Interpreting the interfacial and colloidal stability of bulk nanobubbles

Generation methods, stability, detection techniques, and applications of bulk nanobubbles in agro-food industries: a review and future perspective

Nanobubble (NB) technologies have received considerable attention for various applications due to their low cost, eco-friendliness, scale-up potential, process control, and unique physical characteristics. NB stands for nanoscopic gaseous cavities, typically <1 μm in diameter. NBs can exist on surfaces (surface or interfacial NBs) and be dispersed in a bulk liquid phase (bulk NBs). Compared to the microbubbles, NBs exhibit high specific surface area, negative surface charge, and better adsorption. Bulk NBs can be generated by hydrodynamic/acoustic cavitation, electrolysis, water-solvent mixing, nano-membrane filtration, and so on. NBs exhibit extraordinary longevity compared to microbubbles, prompting the interest of the scientific community aiming for potential applications including medicine, agriculture, food, wastewater treatment, surface cleaning, and so on. Based on the limited amount of research work available regarding the influence of NBs on food matrices, further research, however, needs to be done to provide more insights into its applications in food industries. This review provides an overview of the generation methods for NBs, techniques to evaluate them, and a discussion of their stability and several applications in various fields of science were discussed. However, recent studies have revealed that, despite the many benefits of NB technologies, several NB generating approaches are still limited in their application in specific agro-food industries. Further study should focus on process optimization, integrating various NB generation techniques/combining with other emerging technologies in order to achieve rapid technical progress and industrialization of NB-based technologies.HighlightsNanobubbles (NBs) are stable spherical entities of gas within liquid and are operationally defined as having diameters less than 1 µm.Currently, various reported theories still lack the ability to explain the evidence and stability of NBs in water, numerous NB applications have emerged due to the unique properties of NBs.NB technologies can be applied to various food and dairy products (e.g. yogurt and ice cream) and other potential applications, including agriculture (e.g. seed germination and plant growth), wastewater treatment, surface cleaning, and so on.

How sodium chloride extends lifetime of bulk nanobubbles in water

We present a molecular dynamics simulation study on the effects of sodium chloride addition on stability of a nitrogen bulk nanobubble in water. We find that the lifetime of the bulk nanobubble is extended in the presence of NaCl and reveal the underlying mechanisms. We do not observe spontaneous accumulation or specific arrangement of ions/charges around the nanobubble. Importantly, we quantitatively show that the N₂ molecule selectively diffuses through water molecules rather than pass by any ions after it leaves the nanobubble due to the much weaker water-water interactions than ion-water interactions. The strong ion-water interactions cause hydration effects and disrupt hydrogen bond networks in water, which leave fewer favorable paths for the diffusion of N₂ molecules, and by that reduce the degree of freedom in the dissolution of the nanobubble and prolong its lifetime. These results demonstrate that the hydration of ions plays an important role in stability of the bulk nanobubble by affecting the dynamics of hydrogen bonds and the diffusion properties of the system, which further confirm and interpret the selective diffusion path of N₂ molecules and the extension of lifetime of the nanobubble. The new atomistic insights obtained from the present research could potentially benefit the practical application of bulk nanobubbles.

Alternative methods for cleaning membranes in water and wastewater treatment

Membrane fouling is caused by foulant deposition or adsorption through physical or chemical interactions on the membrane surface, causing the reduction of flux through the membrane. The main drawbacks of chemical agents used for cleaning are cost, damage caused on the membrane, and waste stream making the process unattractive. Alternative, methods such as ultrasound, enzymatic process, and osmotic backwashing were explored for membrane cleaning. Among all mentioned methods, micronanobubbles have been reported as a promising and emergent method for membrane surface cleaning; unfortunately, the information is limited, but preliminary studies have shown it as an efficient, cheap, and environmentally friendly technique. Other methods like electrically and vibratory-enhanced membrane cleaning also could be interesting but currently are unexplored and information is limited. PRACTITIONER POINTS: Chemical cleaning is an efficient option; however, from an environmental point of view, it is not attractive, and high concentrations could cause damage to the membrane. Micronanobubbles are an emergent and suitable technology for membrane and surface cleaning. Membrane modification and functionalization avoid membrane fast fouling, and the cleaning process is easier, but the manufacture cost could be expensive.

The effect of preparation time and aeration rate on the properties of bulk micro-nanobubble water using hydrodynamic cavitation

Fundamental research on bulk micro-nanobubbles (BMNBs) has grown rapidly due to the demand for their industrial applications and potential role in interfacial sciences. This work focuses on examining properties of such bubbles, including the number, concentration, zeta potential, and surface tension in water. For this purpose, BMNBs were generated by the hydrodynamic cavitation (HC) mechanism. Distilled water and air in the experiments were the liquid and gas phases, respectively. The characterization of bulk microbubbles (BMBs) and bulk nanobubbles (BNBs) were performed through focused beam reflectance measurement (FBRM) and nanoparticle tracking analysis (NTA) techniques, respectively. Zeta potential and surface tension of aqueous solutions were measured at different time and aeration rates. The results showed that aeration rate and preparation time had an important role in the properties of BNBs (concentration, bubble size, and surface charge) and BMBs (number, and bubble size). The instability of BMBs led to the rapid changes in the dissolved oxygen (DO) content in the water. The number of BMBs decreased when preparation time and aeration rate increased, but their size remained constant. By enhancing the preparation time and aeration rate, the concentration of BNBs improved first and then reduced. Additionally, the surface tension of an aqueous solution containing BNBs was significantly lower than that of pure water.

