Effect of NaCl on the Lifetime of Micro- and Nanobubbles

Indirect catalytic oxidation of multi-pollutants in diesel exhaust by ozone/micro-nano bubbles system

Wet oxidation absorption is an efficient and promising method of exhaust gas treatment. When the micro-nano bubbles collapse, they can generate hydroxyl radicals with strong oxidising ability, which can oxidise a variety of pollutants in diesel exhaust. Ozone has strong oxidising properties, and the coupling of ozone and micro-nano bubbles can improve the oxidation and removal effects of polluted gases. In this study, the ozone micro-nano bubbles system was used to oxidise NOx, SO2, and CO to gases that were more readily dissolved in water, such as NO2, SO3, and CO2, and the gases were removed through the absorbent solution. The effects of surfactant, catalyst, urea, pH value, and salinity on the removal efficiency of NOx, SO2, and CO from diesel exhaust were investigated. Through experiments, it was found that the removal efficiency of pollutants was enhanced and then weakened with the increasing concentrations of surfactants, catalysts, and salinity, and continued to decrease with increasing concentrations of urea. When the pH value was < 7, the removal efficiency increased first and then weakened with the increase of the pH value. When the pH value was > 7, it mainly depended on the absorption of acid gas by the alkali solution. Under optimal conditions, the removal efficiencies were 86.3% for NO, 92.1% for SO2, and 65.4% for CO. This study could provide important theoretical support for the industrial application of this technology.

Supersaturation dependent nucleation of methane + propane mixed-gas hydrate

Before hydrates can be widely used in industry, we should better understand the problematic issues of hydrate nucleation, particularly its stochastic nature. Here, we report on measurements of the nucleation probability of mixed-gas hydrates in which the guest molecules are a mixture of methane and propane. For the pure cases, at a supersaturation near 1.0, we had previously measured an induction time for the methane hydrate of about 1 h, whereas for the propane hydrate, it was over one day. Using the same experimental setup, we examine here the nucleation probability for a mixture of 90% methane and 10% propane as the guest gas for a range of supersaturations. For the experiments, the temperature was 274 ± 0.5 K and the stirring rate was about 300 rpm. The experiments were repeated at least ten times under the same condition, exchanging the sample water every time. We define the nucleation probability at a given time as the fraction of trials that nucleated by that time and then determine the nucleation probability distribution. The resulting nucleation frequency is found to have a power-law relation to supersaturation. Then, we examine how the nucleation frequency is affected by the existence of ultrafine bubbles in the initial water. We find that the ultrafine bubbles increase the nucleation frequency but much less than that of typical changes in supersaturation.

Evolution of Bulk Nanobubbles under Different Freezing Conditions

The freezing process of aqueous solutions plays a crucial role in various applications including cryopreservation, glaciers, and frozen materials. However, less research has focused on the influence of nanoscale gas bubble formation or collapse in water during freezing, which may significantly impact the formation of ice crystals. Herein, we report for the first time that the freezing process can produce nanobubbles in aqueous solutions, and their size and number concentration could be changed by different cooling rates, i.e., the size would decrease as the cooling rate increased, and the maximum number concentration was found at the -80 °C system. Furthermore, increasing the dissolved gas content in the solution enhanced the production of nanobubbles, whereas for preexisting nanobubbles, the freezing resulted in a decrease in their number concentration, which was negatively correlated with the cooling rate. Our results indicated that a moderate cooling rate of -80 °C favored nanobubble generation, whereas a higher cooling rate was preferable for maintaining preexisting nanobubbles. Conversely, a lower cooling rate could be employed to eliminate preexisting nanobubbles. This study explored the evolution and stability of nanobubbles during the freezing process, providing valuable insights into the application or elimination of nanobubbles.

