Probing Internal Pressures and Long-Term Stability of Nanobubbles in Water

Ionic adsorption on bulk nanobubble interfaces and its uncertain role in diffusive stability

HYPOTHESIS

Bulk nanobubbles have been proposed to improve gas exchange in a variety of applications, such as in water treatment, theragnostics, and microfluidic surface cleaning. However, there is currently no consensus regarding the mechanism responsible for their reportedly long lifetimes, which contradicts classical understanding of diffusive bubble dynamics. Recently, there has been increasing support for an electrostatic stability mechanism, following from experiments that observe negatively charged zeta potentials around nanobubbles.

SIMULATIONS

We use high-fidelity Molecular Dynamics simulations to model bulk nanobubbles under mechanical equilibrium in a sodium iodide electrolyte solution, to investigate ionic adsorption on the liquid-gas interface, and resulting zeta potential. We critically examine the hypothesised electrostatic stress underpinning this previously suggested stability mechanism, which is theorised to stabilise the nanobubbles against dissolution by counteracting the otherwise dominant effects of surface tension, however, has been too difficult to directly measure in experiments.

FINDINGS

Ions adsorb onto the liquid-gas interface, confirming an Electric Double Layer (EDL) distribution around the nanobubble with an estimated zeta potential, in accordance with experiments. However, we find no significant electrostatic stress exerted on the nanobubble surface, as any ion charge density in the EDL is completely neutralised by the rearrangement of the water molecules. As a result, the internal gas pressure is still well predicted by the standard Laplace pressure equation (with a fitted Tolman length correction ), challenging an essential assumption underlying the previously proposed theories, and we instead speculate on alternative mechanisms for electrostatic-based stability.

The Role of Nanobubbles in Protein Unfolding during Electrothermal Supercharging

Nanobubbles (NBs) are tiny gas cavities with diameters around 200 nm that remain stable in solution due to their unique properties, including low buoyancy and negative surface charges. Ammonium bicarbonate (ABC) is an alternative buffer to commonly used ammonium acetate during protein analysis by electrospray ionization (ESI) mass spectrometry. The addition of ABC under high voltage and temperature conditions can lead to protein unfolding, a phenomenon termed electrothermal supercharging (ETS). The role of CO2 bubbles in ETS has been hypothesized and disputed. The solution stability of NBs allows for the direct observation of their effects on protein charge states and unfolding, providing insights into the potential role of CO2 bubbles during ETS. A novel method based on flow regime switching using a Tesla valve is employed to generate stable nanobubbles in solution. NBs were also created by sonication and pressure cycling. Nitrogen and carbon dioxide nanobubbles, when produced by flow regime switching and by pressure cycling, unfold proteins such as cytochrome c and ubiquitin but not to the same extent as with ABC addition to the ESI working solution. Complete unfolding of these proteins by NBs only occurs when the ammonium ion is also present in solution. Myoglobin, known to be less structurally stable, does unfold completely under NB influence. Further, amino acids, previously shown to provide stability to proteins under ETS conditions, also prevent unfolding when NBs are present, providing additional support for the role of gas bubbles during ETS.

Optimizing Nanobubble Production in Ceramic Membranes: Effects of Pore Size, Surface Hydrophobicity, and Flow Conditions on Bubble Characteristics and Oxygenation

Precise control of nanobubble size is essential for optimizing the efficiency and performance of nanobubble applications across diverse fields, such as agriculture, water treatment, and medicine. Producing fine bubbles, including nanobubbles, is commonly achieved by purging gas through porous media, such as ceramic or polymer membranes. Many operational factors and membrane properties can significantly influence nanobubble production and characteristics. This study examines how membrane pore size, surface hydrophobicity, and gas/water flow conditions affect nanobubble size and concentration. Findings reveal that reducing the ceramic membrane pore size from 200 to 10 nm slightly decreased the mean nanobubble diameter from 115 to 89 nm. Furthermore, membranes with a hydrophilic outer surface and hydrophobic pore surface generated smaller nanobubbles with higher concentrations in water. Additionally, a high water cross-flow rate (e.g., >1 L·min-1) increased the nanobubble concentration, though bubble size remained unaffected. In contrast, the gas flow rate had a more pronounced effect. Increasing the gas flow rate from 0.5 to 12 L·min-1 significantly raised the nanobubble concentration from 3.09 × 108 to 1.24 × 109 bubbles·mL-1 while reducing the mean bubble diameter from 100 to 79 nm. An interfacial force model was applied to analyze bubble detachment at the membrane pore outlet, considering factors such as gas flow/pressure, surface tension, and shear forces from the water flow. These findings offer valuable insights into the mechanisms governing nanobubble generation via gas injection through porous membranes.