Development of quantitative and concise measurement method of oxygen in fine bubble dispersion

Fine bubbles (FBs) have attracted significant attention in several research fields. Although some reports have argued that FB dispersion is useful as an oxygen (gas) carrier, only a few reports have examined its properties as an oxygen carrier using experimental data. As one of the reasons for this, there are no standard methods for measuring the oxygen content in FB dispersions. Conventional oxygen measurement methods have certain drawbacks in accuracy or speed; thus, it is difficult to use oxygen content as the primary outcome. In this study, we introduce a Clark-type polarographic oxygen electrode device (OXYG1-PLUS) for oxygen measurement, allowing the dilution of FB dispersion without the influence of ambient air and the adhesion of FBs on the electrode surface due to its special shape. First, the accuracy of our dilution method was evaluated using pure water as a sample, and it was confirmed that our method could measure with an accuracy of ±0.5 mg/L from the results with conventional dissolved oxygen meters. Second, the oxygen content in FB dispersion was evaluated with our method and a chemical titration method (Winkler’s method), and it was found that our method could measure the oxygen content in FB dispersions quantitively. This method satisfies the easiness (4 steps) and quickness (within 8 min) for a wide range of oxygen contents (0 to 332 mg/L, theoretical range) with low coefficient variation (< 4.7%) and requires a small sample volume (50–500 μL); thus, it is a useful method for measuring the oxygen in FB dispersions.

Nanobubble-assisted scaling inhibition in membrane distillation for the treatment of high-salinity brine

In this study, we report the use of nanobubbles (NBs) as a simple and facile approach to effectively delay scaling in membrane distillation (MD) during the treatment of highly saline feed (100 g L-1). Unlike conventional gas bubbling in MD for improving the hydrodynamic flow conditions in the feed channel, here we generated air NBs with an average size of 128.81 nm in the feed stream and examined their impact on membrane scaling inhibition during MD operation. Due to their small size, neutral buoyancy, and negative surface charge, NBs remain in suspension for a longer time (14 days), providing homogenous mixing throughout the entire feed water. The MD performance results revealed that severe membrane scaling happened during the DCMD treatment of high salinity brine in the absence of nanobubbles, which dramatically reduced the distillate flux to zero after 13 h. A one-time addition of air NBs in the saline feed significantly reduced salt precipitation and crystal deposition on the PVDF membrane surface, delayed the occurrence of flux decline, prevented membrane wetting, thereby prolonging the effective MD operating time. With similar feed concentration and operating conditions, only 63% flux decline after 98 h operation was recorded in nanobubble-assisted MD. Two key explanations were suggested for the delayed membrane scaling upon addition of air NBs in the MD feed: (1) NB-induced turbulent flow in the feed channel that increases the surface shear forces at the membrane surface, alleviating both temperature and concentration polarization effect, (2) electrostatic attractions of the counterions to the negatively charged NBs, which reduces the availability of these ions in the bulk feed for scale formation.

Generation and characterization of nanobubbles in ionic liquid for a green extraction of polyphenols from Carya cathayensis Sarg

Nanobubbles (NBs) generated-nanojets membrane poration have gained enormous attention. In this study, NBs were fabricated as a novel green approach to assist ionic liquid (IL) [C₄C₁im][BF₄] extraction of polyphenols from Carya cathayensis Sarg. husk. NBs were successfully generated with mean size of 85.47 ± 5 nm, zeta potential of +39 ± 2.24 mV, and concentration of 21.15 ± 0.75 × 10⁸ particles/mL (stable for over 48 h in IL solution). Compared to common solutions extract, IL-NBs extract showed significantly higher (p < 0.05) antioxidant activity and polyphenols yields with a total polyphenol, total flavonoid, and total tannins contents of 85.67 ± 2.05 mg GAE/g DW, 42.44 ± 1.17 mg CE/g DW, and 8.2 ± 0.05 mg TAE/g DW, respectively. The SEM results confirmed that NBs' nanojets caused morphological destruction of the husk powder. Overall, IL-NBs solution showed better extraction efficiency of polyphenols than other solutions, giving insight into a new "green" nanotechnology-based extraction method.

Ion adsorption stabilizes bulk nanobubbles

The mechanism leading to the extraordinary stability of bulk nanobubbles in aqueous solutions remains an outstanding problem in soft matter, modern surface science, and physical chemistry science. In this work, the stability of bulk nanobubbles in electrolyte solutions under different pH levels and ionic strengths is studied. Nanobubbles are generated via the technique of ultrasonic cavitation, and characterized for size, number concentration and zeta potential under ambient conditions. Experimental results show that nanobubbles can survive in both acidic and basic solutions with pH values far away from the isoelectric point. We attribute the enhanced stability with increasing acidity or alkalinity of the aqueous solutions to the effective accumulation of net charges, regardless of their sign. The kinetic stability of the nanobubbles in various aqueous solutions is evaluated within the classic DLVO framework. Further, by combining a modified Poisson-Boltzmann equation with a modified Langmuir adsorption model, we describe a simple model that captures the influence of ion species and bulk concentration and reproduce the dependence of the nanobubble's surface potential on pH. We also discuss the apparent contradiction between quantitative calculation by ion stabilization model and experimental results. This essentially requires insight into the structure and dynamics of interfacial water on the atomic-scale.

