the flow of liquid and gas/vapor mixtures. These films can form in microseconds and be less than a micrometer thick, while dominating thermal transport in phase-change processes. Here, we report the captured details of these phenomena using a new measurement technique with unprecedented spatial and temporal resolutions of 20 μm and 100 μs, respectively. Thin films with thicknesses ranging from 1 to 20 μm forming around elongated bubbles over a capillary number range of 0.025 to 0.1 are characterized. The measurements suggest that these films thermally develop and evaporate at timescales in the order of 1-10 ms, two orders of magnitude longer than their formation timescale. The formation, reflow and evaporation of the liquid film constitute a complex dynamic involving variations of the film thickness over the periphery of a rectangular channel, leading to a thicker liquid film feeding (through lateral capillary wicking) a much thinner rapidly evaporating film. As a result, the thinner film dictates the rate of surface heat transfer while the thicker film determines the duration of thin film evaporation. A modified Bretherton model provides the best fit to the experimental results.

Microfluidic bioanalytical flow cells for biofilm studies: a review

Bacterial biofilms are among the oldest and most prevalent multicellular life forms on Earth and are increasingly relevant in research areas related to industrial fouling, medicine and biotechnology. The main hurdles to obtaining definitive experimental results include time-varying biofilm properties, structural and chemical heterogeneity, and especially their strong sensitivity to environmental cues. Therefore, in addition to judicious choice of measurement tools, a well-designed biofilm study requires strict control over experimental conditions, more so than most chemical studies. Due to excellent control over a host of physiochemical parameters, microfluidic flow cells have become indispensable in microbiological studies. Not surprisingly, the number of lab-on-chip studies focusing on biofilms and other microbiological systems with expanded analytical capabilities has expanded rapidly in the past decade. In this paper, we comprehensively review the current state of microfluidic bioanalytical research applied to bacterial biofilms and offer a perspective on new approaches that are expected to drive continued advances in this field.

Bacterial Biofilm Material Properties Enable Removal and Transfer by Capillary Peeling

Biofilms, surface-attached communities of bacterial cells, are a concern in health and in industrial operations because of persistent infections, clogging of flows, and surface fouling. Extracellular matrices provide mechanical protection to biofilm-dwelling cells as well as protection from chemical insults, including antibiotics. Understanding how biofilm material properties arise from constituent matrix components and how these properties change in different environments is crucial for designing biofilm removal strategies. Here, using rheological characterization and surface analyses of Vibrio cholerae biofilms, it is discovered how extracellular polysaccharides, proteins, and cells function together to define biofilm mechanical and interfacial properties. Using insight gained from our measurements, a facile capillary peeling technology is developed to remove biofilms from surfaces or to transfer intact biofilms from one surface to another. It is shown that the findings are applicable to other biofilm-forming bacterial species and to multiple surfaces. Thus, the technology and the understanding that have been developed could potentially be employed to characterize and/or treat biofilm-related infections and industrial biofouling problems.

Physico-chemistry from initial bacterial adhesion to surface-programmed biofilm growth

Biofilm formation is initiated by adhesion of individual bacteria to a surface. However, surface adhesion alone is not sufficient to form the complex community architecture of a biofilm. Surface-sensing creates bacterial awareness of their adhering state on the surface and is essential to initiate the phenotypic and genotypic changes that characterize the transition from initial bacterial adhesion to a biofilm. Physico-chemistry has been frequently applied to explain initial bacterial adhesion phenomena, including bacterial mass transport, role of substratum surface properties in initial adhesion and the transition from reversible to irreversible adhesion. However, also emergent biofilm properties, such as production of extracellular-polymeric-substances (EPS), can be surface-programmed. This review presents a four-step, comprehensive description of the role of physico-chemistry from initial bacterial adhesion to surface-programmed biofilm growth: (1) bacterial mass transport towards a surface, (2) reversible bacterial adhesion and (3) transition to irreversible adhesion and (4) cell wall deformation and associated emergent properties. Bacterial transport mostly occurs from sedimentation or convective-diffusion, while initial bacterial adhesion can be described by surface thermodynamic and Derjaguin-Landau-Verwey-Overbeek (DLVO)-analyses, considering bacteria as smooth, inert colloidal particles. DLVO-analyses however, require precise indication of the bacterial cell surface, which is impossible due to the presence of bacterial surface tethers, creating a multi-scale roughness that impedes proper definition of the interaction distance in DLVO-analyses. Application of surface thermodynamics is also difficult, because initial bacterial adhesion is only an equilibrium phenomenon for a short period of time, when bacteria are attached to a substratum surface through few surface tethers. Physico-chemical bond-strengthening occurs in several minutes leading to irreversible adhesion due to progressive removal of interfacial water, conformational changes in cell surface proteins, re-orientation of bacteria on a surface and the progressive involvement of more tethers in adhesion. After initial bond-strengthening, adhesion forces arising from a substratum surface cause nanoscopic deformation of the bacterial cell wall against the elasticity of the rigid peptidoglycan layer positioned in the cell wall and the intracellular pressure of the cytoplasm. Cell wall deformation not only increases the contact area with a substratum surface, presenting another physico-chemical bond-strengthening mechanism, but is also accompanied by membrane surface tension changes. Membrane-located sensor molecules subsequently react to control emergent phenotypic and genotypic properties in biofilms, most notably adhesion-associated ones like EPS production. Moreover, also bacterial efflux pump systems may be activated or mechano-sensitive channels may be opened upon adhesion-induced cell wall deformation. The physico-chemical properties of the substratum surface thus control the response of initially adhering bacteria and through excretion of autoinducer molecules extend the awareness of their adhering state to other biofilm inhabitants who subsequently respond with similar emergent properties. Herewith, physico-chemistry is not only involved in initial bacterial adhesion to surfaces but also in what we here propose to call "surface-programmed" biofilm growth. This conclusion is pivotal for the development of new strategies to control biofilm formation on substratum surfaces, that have hitherto been largely confined to the initial bacterial adhesion phenomena.