Additive-Free Method for Enhancing the Volume Phase Transition Rate in Light-Responsive Hydrogels: A Study of Micro-Nano Bubble Water on PNIPAM-co-AAc Hydrogels

Light-responsive hydrogels containing light-thermal convertible pigments have received interest for their possible applications in light-responsive shutters, valves, drug delivery systems, etc. However, their utility is limited by the slow response time. In this study, we investigated the use of micro-nano bubble water as a preparation solvent to accelerate the volume phase transition kinetics of poly(N-isopropylacrylamide-co-acrylic acid) (PNIPAM-co-AAc) hydrogels. The hydrogels were characterized by dynamic light scattering (DLS) and dissolved oxygen (DO) measurements. The mechanical properties, surface morphology, and chemical composition of the hydrogels were analyzed by Young's modulus measurements, scanning electron microscopy (SEM), and Fourier transform infrared (FT-IR) spectroscopy, respectively. The results showed that hydrogels prepared with bubble water changed the volume transition rate by more than two orders of magnitude by simply changing the standing time of the bubble water for only a few hours. The cooperative diffusion coefficients obtained from the light-induced volume transition kinetics correlated linearly with Young's modulus and metastable state swelling ratio. Our results suggest that bubbles act as efficient water channels, thereby modulating the response rate and providing a simple, additive-free method for preparing hydrogels with a wide range of response rates.

Decrease in the Surface Tension of Nanobubble Dispersion in Water: Results of Surface Excess of Bulk Nanobubbles at Interfaces

The effect of nanobubbles (NBs) on the surface tension of liquid was investigated by three methods of different measuring principles, pendant drop (PD), Wilhelmy, and du Noüy methods, over a wide range of number concentration of bulk NBs (BNBs). In all of the three methods, the surface tension decreased in proportion to the number concentration of BNBs and the proportional constant was different among the three methods. Such behavior was inferred to be caused by the surface excess of BNBs at the gas-liquid or solid-liquid interface. In the PD method, the hydrophobic interaction between BNBs and air around a drop seems to cause the surface excess of BNBs along the surface of water drops. It brings about a subtle change in its profile, resulting in the decrease in surface tension, which takes a time of hundreds of seconds. Meanwhile, in the Wilhelmy and du Noüy methods, electrostatic attractive force between BNBs and a plate or ring is a likely cause of surface excess at the solid-liquid interface, resulting in the instantaneous decrease in surface tension. This study also provides a practical methodology of comparison for surface tension of NB dispersion: surface tension shall be compared among different samples with the same measurement method. Especially in the PD method, retention time of droplets before measurement shall be the same among samples.

Effect of Polyphenols on the Ice-Nucleation Activity of Ultrafine Bubbles

Ultrafine bubbles (UFBs) in water provide a large amount of gas and a large gas-liquid interfacial area, and can release energy through their collapse. Such features may promote ice nucleation. Here, we examined the nucleation of ice in solutions containing polyphenols and UFBs. To reduce the likelihood of nucleation occurring on the container walls over that in previous studies, we used a much larger sample volume of 1 mL. In our experiments, UFBs (when present) had a number concentration of 10⁸ mL^-1. We quantified changes to the nucleation activity by examining the shift in the cumulative freezing (nucleation) probability distribution. Compared to pure water, this freezing curve shifts approximately 0.6 °C higher with the UFBs. Then, to the water, we added three polyphenols (tannic acid TA, tea catechin TC, and oligonol OLG), chosen because they had been reported to reduce the ice-nucleation activity of heterogeneous ice nuclei (e.g., AgI). We found experimentally that, without UFBs, all polyphenols instead shift the pure-water freezing curve to a higher temperature. Then, when UFBs are added, the additional temperature shift in the freezing curve is slightly higher for OLG, essentially unchanged for TA, and slightly lower for TC. To help to explain these differences, we examined the UFB size distributions using dynamic light scattering and freeze-fractured replicas with transmission electron microscopy, finding that OLG and TC alter the UFBs, but that TA does not.