Comment on “Can bulk nanobubbles be stabilized by electrostatic interaction?” by S. Wang, L. Zhou and Y. Gao, Phys. Chem. Chem. Phys., 2021, , 16501

It is shown that electrostatic pressure arising from the repulsion of ions in the Stern layer around space charge of bulk nanobubbles is the main factor of their stability.

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.

Utilization of Ultrafine Gas Bubbles to Investigate the Jones-Ray Effect of Diluted Salt Solutions

The cause of the Jones-Ray effect has been controversially debated for years. Ultrafine gas bubbles were employed to lessen the surface excess of the surface-active impurities adsorbing to the air/water interface of the salt solutions, which would lead to a direct shift in surface tension observable by the Wilhelmy plate method. It was concluded in this study that once the surface excess of the inevitable impurities in the salts is lessened by the introduction of ultrafine gas bubbles, which possess great air/water interfacial area, the Jones-Ray effect becomes nonobservable. Therefore, our finding hypothesized that the Jones-Ray effect might not originate from salts.

Nanobubble boundary layer thickness quantified by solvent relaxation NMR

HYPOTHESIS

The boundary layer holds the key to solve the puzzle of the unusual stability of the nanobubbles in solution. The quantitative determination on its mechanical and structural properties has not been achieved due to its diffusive and dynamic nature, lack of distinctive interfaces, and difficult differentiation from bulk background. Therefore, it is necessary to investigate this boundary using more sensitive interface analysis technologies to effectively differentiate the water molecules at the interface from those in the bulk.

EXPERIMENTS

An in-situ and non-deconstructive method, solvent relaxation nuclear magnetic resonance, was used to investigate the boundary layer on bulk nanobubbles, where the relaxation rate of the water in the layer and its thickness were measured by solvent relaxation NMR and the ratio between the water molecules at the bubble interfaces and those in the bulk and the corresponding boundary layer thickness were determined.

FINDINGS

The spin-spin relaxation time for the water in the layer (∼10¹ms) is found to be two orders of magnitude lower than that of the free water (∼10³ms). As the first attempt, the determined boundary layer thickness is around 35-45 nm and 17.0 %-8.7 % of the effective gaseous size of the nanobubbles, which increases with the decrease of the bubble diameter. As a result, a quantitative measurement model for bubble boundary layer has been established in order to better understand the interfacial properties and stabilization mechanism for bulk nanobubbles.

Novel analytical expressions for determining van der Waals interaction between a particle and air-water interface: Unexpected stronger van der Waals force than capillary force

HYPOTHESIS

Analytical expressions for calculating Hamaker constant (HC) and van der Waals (VDW) energy/force for interaction of a particle with a solid water interface has been reported for over eighty years. This work further developed novel analytical expressions and numerical approaches for determining HC and VDW interaction energy/force for the particle approaching and penetrating air-water interface (AWI), respectively.

METHODS

The expressions of HC and VDW interaction energy/force before penetrating were developed through analysis of the variation in free energy of the interaction system with bringing the particle from infinity to the vicinity of the AWI. The surface element integration (SEI) technique was modified to calculate VDW energy/force after penetrating.

FINDINGS

We explain why repulsive VDW energy exists inhibiting the particle from approaching the AWI. We found very significant VDW repulsion for a particle at a concave AWI after penetration, which can even exceed the capillary force and cause strong retention in water films on a solid surface and at air-water-solid interface line. The methods and findings of this work are critical to quantification and understanding of a variety of engineered processes such as particle manipulation (e.g., bubble flotation, Pickering emulsion, and particle laden interfaces).

Review of Oxygenation with Nanobubbles: Possible Treatment for Hypoxic COVID-19 Patients

The coronavirus disease (COVID-19) pandemic, which has spread around the world, caused the death of many affected patients, partly because of the lack of oxygen arising from impaired respiration or blood circulation. Thus, maintaining an appropriate level of oxygen in the patients' blood by devising alternatives to ventilator systems is a top priority goal for clinicians. The present review highlights the ever-increasing application of nanobubbles (NBs), miniature gaseous vesicles, for the oxygenation of hypoxic patients. Oxygen-containing NBs can exert a range of beneficial physiologic and pharmacologic effects that include tissue oxygenation, as well as tissue repair mechanisms, antiinflammatory properties, and antibacterial activity. In this review, we provide a comprehensive survey of the application of oxygen-containing NBs, with a primary focus on the development of intravenous platforms. The multimodal functions of oxygen-carrying NBs, including antimicrobial, antiinflammatory, drug carrying, and the promotion of wound healing are discussed, including the benefits and challenges of using NBs as a treatment for patients with acute hypoxemic respiratory failure, particularly due to COVID-19.