Advancements in wastewater treatment: A comprehensive review of ozone microbubbles technology

Microbubbles (MBs) possess unique characteristics, including exceptional stability, a high specific surface area, and increased internal pressure. When combined with ozone, these properties significantly enhance the mass transfer and utilization efficiency of ozone, resulting in improved removal of organic pollutants. In recent years, the innovative application of the ozone MBs process has garnered attention as an effective method for wastewater treatment. However, research on its application effects and oxidation mechanisms in this field remains relatively limited. This article provides a comprehensive review of the ozone MBs process, detailing the principles of various MB generation techniques, the oxidation mechanisms of ozone MBs, and the practical applications of this process. Additionally, we address existing controversies and highlight the unique features, efficacy, and limitations of this technology in wastewater treatment. Future research should urgently investigate the pollutant removal mechanisms of the ozone MBs process through device optimization and bubble dynamics, with the aim of enhancing processing efficiency and reducing operating costs. This study presents a viable direction for the advancement and exploration of ozone MB technology, providing scientific support and guidance for its future applications in wastewater treatment.

Ultrahigh concentration exfoliation and aqueous dispersion of few-layer graphene by excluded volume effect

Colloidal properties of nanoparticles are intricately linked to their morphology. Traditionally, achieving high-concentration dispersions of two-dimensional (2D) nanosheets has proven challenging as they tend to agglomerate or re-stack under increased surface contact and Van der Waals attraction. Here, we unveil an excluded volume effect enabled by 2D morphology, which can be coupled with electrostatic repulsion to synthesize high-concentration aqueous graphene dispersions. To achieve this, we designed a sequential process involving edge oxidation, bubble expansion and mechanical shearing, through which graphite flakes were exfoliated into aqueous dispersions with ~94.5 wt.% yield of few-layer graphene, high concentration exceeding 100 mg mL-1, long-term stability over ~550 days, and large-scale wet processability. Structural analysis and theoretical modeling suggested that the 2D morphology of the resultant graphene nanosheets facilitates inter-sheet repulsive excluded volume interactions, leading to a fractal jammed network structure composed of nanosheets and tactoids to prevent their agglomeration. This effect was further leveraged in a continuous stirred tank reactor for the pilot-scale production of concentrated graphene dispersions. Our study unveils the role of excluded volume effect in stabilizing 2D-material colloids for industrial production and processing.

PDL1/HER2-Targeted Lipid-Encapsulated Oxygen Nanobubbles Combined with Photodynamic Therapy for HER2+ Breast Cancer Immunotherapy

Programmed death (PD) 1/PD ligand 1 (PDL1) inhibitors are immune checkpoint inhibitors (ICIs) that may facilitate HER2-positive breast cancer treatment; however, their clinical efficacy remains elusive. Oxygen-enhanced photodynamic therapy (PDT) increases immunogenic cell death (ICD), providing a promising strategy to render the tumor microenvironment more sensitive to the ICIs. Lipid-encapsulated oxygen nanobubbles (Lipo-NBs-O2) obtained using nanobubbles (NBs) water for oxygen delivery in vivo can facilitate enhanced PDT. Here, dual-receptor targeted Lipo-NBs-O2 (DRT@Lipo-NBs-O2) is prepared by modifying Lipo-NBs-O2 with anti-PDL1 scFv and the fusion protein anti-HER2 scFv-tandem-repeat cytochrome c (anti-HER2-nCytc). Copper phthalocyanine is the photosensitizer (PS). DRT@Lipo-PS-NBs-O2 plus near-infrared irradiation leads to robust ICD induction, increasing DC activation and CD8+ T-cell numbers. Modification with anti-PDL1 scFv improves tumor distribution of DRT@Lipo-PS-NBs-O2 and plays the ICI role, invigorating CD8+ T cells and boosting the effects of immunotherapy. Oxygen supplied through DRT@Lipo-PS-NBs-O2 reduces P-glycoprotein expression. Enhanced PDT and Cytc can cause tumor cell death, thereby reducing the immune burden. Under dual receptor targeting and laser local irradiation, tumor cells become subject to the combination effects of PDT, ICD, ICIs, and apoptosis; this effectively suppresses tumor growth and metastasis. Lipo-NBs-O2 affords a combination of oxygen delivery and multidrug therapy to alleviate HER2-positive breast cancer.