Dewetting of Thin Liquid Films Surrounding Air Bubbles in Microchannels

As an air bubble translates in a microchannel, a thin film of liquid is formed on the bounding walls. In a microchannel with a rectangular cross-section, the liquid in the film leaks toward the low-pressure corners of the geometry, which leads to the appearance of local minima in the film thickness in the cross-sectional plane. In such a configuration, theory suggests that the minimum film thickness scales with Ca and Ca^4/3 depending on the distance from the nose of the bubble, where Ca = μU_b/γ is the flow capillary number based on the bubble velocity U_b, liquid viscosity μ, and surface tension γ, and Ca ≪ 1. We show that the film of a partially wetting liquid dewets on the channel wall at the sites of the local minima in the film thickness as it acquires thicknesses around and below 100 nm. Our experiments show that the distance L_w between the nose of the bubble and the initial dewetting location is a function of Ca and surface wettability. For channels of different wettability, L_w always scales proportional to Ca^α, where 1.7 < α < 2 for the range of 10^-5 < Ca < 10^-2. Moreover, L_w increases up to 10 times by enhancing the wettability of the surface at a given Ca. Our present measurements of L_w provide a design constraint on the lengths of bubbles to maintain a liquid wet channel without dry patches on the wall.

A new look at bubbles during biofilm inoculation reveals pronounced effects on growth and patterning

Specially designed microfluidic bioflow cells were used to temporarily trap microbubbles during different inoculation stages of Pseudomonas sp. biofilms. Despite being eliminated many hours before biofilm appearance, templated growth could occur at former bubble positions. Bubble-templated growth was either continuous or in ring patterns, depending on the stage of inoculation when the bubbles were introduced. Templated biofilms were strongly enhanced in terms of their growth kinetics and structural homogeneity. High resolution confocal imaging showed two separate bubble-induced bacterial trapping modes, which were responsible for the altered biofilm development. It is concluded that static bubbles can be exploited for fundamental improvements to bioreactor performance, as well as open new avenues to study isolated bacteria and small colonies.

Measuring a critical stress for continuous prevention of marine biofouling accumulation with aeration

When cleaning the hull of a ship, significant shear stresses are needed to remove established biofouling organisms. Given that there exists a link between the amount of time that fouling accumulates and the stress required to remove it, it is not surprising that more frequent grooming requires less shear stress. Yet, it is unclear if there is a minimum stress needed to prevent the growth of macrofouling in the limit of continuous grooming. This manuscript shows that single bubble stream aeration provides continuous grooming and prevents biofouling accumulation in regions where the average wall stress exceeds ~0.01 Pa. This value was found by comparing observations of biofouling growth from field studies with complementary laboratory measurements that probe the associated flow fields. These results suggest that aeration and other continuous grooming systems must exceed a wall stress of 0.01 Pa to prevent macrofouling accumulation.

Surface-attached molecules control Staphylococcus aureus quorum sensing and biofilm development

Bacteria use a process called quorum sensing to communicate and orchestrate collective behaviours, including virulence factor secretion and biofilm formation. Quorum sensing relies on the production, release, accumulation and population-wide detection of signal molecules called autoinducers. Here, we develop concepts to coat surfaces with quorum-sensing-manipulation molecules as a method to control collective behaviours. We probe this strategy using Staphylococcus aureus. Pro- and anti-quorum-sensing molecules can be covalently attached to surfaces using click chemistry, where they retain their abilities to influence bacterial behaviours. We investigate key features of the compounds, linkers and surfaces necessary to appropriately position molecules to interact with cognate receptors and the ability of modified surfaces to resist long-term storage, repeated infections, host plasma components and flow-generated stresses. Our studies highlight how this surface approach can be used to make colonization-resistant materials against S. aureus and other pathogens and how the approach can be adapted to promote beneficial behaviours of bacteria on surfaces.