Ultrafine bubble water mitigates plant growth in damaged soil

Water containing ultrafine/nano bubbles (UFBs) promoted the growth of tomato (Solanum lycopersicum) in soil damaged by cultivation of tomato in the previous year or bacterial wilt-like disease and also promoted the growth of lettuce (Lactuca sativa) when lettuce was grown in the soil damaged by repeated cultivation of lettuce. On the other hand, UFB supply did not affect plant growth in rock wool or healthy soil. Furthermore, the growth of lettuce was not affected by UFB water treatment in the soil damaged by the cultivation of tomato. UFB water partly suppressed the growth of the pathogen of bacteria wilt disease, Ralstonia solanacearum in vitro. These data suggest that UFB water is effective to recover the plant growth from soil damage.

Dynamic interplay between interfacial nanobubbles: oversaturation promotes anisotropic depinning and bubble coalescence

Probing the dynamics of nanobubbles is essential to understand their longevity and behavior. Importantly, such an observation requires tools and techniques having high temporal resolutions to capture the intrinsic characteristics of the nanobubbles. In this work, we have used the in situ liquid-phase electron microscopy (LPEM) technique to gain insights into nanobubbles' behavior and their interfacial dynamics. Interestingly, we could observe a freely growing-shrinking nanobubble and a pinned nanobubble under the same experimental conditions, suggesting the possibility of multiple nanobubble stabilization theories and pathways. Remarkably, the study reveals that a freely growing-shrinking nanobubble induces anisotropic depinning in the three-phase contact line of a strongly pinned neighboring nanobubble. The anisotropic depinning is attributed to the differential local gas saturation levels, depending on the relative positioning of the freely growing-shrinking nanobubble. Furthermore, we also observed a unique pull-push phenomenon exhibited by the nanobubble's interfaces, which is attributed to the van der Waals interactions and the electric double layer collectively. The role of the electric double layer in suppressing and delaying the merging is also highlighted in this study. The present work aims to reveal the role of locally varying gas saturation in the depinning of nanobubbles, their longevity due to the electric double layer, and the consequent coalescence, which is crucial to understand the behavior of the nanobubbles. Our findings will essentially contribute to the understanding of these novel nanoscale gaseous domains and their dynamics.

Generation of Bulk Nanobubbles by Self-Developed Venturi-Type Circulation Hydrodynamic Cavitation Device

Bulk nanobubbles (BNBs) have attracted substantial interest from academia and industry owing to their peculiar properties and extensive potential applications. However, a scalable engineering method needs to be developed. Herein, we developed a nanobubble generator based on venturi-type recirculating hydrodynamic cavitation. The existence of nanobubbles produced by our generator was confirmed using physicochemical test methods, including the Tyndall effect, multiple freeze-thaw degassing experiments, and trace metal analysis. Subsequently, the effects of different operating parameters (circulation time and operating pressure) on bulk nanobubble production and properties, as well as their stability, were investigated. The results suggest that the characteristics of BNBs varied with the circulation time (5-20 min) and operating pressure (2-5 bar). However, all the particle size distribution of BNBs had a bimodal distribution with a mean diameter of 180-210 nm for the different circulation time and operating pressures. For example, by increasing the circulation time from 5 to 20 min, the peak value of size distribution decreased from 333/122 nm to 218/52 nm, and the average sample scattering signal count rate (Avg. Count Rate) increased from 133 to 303 Kcps. The evaluation of the stability of the BNBs formed for the circulation time of 15 min and the operating pressure of 3 bar showed that they could continue existence and stability in the suspension for 72 h. The study results might provide a valuable method for further investigation of industrial applications of venturi-type nanobubble generators.

Biomedical nanobubbles and opportunities for microfluidics

The use of bulk nanobubbles in biomedicine is increasing in recent years, which is attributable to the array of therapeutic and diagnostic tools promised by developing bulk nanobubble technologies. From cancer drug delivery and ultrasound contrast enhancement to malaria detection and the diagnosis of acute donor tissue rejection, the potential applications of bulk nanobubbles are broad and diverse. Developing these technologies to the point of clinical use may significantly impact the quality of patient care. This review compiles and summarizes a representative collection of the current applications, fabrication techniques, and characterization methods of bulk nanobubbles in biomedicine. Current state-of-the-art generation methods are not designed to create nanobubbles of high concentration and low polydispersity, both characteristics of which are important for several bulk nanobubble applications. To date, microfluidics has not been widely considered as a tool for generating nanobubbles, even though the small-scale precision and real-time control offered by microfluidics may overcome the challenges mentioned above. We suggest possible uses of microfluidics for improving the quality of bulk nanobubble populations and propose ways of leveraging existing microfluidic technologies, such as organ-on-a-chip platforms, to expand the experimental toolbox of researchers working to develop biomedical nanobubbles.

The use of bulk nanobubbles in biomedicine is increasing in recent years. This translates into new opportunities for microfluidics, which may enable the generation of higher quality nanobubbles that lead to advances in diagnostics and therapeutics.

From Bubbles to Nanobubbles

Nanobubbles are classified into surface and bulk. The main difference between them is that the former is immobile, whereas the latter is mobile. The existence of sNBs has already been proven by atomic force microscopy, but the existence of bNBs is still open to discussion; there are strong indications, however, of its existence. The longevity of NBs is a long-standing problem. Theories as to the stability of sNBs reside on their immobile nature, whereas for bNBs, the landscape is not clear at the moment. In this preliminary communication, we explore the possibility of stabilizing a bNB by Brownian motion. It is shown that a fractal walk under specific conditions may leave the size of the bubble invariant.