Controllable generation of interfacial gas structures on the graphite surface by substrate hydrophobicity and gas oversaturation in water

Spherical nanobubbles and flat micropancakes are two typical states of gas aggregation on solid-liquid surfaces. Micropancakes, which are quasi-two-dimensional gaseous structures, are often produced accompanied by surface nanobubbles. Compared with surface nanobubbles, the intrinsic properties of micropancakes are barely understood due to the challenge of the highly efficient preparation and characterization of such structures. The hydrophobicity of the substrate and gas saturation of solvents are two crucial factors for the nucleation and stability of interfacial gas domains. Herein, we investigated the synergistic effect of the surface hydrophobicity and gas saturation on the generation of interfacial gas structures. Different surface hydrophobicities were achieved by the aging process of highly oriented pyrolytic graphite (HOPG). The results indicated that higher surface hydrophobicity and gas oversaturation could create surface nanobubbles and micropancakes with higher efficiency. Strong surface hydrophobicity could promote nanobubble nucleation and higher gas saturation would induce bigger nanobubbles. Degassed experiments could remove most of these structures and prove that they are actually gaseous domains. Finally, we draw a region diagram to describe the formation conditions of nanobubbles, micropancakes based on observations. These results would be very helpful for further understanding the formation of interfacial gas structures on the hydrophobic surface under different gas saturation.

Coupling Effects of Ionic Surfactants and Electrolytes on the Stability of Bulk Nanobubbles

As interest in the extensive application of bulk nanobubbles increases, it is becoming progressively important to understand the key factors affecting their anomalous stability. The scientific intrigue over nanobubbles originates from the discrepancy between the Epstein-Plesset prediction and experimental observations. Herein, the coupling effects of ionic surfactants and electrolytes on the stability of bulk nanobubbles is studied. Experimental results show that ionic surfactants not only reduce the surface tension but also promote the accumulation of net charges, which facilitate the nucleation and stabilization of bulk nanobubbles. The addition of an electrolyte in a surfactant solution further results in a decrease in the zeta potential and the number concentration of nanobubbles due to the ion shielding effect, essentially colloidal stability. An adsorption model for the coexistence of ionic surfactants and electrolytes in solution, that specifically considers the effect of the adsorption layer thickness within the framework of the modified Poisson-Boltzmann equation, is developed. A quantitative agreement between the predicted and experimental surface tension is found in a wide range of bulk concentrations. The spatial distribution of the surface potential, surfactant ions and counterions in the vicinity of the interface of bulk nanobubbles are described. Our study intrinsically paves a route to investigate the stability of bulk nanobubbles.

Interactions between O2 Nanobubbles and the Pulmonary Surfactant in the Presence of Inhalation Medicines

A dispersion of oxygen nanobubbles (O2-NBs) is an extraordinary gas-liquid colloidal system where spherical gas elements can be considered oxygen transport agents. Its conversion into inhalation aerosol by atomization with the use of nebulizers, while maintaining the properties of the dispersion, gives new opportunities for its applications and may be attractive as a new concept in treating lung diseases. The screening of O2-NBs interactions with lung fluids is particularly needed in view of an O2-NBs application as a promising aerosol drug carrier with the additional function of oxygen supplementation. The aim of the presented studies was to investigate the influence of O2-NBs dispersion combined with the selected inhalation drugs on the surface properties of two types of pulmonary surfactant models (lipid and lipid-protein model). The characteristics of the air-liquid interface were carried out under breathing-like conditions using two selected tensiometer systems: Langmuir-Wilhelmy trough and the oscillating droplet tensiometer. The results indicate that the presence of NBs has a minor effect on the dynamic characteristics of the air-liquid interface, which is the desired effect in the context of a potential use in inhalation therapies.

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.

Recent Developments in Generation, Detection and Application of Nanobubbles in Flotation