Mitigating environmental toxicity with hydrogen nanobubbles: A mitochondrial function-based approach to ecological restoration

In biological systems, nanobubbles (NBs) effectively enhance hydrogen molecule retention and scavenging reactive oxygen species (ROS), but the underlying mechanisms remain elusive. To investigate this, we prepared hydrogen NB water samples with consistent dissolved hydrogen levels but varying NB densities to explore their physicochemical properties and effects on green algae (Chlorella vulgaris) under oxidative stress induced by copper ions (Cu2+) and cadmium ions (Cd2+). The results indicated a strong correlation between the hydrogen NB number density and the 25 % inhibitory concentration of Cu2+ over 24 h, with ROS removal efficiency increased with the NB number density. Gas chromatography showed that the hydrogen NBs in the solution had a high gas density that enhanced hydrogen transport into C. vulgaris. With regard to mitochondrial activity, hydrogen NBs were observed to enhance the function of mitochondrial complexes I and V and increase the mitochondrial membrane potential. Experiments with C. vulgaris mitochondrial electrodes showed that the electron transfer rates increased significantly in the presence of hydrogen NBs. We concluded that the high gas density of hydrogen NBs augments intracellular hydrogen delivery and strengthens mitochondrial functions.

Polarizing Perspectives: Ion- and Dipole-Induced Dipole Interactions Dictate Bulk Nanobubble Stability

The origin of the stability of bulk Nanobubbles (NBs) has been the object of scrutiny in recent years. The interplay between the surface charge on the NBs and the Laplace pressure resulting from the surface tension at the solvent-NB interface has often been evoked to explain the stability of the dispersed NBs. While the Laplace pressure is well understood in the community, the nature of the surface charge on the NBs has remained obscure. In this work, we aim to show that the solvent and the present ions can effectively polarize the NB surface by inducing a dipole moment, which in turn controls the NB stability. We show that the polarizability of the dispersed gas and the polarity of the dispersing solvent control the dipole-induced dipole interactions between the solvent and the NBs, and that, in turn, determines their stability in solution.

Synergistic roles of carbon dioxide nanobubbles and biochar for promoting direct CO2 assimilation by plants and optimizing nutrient uptake efficiency

This study investigates the synergistic role of carbon dioxide nanobubbles (CNBs) and biochar (BC) on seed germination, plant growth, and soil quality, employing Solanum lycopersicum (tomato) and Phaseolus vulgaris (beans) as test plant species. CNBs, generated and dispersed in both distilled water (DW) and tap water (TW), exhibited distinct characteristics, with TW-CNBs being larger and more stable (peak values of around 18.17 nm and 299.5 nm, zeta potential (ZP) of -5.91 mV), while DW-CNBs have peak values of around 1.63 nm and 216.1 nm, ZP of -3.23 mV. The results suggest CNBs enhance seed germination by upto 20%. CNBs in BC amended soil further promoted plant height and leaf number. CNBs increased dissolved CO2 levels to 2-24 ppm within 40 min, while BC enriched soil organic carbon from 19.20 to 24.96 ppm in beans and 18.33 to 22.35 ppm in tomatoes. The pH levels decreased from 7.68 to 3.78 for TW-CNBs and from 7.41 to 2.13 for DW-CNBs. Additionally, the electrical conductivity (EC) decreased from 112.1 to 99.6 for TW-CNBs, while it increased from 4.15 to 32.1 for DW-CNBs. Together they significantly increased soil available phosphorus and potassium to 4.03-8.06 and 3.58-7.16 kg ha-1; and 5.67-55.74 and 17.57-43.79 kg ha-1 in bean and tomato, respectively. Variations in nutrient concentrations were observed, with substantial increase in Na (16.27% and 6.58%), Zn (3.39% and 0.46%), and Mg (5.05% and 1.44%) content for beans and tomatoes, respectively. Structural equation model and principal component analysis revealed differences between CNB and BC treated soils, highlighting positive impact on soil quality and plant growth compared to control. Integration of CNBs and BC presents a multifaceted approach to enhance soil quality and promote plant growth, offering promising solutions for sustainable agriculture and environmental management.