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.

Contrast Variation Small-Angle Neutron Scattering Study of Solubilization of Perfumes in Cationic Surfactant Micelles

Perfume solubilization is an important process in the production of commercial products such as beverages, foods, and cosmetics. In the present study, small-angle neutron scattering (SANS) experiments were performed to investigate the solubilization behavior of perfumes in cetyltrimethylammonium bromide (CTAB) micelles. The solubilization of linalool (LL) and l-menthol (MT), which are relatively hydrophilic perfumes, did not change the size of the CTAB micelles although the perfumes were incorporated in the micelles, as indicated by a decrease in scattering length density. On the other hand, the solubilization of d-limonene (LN), a hydrophobic perfume, led to the swelling of CTAB micelles. An internal contrast variation SANS study was performed by the deuteration of CTAB molecules to directly observe the perfumes in the micelles. The radius of d-CTAB micelles solubilizing LL or MT corresponds to that of h-CTAB, which indicates that these perfumes are accommodated in the palisade layers of the micelles and are homogeneously distributed in the micelles. On the other hand, LN formed small droplets, as indicated by the SANS profile, which implies the solubilization of LN molecules in the core of the CTAB micelles. We found that the relatively hydrophilic perfumes (LL and MT) show less impact on the sizes of the cationic micelles in comparison to nonionic micelles. Thus, the internal contrast variation method of SANS allowed the direct observation of the solubilization sites of perfumes with different hydrophilicity-hydrophobicity balances. This method is a powerful tool to determine the solubilization states that affect the solubilization capacity, volatilization, or release speed of perfumes.

Micellization Transformations of Sodium Oleate Induced by Gas Nucleation

The interfacial properties of surfactant solutions are closely related to the micellization of surfactants. Temperature, salt type and concentration, pH, and other parameters affecting the micellization of surfactants have all been extensively investigated previously. However, the effect of dissolved gas on surfactant micellization and associated interfacial properties' transformations is not completely understood yet. In this study, sodium oleate (NaOl) was chosen as the research object, and the role of gas/gas nucleation in NaOl micellization was systematically investigated. The results indicated that the solution changed to be more turbid and the dissolved oxygen content increased after NaOl solutions were subjected to compression-decompression treatments. Meanwhile, the surface tension of the NaOl solution was altered, which was more pronounced when the concentration of NaOl was close to the critical micelle concentration. Given that the surface tension was a good indicator of the assembly and distribution state of the soluble monomers and insoluble micelles of NaOl, interactions between nucleated bubbles originating from the gas nucleation and NaOl molecules were unveiled through the analysis of the size distribution and zeta potential of sub-micro- and nanoscale particles in bulk solutions. Finally, possible micellization models of NaOl molecules, fully considering the role of gas/gas nucleation, were proposed under varying NaOl concentration conditions.

Can bulk nanobubbles be stabilized by electrostatic interaction?

It has been suggested that electrostatic stress arising from charges accumulated at the surface of nanobubbles might balance Laplace pressure leading to their stability. This mechanism has been widely discussed in the nanobubble field for the past decade. However, the stress in the diffusive double layer was overlooked when calculating the electrostatic effect in previous theories. In this communication, we recalculated this effect using the classical double layer theory. Combined with experimentally measured zeta potential, we find that the ratio of electrostatic pressure to Laplace pressure is much less than 10-2, which suggests that electrostatic interaction may not be the main factor for stabilizing bulk nanobubbles.

Bactericidal Activity of Bulk Nanobubbles through Active Oxygen Species Generation

We investigated the bactericidal activity of bulk nanobubbles (NBs) using E. coli, a model bacterium. Bulk NBs were produced by forcing gas through a porous alumina membrane with an ordered arrangement of nanoscale straight holes in contact with water. NBs with different gas contents, including CO2, O2, and N2, were generated and evaluated for their bactericidal effects. The survival rate of E. coli was significantly reduced in a suspension of CO2-containing NB (CO2-NB water). The N2-NB water demonstrated a small amount of bactericidal behavior, but its impact was not as significant as that of CO2-NB water. When E. coli was retained in O2-NB water, the survival rate was even higher than that in pure water (PW). We investigated the generation of reactive oxygen species (ROS) in NB suspensions by electron spin resonance spectroscopy. The main ROS generated in the NB water were hydroxyl radicals and OH·, and the production of ROS was the strongest in CO2-NB water, which was consistent with the results of the bactericidal effect measurements. We assumed that NB mediated by ROS would exhibit bactericidal behavior and proposed a kinetic model to explain the retention time variation of the survival rate. The results calculated based on the proposed model matched closely with the experimental results.

The effect of ultrasound on bulk and surface nanobubbles: A review of the current status

The generation, and stability of nanobubbles are of particular interest for fundamental research and have potential application in numerous fields. Several attempts were made in the literature to produce nanobubbles through acoustic cavitation. However, the generation and stability mechanisms of nanobubbles in the acoustic field are unclear. Here, we review the effect of ultrasound parameters on bulk nanobubbles and surface nanobubbles. On this basis, we discuss the proposed generation and stability mechanisms of nanobubbles from the perspective of transient and stable acoustic cavitation. Moreover, we propose some future research directions for a deeper understanding of the role of ultrasound in the generation and stability of nanobubbles.