This paper reviews recent developments in the fundamental understating of ultrafine (nano) bubbles (NBs) and presents technological advances and reagent types used for their generation in flotation. The generation of NBs using various approaches including ultrasonication, solvent exchange, temperature change, hydrodynamic cavitation, and electrolysis was assessed. Most importantly, restrictions and opportunities with respect to the detection of NBs were comprehensively reviewed, focusing on various characterization techniques such as the laser particle size analyzer (LPSA), nanoparticle tracking (NTA), dynamic light scattering (DLS), zeta-phase light scattering (ZPALS), and zeta sizer. As a key feature, types and possible mechanisms of surfactants applied to stabilize NBs were also explored. Furthermore, flotation-assisted nano-bubbles was reported as an efficient method for recovering minerals, with a special focus on flotation kinetics. It was found that most researchers reported the existence and formation of NBs by different techniques, but there is not enough information on an accurate measurement of their size distribution and their commonly used reagents. It was also recognized that a suitable method for generating NBs, at a high rate and with a low cost, remains a technical challenge in flotation. The application of hydrodynamic cavitation based on a venturi tube and using the LPSA and NTA in laboratory scales were identified as the most predominant approaches for the generation and detection of NBs, respectively. In this regard, neither pilot- nor industrial-scale case studies were found in the literature; they were only highlighted as future works. Although the NB-stabilizing effects of electrolytes have been well-explored, the mechanisms related to surfactants remain the issue of further investigation. The effectiveness of the NB-assisted flotation processes has been mostly addressed for single minerals, and only a few works have been reported for bulk materials. Finally, we believe that the current review paves the way for an appropriate selection of generating and detecting ultrafine bubbles and shines the light on a profound understanding of its effectiveness.

Molecular simulations on the stability and dynamics of bulk nanobubbles in aqueous environments

Nanobubbles have attracted significant attention due to their unexpectedly long lifetimes and stabilities in liquid solutions. However, explanations for the unique properties of nanobubbles at the molecular scale are somewhat controversial. Of special interest is the validity of the Young-Laplace equation in predicting the inner pressure of such bubbles. In this work, large-scale molecular dynamics simulations were performed to study the stability and diffusion of nanobubbles of methane in water. Two types of force field, atomistic and coarse-grained, were used to compare the calculated results. In accordance with predictions from the Young-Laplace equation, it was found that the inner pressure of the nanobubbles increased with decreasing nanobubble size. Consequently, a large pressure difference between the nanobubble and its surroundings resulted in the high solubility of methane molecules in water. The solubility was considered to enable nanobubble stability at exceptionally high pressures. Smaller bubbles were observed to be more mobile via Brownian motion. The calculated diffusion coefficient also showed a strong dependence on the nanobubble size. However, this active mobility of small nanobubbles also triggered a mutable nanobubble shape over time. Nanobubbles were also found to coalesce when they were sufficiently close. A critical distance between two nanobubbles was thus identified to avoid coalescence. These results provide insight into the behavior of nanobubbles in solution and the mechanism of their unique stability while withstanding high inner pressures.

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.

Vibration influence on the O2-dependent processes activity in human erythrocytes

The early signs of vibration effects on the human body are microcirculation and transcapillary metabolism disorders, accompanied by disruption of the supply to and utilization of oxygen in the tissues and organs. However, there are few experimental studies aimed at finding targets of vibration in cells and determining the action mechanism of vibration. In in vitro experiments, human erythrocytes in buffer solution were exposed to low-frequency vibration (frequency range 8–32 Hz, amplitudes 0.5–0.9 mm) for 3 hours. The dynamics of the accumulation of membrane-bound catalase and hemoglobin and the distribution of ligand hemoglobin in the membrane-bound fraction were studied as the indicators of functional activity of cells. The choice of these indicators is justified by the participation of catalase and hemoglobin in O2-dependent cellular reactions as a part of protein complexes. Since pО2 is a trigger of conformational transitions in the hemoglobin molecule, simultaneously with oxygen transport, hemoglobin signals to different metabolic systems about oxygen conditions in the environment. The studies revealed that in the conditions of vibration, the activity of membrane-associated catalase increased by 40–50% in the frequency range of 12–24 Hz (amplitude 0.5 ± 0.04 mm), by 20–30% in the amplitude of 0.9 mm, but after about 100–120 min exposure the enzyme activity decreased even below the control level. There was a dose-dependent accumulation of membrane-bound hemoglobin during exposure to vibration. In the membrane-bound fraction of hemoglobin, oxyhemoglobin had the highest content (60–80%), while the content of methemoglobin varied 5–20%. During vibrations in the frequency range 12–28 Hz, 0.5 mm, we recorded 10–30% increase in oxyhemoglobin. With increase in the vibration amplitude (0.9 mm) in the frequency range of 16–32 Hz, constant content of oxyhemoglobin was noted at the beginning of the experiment, which tended to decrease during the last exposure time. Frequency of 32 Hz caused increase in the deoxyhemoglobin content in the membrane-bound fraction. The content of methemoglobin (metHb) in erythrocytes significantly increased during exposure to the frequency range of 12–24 Hz, with the amplitude of 0.5 mm (1.3–2.4 times). During the exposure to frequencies of 28 and 32 Hz, we observed the transition of methemoglobin to hemichrome. The content of methemoglobin in the cells was lower and decreased at the end of the experiment when the vibration amplitude was 0.9 mm. In these experimental conditions, no increase in hemichrome content in the membrane-bound fraction was recorded. Therefore, the degree of binding of catalase and hemoglobin with the membrane of erythrocytes that were exposed to vibration and the changes in the content of ligand forms in the composition of membrane-bound hemoglobin are dose-dependent. Low-frequency vibration initiates O2-dependent processes in erythrocytes. Targets of such an influence are nanobubbles of dissolved air (babstons), retained on the surface of erythrocytes due to Coulomb interactions, capable of coagulation and increase in size under the action of vibration. At first, the consequences of these processes are increase in oxygen content in the surface of erythrocytes, and then decrease as a result of degassing. Thus, increase in oxygen content on the surface initiates redox reactions, whereas decrease in oxygen content leads to reconstruction of metabolic processes oriented at overcoming hypoxia.