Unveiling the potential of nanobubbles in water: Impacts on tomato's early growth and soil properties

Nanobubbles (NBs) in water have been proven to improve plant growth and seed germination, potentially reducing both water and fertilizer consumption. To unravel the promotion mechanism of NBs on plant growth, this study investigated the characteristics of NBs in tap water and their impacts on tomato's early growth, soil chemical properties, enzymatic activity and electrochemical properties of plant roots. Oxygen NBs (ONBs) were found to increase the seed germination by 10 % and plant growth by 30 %-50 % (e.g., stem and diameter), whereas nitrogen NBs (NNBs) only had a significant promotion (7 %-34 %) on plant height. Additionally, compared to control group, irrigation with ONBs increased the peroxidase activities by 500 %-1000 % in tomato leaves, which may increase the expression of genes for peroxidase and promote cell proliferation and plant growth. Moreover, electrical impedance spectroscopy (EIS) revealed that the ONBs could reduce the interfacial impedance due to the increased active surface area and electrical conductivity of root.

Nanobubbles in water and wastewater treatment systems: Small bubbles making big difference

Since the discovery of nanobubbles (NBs) in 1994, NBs have been attracting growing attention for their fascinating properties and have been studied for application in various environmental fields, including water and wastewater treatment. However, despite the intensive research efforts on NBs' fundamental properties, especially in the past five years, controversies and disagreements in the published literature have hindered their practical implementation. So far, reviews of NB research have mainly focused on NBs' role in specific treatment processes or general applications, highlighting proof-of-concept and success stories primarily at the laboratory scale. As such, there lacks a rigorous review that authenticates NBs' potential beyond the bench scale. This review aims to provide a comprehensive and up-to-date analysis of the recent progress in NB research in the field of water and wastewater treatment at different scales, along with identifying and discussing the challenges and prospects of the technology. Herein, we systematically analyze (1) the fundamental properties of NBs and their relevancy to water treatment processes, (2) recent advances in NB applications for various treatment processes beyond the lab scale, including over 20 pilot and full-scale case studies, (3) a preliminary economic consideration of NB-integrated treatment processes (the case of NB-flotation), and (4) existing controversies in NBs research and the outlook for future research. This review is organized with the aim to provide readers with a step-by-step understanding of the subject matter while highlighting key insights as well as knowledge gaps requiring research to advance the use of NBs in the wastewater treatment industry.

Effect of chemical species and temperature on the stability of air nanobubbles

The colloidal stability of air nanobubbles (NBs) was studied at different temperatures (0-30 °C) and in the presence of sulfates, typically found in mining effluents, in a wide range of Na2SO4 concentrations (0.001 to 1 M), along with the effect of surfactants (sodium dodecyl sulfate), chloride salts (NaCl), and acid/base reagents at a pH range from 4 to 9. Using a nanobubble generator based on hydrodynamic cavitation, 1.2 × 108 bubbles/mL with a typical radius of 84.66 ± 7.88 nm were generated in deionized water. Multiple evidence is provided to prove their presence in suspension, including the Tyndall effect, dynamic light scattering, and nanoparticle size analysis. Zeta potential measurements revealed that NBs are negatively charged even after two months (from - 19.48 ± 1.89 to - 10.13 ± 1.71 mV), suggesting that their stability is due to the negative charge on their surface. NBs were found to be more stable in alkaline solutions compared to acidic ones. Further, low amounts of both chloride and sulfate dissolved salts led to a reduction of the size of NBs. However, when high amounts of dissolved salts are present, NBs are more likely to coalesce, and their size to be increased. Finally, the investigation of the stability of air NBs at low temperatures revealed a non-monotonic relationship between temperature and NBs upon considering water self-ionization and ion mobility. This research aims to open a new frontier towards the application of the highly innovative NBs technology on the treatment of mining, mineral, and metal processing effluents, which are challenging aqueous solutions containing chloride and sulfate species.