Influence of bulk nanobubble concentration on the intensity of sonoluminescence

The present study mainly examined the effects of the volumetric concentration of nanobubbles (ultrafine bubbles) on the intensity of sonoluminescence (SL). The addition of nanobubbles at high acoustic amplitude enhanced the SL intensity for various bubble concentrations in comparison with that in pure water. This probably means that the resulting high amplitude is over the Blake threshold, and accordingly nanobubbles expand to some extent, leading to higher SL intensity. Therefore, nanobubbles have the potential to provide nucleation sites for sonochemistry. The influence of bubble size on the intensity of SL was also evaluated.

Stimuli-responsive nanobubbles for biomedical applications

Stimuli-responsive nanobubbles have received increased attention for their application in spatial and temporal resolution of diagnostic techniques and therapies, particularly in multiple imaging methods, and they thus have significant potential for applications in the field of biomedicine. This review presents an overview of the recent advances in the development of stimuli-responsive nanobubbles and their novel applications. Properties of both internal- and external-stimuli responsive nanobubbles are highlighted and discussed considering the potential features required for biomedical applications. Furthermore, the methods used for synthesis and characterization of nanobubbles are outlined. Finally, novel biomedical applications are proposed alongside the advantages and shortcomings inherent to stimuli-responsive nanobubbles.

Effect of electrolytes and surfactants on generation and longevity of carbon dioxide nanobubbles

The objective of this research was to investigate the effect of individual additions of mono- and divalent electrolytes (NaCl and CaCl₂), anionic (sodium dodecyl sulfate, SDS) and non-ionic surfactants (polysorbate 80, Tween 80) at varied concentrations on the generation and stability of bulk nanobubbles (NBs) from carbon dioxide (CO₂) gas in aqueous system. Overall, NBs generated in the small-amount salt fluids exhibited significantly (p ≤ 0.05) lower size range (150-350 nm). Smaller diameter and higher zeta potential magnitudes (18-24 mV) of the NBs in SDS medium were also observed and related to the higher CO₂ concentration (~1850 ppm) and lower surface tension (~64 mN/m) of the solution. However, the gas NBs were disappeared with the incorporation of Tween 80. The outcomes provide some more research-based details about the impact of potential nano-bubble stabilising agents on characteristics of NBs contributing to the green and sustainable NB-related applications in food sectors.

On the clustering of bulk nanobubbles and their colloidal stability

Bulk nanobubbles which are usually observed in pure water have a mean diameter typically around 100 nm. We use a combination of physical and chemical techniques to prove the hypothesis that the nanoentities observed in pure water are stable clusters of much smaller stable nanobubbles. The stability of bulk nanobubble clusters is affected by factors such as ionic strength or internal energy of the system. We show that bulk nanobubbles on the order of 100 nm exist in a stable cluster form in neutral or basic media, and dissociate into tiny primary nanobubbles on the order of 1 nm in acidic media, or in the presence of small amounts of salt. These new findings suggest that bulk nanobubbles which have a high surface energy unsurprisingly tend to behave in a similar manner to solid nanoparticles in terms of their agglomeration tendency, which is confirmed by the DLVO theory. The results will have important implications for our understanding and interpretation of the behaviour of bulk nanobubbles, in particular their interfacial and colloidal stability.

Long-Term Stability of Different Kinds of Gas Nanobubbles in Deionized and Salt Water

Nanobubbles have many potential applications depending on their types. The long-term stability of different gas nanobubbles is necessary to be studied considering their applications. In the present study, five kinds of nanobubbles (N₂, O₂, Ar + 8%H₂, air and CO₂) in deionized water and a salt aqueous solution were prepared by the hydrodynamic cavitation method. The mean size and zeta potential of the nanobubbles were measured by a light scattering system, while the pH and Eh of the nanobubble suspensions were measured as a function of time. The nanobubble stability was predicted and discussed by the total potential energies between two bubbles by the extended Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. The nanobubbles, except CO₂, in deionized water showed a long-term stability for 60 days, while they were not stable in the 1 mM (milli mol/L) salt aqueous solution. During the 60 days, the bubble size gradually increased and decreased in deionized water. This size change was discussed by the Ostwald ripening effect coupled with the bubble interaction evaluated by the extended DLVO theory. On the other hand, CO₂ nanobubbles in deionized water were not stable and disappeared after 5 days, while the CO₂ nanobubbles in 1 mM of NaCl and CaCl₂ aqueous solution became stable for 2 weeks. The floating and disappearing phenomena of nanobubbles were estimated and discussed by calculating the relationship between the terminal velocity of the floating bubble and bubble size.