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.

Promoting Effect of Ultra-Fine Bubbles on CO2 Hydrate Formation

When gas hydrates dissociate into gas and liquid water, many gas bubbles form in the water. The large bubbles disappear after several minutes due to their buoyancy, while a large number of small bubbles (particularly sub-micron-order bubbles known as ultra-fine bubbles (UFBs)) remain in the water for a long time. In our previous studies, we demonstrated that the existence of UFBs is a major factor promoting gas hydrate formation. We then extended our research on this issue to carbon dioxide (CO2) as it forms structure-I hydrates, similar to methane and ethane hydrates explored in previous studies; however, CO2 saturated solutions present severe conditions for the survival of UFBs. The distribution measurements of CO2 UFBs revealed that their average size was larger and number density was smaller than those of other hydrocarbon UFBs. Despite these conditions, the CO2 hydrate formation tests confirmed that CO2 UFBs played important roles in the expression of the promoting effect. The analysis showed that different UFB preparation processes resulted in different promoting effects. These findings can aid in better understanding the mechanism of the promoting (or memory) effect of gas hydrate formation.

Long-Term Effect of Low-Frequency Electromagnetic Irradiation in Water and Isotonic Aqueous Solutions as Studied by Photoluminescence from Polymer Membrane

The swelling of a polymer membrane Nafion^TM in deionized water and isotonic NaCl and Ringer’s solutions was studied by photoluminescent spectroscopy. According to our previous studies, the surface of this membrane could be considered as a model for a cellular surface. Liquid samples, in which the membrane was soaked, were subjected to preliminary electromagnetic treatment, which consisted of irradiating these samples with electric rectangular pulses of 1 µs duration using platinum electrodes immersed in the liquid. We used a series of pulses with a repetition rate of 11–125 Hz; the pulse amplitudes were equal to 100 and 500 mV. It turned out that at certain pulse repetition rates and their amplitudes, the characteristic swelling time of the polymer membrane significantly differs from the swelling time in untreated (reference) samples. At the same time, there is no effect for certain frequencies/pulse amplitudes. The time interval between electromagnetic treatment and measurements was about 20 min. Thus, in our experiments the effects associated with the long-term relaxation of liquids on the electromagnetic processing are manifested. The effect of long-term relaxation could be associated with a slight change in the geometric characteristics of bubston clusters during electromagnetic treatment.

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.