Physicochemical Characteristics and the Scale Inhibition Effect of Air Nanobubbles (A-NBs) in a Circulating Cooling Water System

Air nanobubbles (A-NBs) in a circulating cooling water system have not been investigated, although their role is significant. In this paper, the influences of the contents of main salts and other parameters on the physicochemical characteristics and scale inhibition performance of A-NBs in circulating cooling water were investigated and the scale inhibition mechanism of A-NBs in a simulated circulating cooling water system was explored. A-NBs realized a higher scale inhibition rate of 90%, which was higher than that of 1-hydroxyethane-1,1-diphosphonic acid (40%), and A-NBs stably existed for more than 5 days in the complex water environment. Four interface functions were proposed to interpret the scale inhibition effect of A-NBs in circulating cooling water as follows. First, the negatively charged surface of A-NBs adsorbed cations (Ca²⁺) reduced the concentration of scaling ions. Second, the negatively charged surface of A-NBs could also adsorb microcrystals, and their crystal-like seed action was conducive to the formation of large-size crystals, broke the rules of crystal growth, and reduced the adhesion of scales to the pipe wall. Third, A-NBs could also form a bubble layer after they were adsorbed on the inner surface of pipes, thereby preventing the deposition of scales on the surface. Fourth, A-NB burst caused local turbulence, increased the shear force onto the pipe surface, and reduced the scales adhering to the pipe surface. The interface effect of A-NBs in metal pipes is important in many industrial applications. This study laid the basis for the development of a new green A-NB scale-inhibiting technology.

Enhanced Removal of Free Radicals by Aqueous Hydrogen Nanobubbles and Their Role in Oxidative Stress

Elevated levels of reactive oxygen radicals caused by environmental stress are the key triggers of inflammation, aging, and disease; thus, it is critical to develop novel reactive oxygen radical scavenging methods with high efficiency and low toxicity. As a result of their selective reactive oxygen radical removal, hydrogen molecules are strong candidates, but their application is limited by the small hydrogen supply and short duration of action. In this study, we for the first time combined nanobubble (NB) technology and hydrogen water to remove reactive oxygen species (ROS) using copper ions as a representative environmental pollutant and Tetrahymena thermophila as a model organism. Hydrogen NBs displayed a remarkable capability of removing H₂O₂ and O₂^•- at molar ratios of 8:1 and 240:1, respectively, which were unable to be removed by dissolved hydrogen molecules only. During the oxidative defense phase, hydrogen NB water either directly removed ROS or increased the activity and relative expression of glutathione peroxidase (GSH-Px). During the oxidative inhibition phase, hydrogen NB water exerted antioxidant effects mainly by increasing the activities of superoxide dismutase and GSH-Px as well as the expression of the corresponding genes. Our results provide an important theoretical support for the wide application of hydrogen NBs in empowering the antioxidant defense system.

High resolution imaging of ultrafine bubbles in water by Atmospheric SEM-CL

Various analytical methods such as high-resolution observation of ultrafine bubbles in water are required to clarify the mechanisms and interrelationships of various effects brought about by ultrafine bubbles. In this study, we used atmospheric scanning electron microscopy-cathodoluminescence (ASEM-CL) method for observing ultrafine bubbles in water. ASEM can observe samples in water, and the fine electron beam provides high spatial resolution. Furthermore, the gas in the bubble can be estimated from the CL emission spectrum. We have measured characteristics such as bubble size and particle number density. Also, the CL spectra has shown that the ultrafine bubbles contained nitrogen.