Acoustic Measurements of Nucleus Size Distribution at the Cavitation Threshold

An understanding of the acoustic cavitation threshold is essential for minimizing cavitation bio-effects in diagnostic ultrasound and for controlling cavitation-mediated tissue ablation in focused ultrasound procedures. The homogeneous cavitation threshold is an intrinsic material property of recognized importance to biomedical ultrasound as well as a variety of other applications requiring cavitation control. However, measurements of the acoustic cavitation threshold in water differ from those predicted by classic nucleation theories. This persistent discrepancy is explained by combining recently developed methods for acoustically nucleating single bubbles at threshold with numerical modeling to obtain a nucleus size distribution consistent with first-principles estimates for ion-stabilized nuclei. We identify acoustic cavitation at threshold as a reproducible subtype of heterogeneous cavitation with a characteristic nucleus size distribution. Knowledge of the nucleus size distribution could inspire new approaches to achieving cavitation control in water, tissue and a variety of other media.

Metastable Nanobubbles

Water containing suspended nanobubbles is utilized in various applications. The observed lifetime of suspended nanobubbles is several weeks, whereas, according to the classical theory of bubble stability, a nanosized bubble should dissolve within microseconds. Explanations for the longevity of nanosized bubbles have been proposed but none of them has gained general acceptance. In this study, we derive an explanation for the existence of metastable nanobubbles solely from the thermodynamic principles. According to our analysis, the dissolution of nanosized aqueous bulk bubbles is nonspontaneous below 180 nm diameter due to the energy requirement of gas dissolution. Hydrophobic surfaces have a further stabilizing effect, and the dissolution becomes nonspontaneous in surface nanobubbles having a diameter below 600 nm.

Destabilization of ultrafine bubbles in water using indirect ultrasonic irradiation

Ultrafine bubble (UFB) is a bubble with a diameter of less than 1 μm. Little attention has been paid to the defoaming and removal of UFBs. This study proposes a method to destabilize UFBs by using indirect ultrasonic irradiation. Besides, the destabilization mechanism of UFB was investigated. The ultrasonic frequency was 1.6 MHz and the dissipated power was 30 W. UFB dispersions were prepared using two different types of bubble generators: pressurized dissolution method and swirling liquid flow method. The effects of ultrasonic irradiation on the stability of UFBs were evaluated by particle tracking analysis (PTA) and electrophoretic zeta potential measurement. Results showed that the indirect ultrasonic irradiation for 30 min reduced the number concentration of UFBs by 90% regardless of the generation method. This destabilization was attributed to a decrease in the magnitude of zeta potential of UFBs due to the changes in pH and electrical conductivity. These changes in the electrochemical properties were caused by the formation of nitric acid. To study the destabilization mechanism, the pH of the UFB dispersions were modified by titration; the chemical and mechanical effects of ultrasound were separately examined. It was found that not only the chemical effect caused by the formation of nitric acid but also the mechanical effect contributed to the destabilization of UFB. Feasibility studies were also performed for UFBs in an aqueous surfactant solution and UFBs in a solid particle dispersion. The proposed method selectively destabilized UFBs in the solutions.

Interaction of Bulk Nanobubbles (Ultrafine Bubbles) with a Solid Surface

The experimental results [Kanematsu, W. Chem. Eng. Sci. 2020, 219, 115594] on the temporal variations of number concentrations of bulk nanobubbles (ultrafine bubbles) in contact with polymer materials are theoretically analyzed based on the dynamic equilibrium model of bulk nanobubbles partly covered with hydrophobic materials (impurities). It is suggested that bulk nanobubbles are adsorbed on a polymer surface by attractive hydrophobic interaction between a polymer surface and a hydrophobic material partly covering the bubble surface, overcoming the repulsive double-layer interaction. There are two mysteries. One is that the maximum surface number concentration of bulk nanobubbles of about 70 nm in diameter adsorbed on a hydrophobic polymer surface is more than an order of magnitude lower than the typical value for colloid particles of a similar or larger size. The other is that the experimental adsorption rate of bulk nanobubbles on hydrophobic polymer surface is several orders of magnitude lower than the theoretically estimated one. The mysteries are resolved if many of the bulk nanobubbles adsorbed on a hydrophobic polymer surface change to surface nanobubbles with a footprint diameter of about 1 μm.

Generating Bulk Nanobubbles in Alcohol Systems

Bulk nanobubbles (NBs) have attracted wide attention due to their peculiar physicochemical properties and great potential in applications in various fields. However, so far there are no reports on bulk NBs generated in pure organic systems, which we think is very important as NBs would largely improve the efficiency of gas-liquid mass transfer and facilitate chemical reactions to take place. In this paper, we verified that air and N₂ NBs could be generated in a series of alcohol solutions by using various methods including acoustical cavitation, pressurization-depressurization, and vibration. The experiments proved that NBs existed in alcohol solutions, with a highest density of 5.8 × 10⁷ bubble/mL in propanol. Our results also indicated that bulk NBs could stably exist for at least hours in alcohol systems. The parameters in generating NBs in alcohols were optimized. Our findings open up an opportunity for improving gas-liquid mass transfer efficiency in the field of the chemical industry.