Remediation of contaminated sediments containing both organic and inorganic chemicals using ultrasound and ozone nanobubbles

Most river sediments are contaminated with organic and inorganic pollutants and cause significant environmental damage and health risks. This research is evaluated an in-situ sediment remediation method using ultrasound and ozone nanobubbles to remove organic and inorganic chemicals in contaminated sediments. Contaminated sediment is prepared by mixing synthetic fine sediment with an organic (p-terphenyl) and an inorganic chemical (chromium). The prepared contaminated sediment is treated with ultrasound and ozone nanobubbles under different operating conditions. For the samples with the maximum initial concentration of 4211 mg/kg Cr and 1875 mg/kg p-terphenyl, average removal efficiencies are 71% and 60%, respectively, with 240 min of sonication with 2-min pulses, whereas 97.5% and 91.5% removal efficiencies are obtained for the same, respectively, as a single contaminant in the sediment. For the same maximum concentrations, the highest removal of p-terphenyl is 82.7% with 127.2 J/ml high energy density, and for Cr, it is 77.1% using the highest number of the treatment cycle and ozone usage with 78.75/ml energy density. The Cr highest removal efficiency of 87.2% is recorded with the reduced initial concentration of 1227 mg/kg with the highest treatment cycles. The Cr removal efficiency depends on the availability of oxidizing agents and the number of washing cycles of sediments, whereas P-terphenyl degradation is most likely influenced by the combined effects of oxidation and ultrasound-assisted pyrolysis and combustion of organics.

Effect of acidic conditions on decomposition of methylene blue in aqueous solution by air microbubbles

Methylene blue (MB) has recently been proposed as a new chemical probe to colorimetrically verify the presence of hydroxyl radicals and was applied to the microbubble technique in this report. The degradation of MB in water was observed by air microbubbles under acidic condition or adding H₂O₂. The discoloration of MB induced by air microbubbles supports that hydroxyl radicals are generated by the collapse of microbubbles in water. The decomposition rate of MB increases with increasing the concentration of acids (HCl, HNO₃, H₂SO₄) and H₂O₂ up to ab. 2 mM, and becomes almost constant above ab. 2 mM. Note that the decomposition rate of MB mainly depends on pH, not the kind of acid and that adding salt slightly enhanced the decomposition of MB. The fact that the blue-shift of the absorbance band of MB was observed for not H₂O₂ but acids proposed that the methyl groups of MB are preferentially dissociated under acidic conditions due to the protonation of dimethylamino group.

Stability of Engineered Micro or Nanobubbles for Biomedical Applications

A micro/nanobubble (MNB) refers to a bubble structure sized in a micrometer or nanometer scale, in which the core is separated from the external environment and is normally made of gas. Recently, it has been confirmed that MNBs can be widely used in angiography, drug delivery, and treatment. Thus, MNBs are attracting attention as they are capable of constructing a new contrast agent or drug delivery system. Additionally, in order to effectively use an MNB, the method of securing its stability is also being studied. This review highlights the factors affecting the stability of an MNB and the stability of the MNB within the ultrasonic field. It also discusses the relationship between the stability of the bubble and its applicability in vivo.

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.

Contribution of Ultra-Fine Bubbles to Promoting Effect on Propane Hydrate Formation

To investigate experimentally how ultra-fine bubbles (UFBs) may promote hydrate formation, we examined the formation of propane (C₃H₈) hydrate from UFB-infused water solution using two preparation methods. In one method, we used C₃H₈-hydrate dissociated water, and in the other, C₃H₈-UFB-included water prepared with a generator. In both solutions, the initial conditions had a UFB number density of up to 10⁹ mL⁻¹. This number density decreased by only about a half when stored at room temperature for 2 days, indicating that enough amount of UFBs were stably present at least during the formation experiments. Compared to the case without UFBs, the nucleation probabilities within 50 h were ~1.3 times higher with the UFBs, and the induction times, the time period required for the bulk hydrate formation, were significantly shortened. These results confirmed that UFB-containing water promotes C₃H₈-hydrate formation. Combined with the UFB-stability experiments, we conclude that a high number density of UFBs in water contributes to the hydrate promoting effect. Also, consistent with previous research, the present study on C₃H₈ hydrates showed that the promoting effect would occur even in water that had not experienced any hydrate structures. Applying these findings to the debate over the promoting (or “memory”) effect of gas hydrates, we argue that the gas dissolution hypothesis is the more likely explanation for the effect.