A review on future wastewater treatment technologies: micro-nanobubbles, hybrid electro-Fenton processes, photocatalytic fuel cells, and microbial fuel cells

The future prospect in wastewater treatment technologies mostly emphasizes processing efficiency and the economic benefits. Undeniably, the use of advanced oxidation processes in physical and chemical treatments has played a vital role in helping the technologies to remove the organic pollutants efficiently and reduce the energy consumption or even harvesting the electrons movements in the oxidation process to produce electrical energy. In the present paper, we review several types of wastewater treatment technologies, namely micro-nanobubbles, hybrid electro-Fenton processes, photocatalytic fuel cells, and microbial fuel cells. The aims are to explore the interaction of hydroxyl radicals with pollutants using these wastewater technologies, including their removal efficiencies, optimal conditions, reactor setup, and energy generation. Despite these technologies recording high removal efficiency of organic pollutants, the selection of the technologies is dependent on the characteristics of the wastewater and the daily production volume. Hence the review paper also provides comparisons between technologies as the guidance in technology selection.

Does salting-out effect nucleate nanobubbles in water: Spontaneous nucleation?

The solubility of gases in aqueous salt solution decreases with the salt concentration, often termed the "salting-out effect." The dissolution of salt in water is followed by dissociation of salt and further solvation of ions with water molecules. The solvation weakens the affinity of gaseous molecules, and thus it releases the excess dissolved gas. Now it is interesting to know that what happens to the excess gas released during salting-out? Since it is imperative to note that the transfer of the dissolved gas in the bulk liquid may often occur in the form of nanobubbles. In this work, we have answered this question by investigating the nano-entities nucleation during the salting-out effect. The solubility of gases in aqueous salt solution decreases with the salt concentration, and it is often termed as the "salting-out effects." The dissolution of salt in water undergoes dissociation of salt and further solvation of ions with water molecules. The solvation weakens the affinity of gaseous molecules, and thus it releases the excess dissolved gas. Now it is interesting to know that what happens to the excess gas released during salting-out? While it is also imperative to note that the gas transfer in the bulk liquid often occurs in the form of bubbles. With this hypothesis, we have experimentally investigated that whether the salting-out effect nucleates nanobubble or not. What is the strong scientific evidence to prove that they are nanobubbles? Does the salting-out parameter affect the number density? The answers to such questions are essential for the fundamental understanding of the origin and driving force for nanobubble generation. We have provided three distinct proofs for the nano-entities to be the nanobubbles, namely, (1) by freezing and thawing experiments, (2) by destroying the nanobubbles under ultrasound field, and (3) we also proposed a novel method for refractive index estimation of nanobubbles to differentiate them from nano drops and nanoparticles. The refractive index (RI) of nanobubbles was estimated to be 1.012 for mono- and di-valent salts and 1.305 for trivalent salt. The value of RI closer to 1 provides strong evidence of gas-filled nanobubbles. Both positive and negative charged nanobubbles nucleate during the salting-out effect depending upon the valency of salt. The nanobubbles during the salting-out effect are stable only for up to three days. This shorter stability could plausibly be due to reduced colloidal stability at a low surface charge.

Nanobubbles promote nutrient utilization and plant growth in rice by upregulating nutrient uptake genes and stimulating growth hormone production

Excessive application of chemical fertilizers can lead to serious environmental problems. In this study, we explored the use of nanobubble water for irrigation of crop rice as a means of reducing fertilizer use. The effect of nanobubbles on plant growth and nutrient uptake was evaluated in the laboratory, while crop yield and the efficiency of fertilizer use were evaluated in a field study. The laboratory experiments indicated that nanobubbles significantly improve plant height and root length in rice seedlings. Nanobubble treatment stimulated synthesis of the growth hormone gibberellin and upregulated the plant nutrient absorption genes OsBT, PiT-1 and SKOR, resulting in increased nutrient uptake and utilization by the roots. The field experiments verified the laboratory observations, showing that nanobubble treatment significantly increases rice yield by almost 8% when using similar levels of fertilizer as controls. Moreover, the same yield as controls was achieved with approximately 25% less fertilizer. As well as their impact on growth hormones and nutrient absorption genes, nanobubbles, due to hydrophobic and surface charge properties, enhance the release and absorption of soil nutrients, thereby reducing fertilizer demand. Overall, this study highlights a new and sustainable water irrigation strategy for enhancing crop yield and reducing chemical fertilizer waste.

Aeration and dissolution behavior of oxygen nanobubbles in water

HYPOTHESIS

Nanobubbles (NBs) in water elicit unique physicochemical and colloidal properties (e.g., high stability and longevity). Aeration kinetics and dissolution behavior of oxygen (O₂) NBs are assumed to be bubble size dependent.