Response to "Comment on Bulk Nanobubbles or Not Nanobubbles: That is the Question"

Advanced techniques that combine high spatial resolution with chemical sensitivity to directly probe the observed nanoentities and provide direct evidence that they are truly gas-filled nanobubbles do not exist. Therefore, in our paper, we focused on providing, for the first time, multiple types of indirect evidence using a variety of physical and chemical techniques that the nanoentities are not due to contamination and, hence, they must be bulk nanobubbles (BNBs). It should be noted that such techniques require good experimental skills, sound protocols, good scientific expertise, and reliable equipment. While no single piece of indirect evidence on its own can be considered as conclusive proof, we estimate that our results combined provide strong evidence that bulk nanobubbles do exist and they are stable. The work presented in our paper is the culmination of a series of studies, and many authors have either directly or indirectly confirmed our findings. Nonetheless, in their Comment, Rak & Sedlak reject all of the work we reported. We here address their comments point by point and show that their criticisms are unwarranted and unfounded, as follows.

Concentration and Dilution of Ultrafine Bubbles in Water

Submicron-sized bubbles are now officially called ultrafine bubbles (UFBs) by the international standard. The concentration of UFBs is generally low (<109 particles/mL; <0.001 vol%) compared to other colloidal dispersions. To overcome this practical problem, we concentrated UFBs in ultrapure water prepared by a commercial UFB generator and quantified the effect of rotary evaporation of the dispersion media on the stability of UFBs. The UFB dispersions were characterized by a particle tracking analysis (PTA) instrument. The experimental results showed that the UFBs can be diluted and concentrated without changing the size distribution and there was little or no loss of UFBs. By using a rotary evaporator, UFB dispersions were about 30-fold concentrated and the resultant number concentration reached over 3 × 1010 particles/mL. Increasing the concentration of UFBs allowed for satisfactory dynamic light scattering (DLS) measurements. The differences among the three algorithms for analyzing the raw data, i.e., autocorrelation function, obtained by DLS are discussed, along with the characteristics of the particle size distribution derived from each algorithm.

Hydroxyl ion stabilization of bulk nanobubbles resulting from microbubble shrinkage

HYPOTHESIS

The shrinkage of microbubbles that are less than about 50 µm in diameter is a well-known phenomenon that results from the surface tension. It has also been shown recently that hydroxyl ions have an extremely strong affinity for gas-water interfaces including bubble surfaces. A theoretical model is proposed that predicts bulk nanobubble stability in water, based on a force balance that results from the shrinkage of microbubbles. This model was designed to test the hypothesis that the surface tension of a shrinking microbubble can ultimately be balanced by the repulsion of the hydroxyl ions that initially adsorb onto the microbubble surface prior to shrinking.

THEORY

The present model considers the forces due to ionic repulsion as microbubbles shrink under the surface tension. No special assumptions are required in the present model other than the recently reported strong affinity hydroxyl ions have to gas/water interfaces. The Debye-Hückel theory was used to determine the number of ions on the microbubble surface.

FINDINGS

The results of this model predict a stable balance between the surface tension and the electrostatic repulsion of hydroxyl ions for nanobubble diameters less than 1100 nm. This predicted maximum in nanobubble size is shown to be consistent with experimental findings.

On the thermodynamic stability of bubbles, immiscible droplets, and cavities

Nanobubbles filled with air or a variety of pure gases are observed to persist in bulk water for weeks and months. Nanoemulsions consisting of oil droplets in water are also remarkably stable against coagulation, with lifetimes up to weeks even if not coated with surfactants. The inverse system of nanodroplets of water in oil is also accessible for study and application. Voids on the nanoscale are formed in simulations of water under strong tension and are stable during the time of the simulation. The stability of these nano-entities is ultimately determined by the molecular-level structure of their interfaces. However, a thermodynamic theory might also be capable of providing some insight. We therefore consider spherical gas nanobubbles, immiscible liquid nanodroplets, and nanocavities formed in water under negative pressure on the same footing, and give a unified thermodynamic analysis of these systems. In all cases, mechanical equilibrium (local free energy maximum or minimum) is expressed by the Laplace equation, and thermodynamic stability (local free energy minimum) follows from the radius dependence of surface tension. All of them would be unstable if their surface tensions were constant. Data from the literature allow construction of numerical examples for cavities and gas nanobubbles. Spectroscopic data are cited in support of an interfacial water structure in gas nanobubbles and water droplets in oil that differ from their flat surface counterparts. The observed longevity of nanobubbles in particular has been thought to violate fundamental principles of diffusion and solubility. A close look at the Laplace equation and its derivation shows why this widespread belief is incorrect.

A Henry's law method for generating bulk nanobubbles

A new technique for generating bulk nanobubble suspensions has been developed based on Henry's law which states that the amount of dissolved gas in a liquid is proportional to its partial pressure above the liquid. This principle which forms the basis of vacuum degasification has been exploited here to produce stable bulk nanobubbles in excess of 10⁹ bubble mL^-1 in pure water, through successive expansion/compression strokes inside a sealed syringe. We provide evidence that the observed nano-entities must be gas-filled nanobubbles by showing that: (i) they cannot be attributed to organic or inorganic impurities; (ii) they disappear gradually over time whilst their mean size remains unchanged; (iii) their number density depends on the concentration of dissolved gas in water and its solubility; and (iv) added sparging of gas enhances process yield. We study the properties of these nanobubbles including the effects of type of dissolved gas, water pH and the presence of different valence salts on their number density and stability. Given the potential of the technique for large scale production of nanobubble suspensions, we describe a successfully tested automated model and outline the basis for process scale-up.