Surface enrichment of ions leads to the stability of bulk nanobubbles

Numerous experiments have shown that bulk nanobubble suspensions are often characterized by a high magnitude of zeta potential. However, the underlying physical mechanism of how the bulk nanobubbles can stably exist has remained unclear so far. In this paper, based on theoretical analysis, we report a stability mechanism for charged bulk nanobubbles. The strong affinity of negative charges for the nanobubble interface causes charge enrichment, and the resulting electric field energy gives rise to a local minimum for the free energy cost of bubble formation, leading to thermodynamic metastability of the charged nanobubbles. The excess surface charges mechanically generate a size-dependent force, which balances the Laplace pressure and acts as a restoring force when a nanobubble is thermodynamically perturbed away from its equilibrium state. With this negative feedback mechanism, we discuss the nanobubble stability as a function of surface charge and gas supersaturation. We also compare our theoretical prediction with recent experimental observations, and a good agreement is found. This mechanism provides new fundamental insights into the origin of the unexplained stability of bulk nanobubbles.

Application of the Diffused Double Layer Theory to Nanobubbles

Nanobubbles have electrically charged interfaces; hence, the diffused double layer theory can be applied to explain the behavior of nanobubbles in different electrolytic solutions. In this research, oxygen nanobubbles were generated in NaCl solutions of different concentrations, and bubble size and ζ potentials were measured just after the generation and after 1 week. The measured data and diffused double layer theory were used to compute the surface charge density, the potential due to the surface charge, and the interaction energy between bubbles. With the increased NaCl concentration, bubble size, surface charge density, and the number of negative charges increased, while the magnitude of ζ potential/surface potential, double layer thickness, internal pressure, and the electrostatic repulsion force decreased. The same trend was observed after 1 week. The net total energy calculation for the 0.001 M NaCl solution showed that the bubble repulsion for an intermediate separation distance had a 6.99 × 10^-20 J energy barrier, which prevented bubble coalescence. Hence, the 0.001 M NaCl solution produced stable nanobubbles. The calculation of internal pressure inside nanobubbles showed a reduction in the interfacial pressure difference with the increased NaCl concentration. The test results, as well as diffuse double layer and net total energy calculations, showed that the most stable bubbles were obtained with 0.001 M NaCl concentration and the least stability was recorded with the highest amount (0.1 M) of NaCl concentration.

Generation and Stability of Size-Adjustable Bulk Nanobubbles Based on Periodic Pressure Change

Recently, bulk nanobubbles have attracted intensive attention due to the unique physicochemical properties and important potential applications in various fields. In this study, periodic pressure change was introduced to generate bulk nanobubbles. N₂ nanobubbles with bimodal distribution and excellent stabilization were fabricated in nitrogen-saturated water solution. O₂ and CO₂ nanobubbles have also been created using this method and both have good stability. The influence of the action time of periodic pressure change on the generated N₂ nanobubbles size was studied. It was interestingly found that, the size of the formed nanobubbles decreases with the increase of action time under constant frequency, which could be explained by the difference in the shrinkage and growth rate under different pressure conditions, thereby size-adjustable nanobubbles can be formed by regulating operating time. This study might provide valuable methodology for further investigations about properties and performances of bulk nanobubbles.

Effects of nanobubble water on the growth ofLactobacillus acidophilus1028 and its lactic acid production

Nanobubbles (NBs) in the culture medium may offer a new way of accelerating bacterial growth in the lag and logarithmic phases.