EXPERIMENTS

As an indicator for aeration efficiency, volumetric mass transfer coefficient (K_L·a) was assessed by measuring the dissolved oxygen (DO) levels during aeration using O₂ NBs with different sizes. Mass transfer coefficient (K_L) was estimated by correlation analysis. Moreover, a modified Epstein-Plesset (EP) model was developed to predict the dissolution behavior by monitoring the DO and size changes during the dissolution of O₂ NBs in water.

FINDINGS

A higher rate of DO increase and a higher equilibrium DO level were both observed after aeration with NBs that present higher surface areas for the mass transfer of O₂ and a higher vapor pressure of O₂ to drive the partitioning equilibrium. Dissolution kinetics of O₂ NBs were highly dependent on the initial bubble size as indicated by the changes of bubble size and DO. Smaller NBs raised up DO faster, whereas larger NBs could lead to higher equilibrium DO levels. Moreover, the rate of DO decline and the quasi-steady DO levels both decreased when the dilution ratio increased, confirming that O₂ NBs dictates the DO level in water. Finally, the dissolving NBs may either swell or shrink according to the model prediction.

Understanding the Stabilization of a Bulk Nanobubble: A Molecular Dynamics Analysis

Bulk nanobubbles (NBs) have received considerable attention because of their extensive potential applications, such as in ultrasound imaging and water management. Although multiple types of experimental evidence have supported the existence and stabilization of bulk NBs, the underlying mechanism remains unclear. This study numerically investigates the bulk NB stabilization with molecular dynamics (MD) methods: the all-atom (AA) MD simulation is used for NBs of several nanometers diameter; the coarse-grained (CG) MD simulation is for the NBs of about 100 nm. The NB properties are statistically obtained and analyzed, including the inner density, inner pressure, surface charge, interfacial hydrogen bond (HB), and gaseous diffusion. The results show that the gas inside an NB has ultrahigh density (tens of kilograms per cubic meter). A double-layer surface charge exists on the NB. The inner/outer layer is positively/negatively charged, and the electrostatic stress can counteract part of the surface tension. In addition, the interfacial HB is weakened by the interaction between gas and water molecules, causing less surface tension. The above features are beneficial to NB stabilization. The NB equilibrium radii solved by the interfacial mechanical equilibrium equation agree with the MD results, indicating that this equation can describe the force balance of an NB as small as several nanometers. Besides, supersaturation appears to be necessary for the NB thermodynamic equilibrium. Based on Henry's law and the ideal gas law, the theoretical analysis suggests that the stability of the NB thermodynamic equilibrium is conditional: the number of gas molecules in NBs should be more than half that dissolved in liquid. This study unravels a stabilized bulk NB's properties and discusses the NB equilibrium and stabilization mechanism, which will advance the understanding and application of bulk NBs.

High-Performance Ultrafine Bubble Aeration on Janus Aluminum Foil Prepared by Laser Microfabrication

Aeration is a mass transfer process, in which gas is dispersed into a liquid by utilizing air inflation or agitation. Typically, a microporous device is often used for aeration. Increasing the gas flow rate and decreasing the pore size reduce the bubble size, but the surface wettability of the gas/solid interface also has a significant impact on the bubble size, which is rarely studied. In this study, a superhydrophilic/superhydrophobic Janus aluminum foil (JAF) is fabricated by laser microstructuring and low surface energy modification. The gas-repelling superhydrophilic surface not only facilitates ultrafine bubble generation but also allows the bubbles to detach from the aerator surface quickly, while the superhydrophobic surface prevents water from infiltrating into the aeration chamber and reduces the mass transfer resistance. The micropores with different diameters are obtained by adjusting the laser processing parameters. The pore prepared by the laser is uniform, consequently leading to the uniform bubble size. When the pore diameter is set to 30 μm, the diameter of bubbles released from the superhydrophilic surface of the JAF is only 0.326 mm, and the gas dissolution rate is about six times that of the double-sided superhydrophobic aluminum foil. This simple, low-cost, and controllable method of the laser processing JAF has broad applications in wastewater treatment, energy production, and aquaculture.