Stable Air Nanobubbles in Water: the Importance of Organic Contaminants

Nanobubbles, surprisingly stable submicrometer gas bubbles in water, appear to be common in water and biological fluids and are of great interest in technical applications ranging from ultrasound contrast agents to flotation in the mining industry. Nanobubbles on surfaces have been more researched than freely floating bulk nanobubbles, and the reason for their stability appears to be better explained. The stability of bulk nanobubbles is less well explained, several theories exist, and even their existence is sometimes questioned. In the present study, an attempt was made to generate nanobubbles through hydrodynamic cavitation as well as through vigorous shaking in test tubes, and it was found that none of these methods generated a detectable concentration of possible bulk nanobubbles if pure water was used, with or without a small addition of NaCl, the equipment was cleaned properly, and certain plastic materials were avoided. These results indicate that trace organic contaminants are necessary for nanobubble stabilization. Experiments were also made with the dissolution of a high concentration of inorganic salts, which generated bubbles by creating air supersaturation. Light scattering submicron particles were found in all solutions and appeared to be actual gas bubbles in at least one case. However, in many cases, these light scattering particles were unaffected by vacuum and pressure and appear, therefore, to be something else other than air bubbles. It is concluded that, in future research on nanobubble stability, it is very important to avoid contamination, as well as to ascertain that light scattering objects really are bubbles and not oil droplets or particles.

Ultra-Fine Bubble Distributions in a Plant Factory Observed by Transmission Electron Microscope with a Freeze-Fracture Replica Technique

Water containing ultra-fine bubbles (UFB) may promote plant growth. But, as UFBs are too small to distinguish from other impurities in a nutrient solution, it is not known if UFBs survive transport from the water source to the rhizosphere. Here we use the freeze-fracture replica method and a transmission electron microscope (TEM) to observe UFBs in the nutrient solutions used in a crop-growing system known as a plant factory. In this factory, TEM images taken from various points in the supply line indicate that the concentration of UFBs in the nutrient solution is conserved, starting from their addition to the nutrient solution in the buffer tank, through the peat-moss layer, all the way to the rhizosphere. Measurements also show that a thin film formed on the surface of UFBs in the nutrient solution, with greater film thickness at the rhizosphere. This film is considered to be made from the accumulation of impurities coming from solute and the peat-moss layer.

Xylem Surfactants Introduce a New Element to the Cohesion-Tension Theory

Vascular plants transport water under negative pressure without constantly creating gas bubbles that would disable their hydraulic systems. Attempts to replicate this feat in artificial systems almost invariably result in bubble formation, except under highly controlled conditions with pure water and only hydrophilic surfaces present. In theory, conditions in the xylem should favor bubble nucleation even more: there are millions of conduits with at least some hydrophobic surfaces, and xylem sap is saturated or sometimes supersaturated with atmospheric gas and may contain surface-active molecules that can lower surface tension. So how do plants transport water under negative pressure? Here, we show that angiosperm xylem contains abundant hydrophobic surfaces as well as insoluble lipid surfactants, including phospholipids, and proteins, a composition similar to pulmonary surfactants. Lipid surfactants were found in xylem sap and as nanoparticles under transmission electron microscopy in pores of intervessel pit membranes and deposited on vessel wall surfaces. Nanoparticles observed in xylem sap via nanoparticle-tracking analysis included surfactant-coated nanobubbles when examined by freeze-fracture electron microscopy. Based on their fracture behavior, this technique is able to distinguish between dense-core particles, liquid-filled, bilayer-coated vesicles/liposomes, and gas-filled bubbles. Xylem surfactants showed strong surface activity that reduces surface tension to low values when concentrated as they are in pit membrane pores. We hypothesize that xylem surfactants support water transport under negative pressure as explained by the cohesion-tension theory by coating hydrophobic surfaces and nanobubbles, thereby keeping the latter below the critical size at which bubbles would expand to form embolisms.

A History of Nanobubbles

We follow the history of nanobubbles from the earliest experiments pointing to their existence to recent years. We cover the effect of Laplace pressure on the thermodynamic stability of nanobubbles and why this implies that nanobubbles are thermodynamically never stable. Therefore, understanding bubble stability becomes a consideration of the rate of bubble dissolution, so the dominant approach to understanding this is discussed. Bulk nanobubbles (or fine bubbles) are treated separately from surface nanobubbles as this reflects their separate histories. For each class of nanobubbles, we look at the early evidence for their existence, methods for the production and characterization of nanobubbles, evidence that they are indeed gaseous, or otherwise, and theories for their stability. We also look at applications of both surface and bulk nanobubbles.

Nanobubbles, cavitation, shock waves and traumatic brain injury

Shock wave induced cavitation denaturates blood–brain barrier tight junction proteins; this may result in various neurological complications.