Dynamic tracking of bulk nanobubbles from microbubbles shrinkage to collapse

Surface Cleaning of Oil Contaminants Using Bulk Nanobubbles

The removal of oil from solid surfaces, such as textiles and plates, remains a challenge due to the strong binding affinity of the oil. Conventional methods for surface cleaning often require surfactants and mechanical abrasion to enhance the cleaning process. However, in excess, these can pose adverse effects on the environment and to the material. This study investigated how bulk nanobubble water can clean oil microdroplets deposited on surfaces like glass coverslips and dishes. Microscopy imaging and further image analysis clearly revealed that these microdroplets detached from both hydrophobic and hydrophilic surfaces when washed with bulk nanobubble water within a fluidic microchannel. Oil contaminant cleaning was also conducted in water as mobile phase to mimic the circumstances that occur in a dishwasher and washing machine. Cleaning on a larger scale also proved very successful in the removal of oil from a porcelain bowl. These results indicate that nanobubble water can easily remove oil contaminants from glass and porcelain surfaces without the assistance of surfactants. This is in stark contrast to negligible results obtained with a control solution without nanobubbles. This study indicates that nanobubble technology is an innovative, low-cost, eco-friendly approach for oil removal, demonstrating its potential for broad practical applications.

Interaction and Stability of Nanobubbles and Prenucleation Calcium Clusters during Ultrasonic Treatment of Hard Water

To investigate the stability of nanobubbles in natural hard water, a series of eight samples ranging in hardness from 0 to 332 mg/L CaCO3 were sonicated for periods of 5-45 min with an ultrasonic horn. Conductivity, temperature, ζ-potential, composition, and pH of the water were analyzed, together with the crystal structure of any calcium carbonate precipitate. Quasi-stable populations of bulk nanobubbles in Millipore and soft water are characterized by a ζ-potential of -35 to -20 mV, decaying over 60 h or more. After sonicating the hardest waters for about 10 min, they turn cloudy due to precipitation of amorphous calcium carbonate when the water temperature reaches 40 °C; the ζ-potential then jumps from -10 to +20 mV and remains positive for several days. From an analysis of the change of conductivity of the hard water before and after sonication, it is estimated that 37 ± 5% of calcium was not originally in solution but existed in nanoscale prenucleation clusters, which decorate the nanobubbles formed in the early stages of sonication. Heating and charge screening in the nanobubble colloid cause the decorated bubbles to collapse or disperse, leaving an amorphous precursor of aragonite. Sonicating the soft supernatant increases its conductivity and pH and restores the negative ζ-potential associated with bulk nanobubbles, but there is no further precipitation. Our study of the correlation between nanobubble production and calcium agglomeration spanning the hardness and composition ranges of natural waters shows that the sonication method for introducing nanobubbles is viable only for hard water if it is kept cold; the stability of the nanobubble colloid will be reduced in any case by the presence of dissolved calcium and magnesium.

The Number of Microbubbles Generated During Cardiopulmonary Bypass Can Be Estimated Using Machine Learning From Suction Flow Rate, Venous Reservoir Level, Perfusion Flow Rate, Hematocrit Level, and Blood Temperature

GOAL

Microbubbles (MBs) are known to occur within the circuits of cardiopulmonary bypass (CPB) systems, and higher-order dysfunction after cardiac surgery may be caused by MBs as well as atheroma dispersal associated with cannula insertion. As complete MB elimination is not possible, monitoring MB count rates is critical. We propose an online detection system with a neural network-based model to estimate MB count rate using five parameters: suction flow rate, venous reservoir level, perfusion flow rate, hematocrit level, and blood temperature.

METHODS

Perfusion experiments were performed using an actual CPB circuit, and MB count rates were measured using the five varying parameters.

RESULTS

Bland-Altman analysis indicated a high estimation accuracy (R2 > 0.95, p < 0.001) with no significant systematic error. In clinical practice, although the inclusion of clinical procedures slightly decreased the estimation accuracy, a high coefficient of determination for 30 clinical cases (R2 = 0.8576) was achieved between measured and estimated MB count rates.

CONCLUSIONS

Our results highlight the potential of this system to improve patient outcomes and reduce MB-associated complication risk.

Characteristics and Stability of Ozone Nanobubbles in Freshwater Conditions

The characteristics and stability of ozone nanobubbles (NBs) were investigated for the first time under different preparation conditions and freshwater conditions (i.e., pH, natural organic matter [NOM], carbonate, calcium, and temperature) for an extended period. Two oxygen gas flow rates (4 and 1 L/min) used in ozone NB generation affected the characteristics and stability of ozone NBs. The ozone NBs generated at a high initial dissolved ozone (12.5 mg/L) concentration showed a much higher brightness during measurements than the ozone NBs generated at a low initial dissolved ozone concentration (1 mg/L). The former also exhibited a higher negative surface charge and higher stability in comparison to the latter. The stability and half-lives of ozone NBs followed the order of 3 mM Ca2+ < pH 3 < NOM with high specific ultraviolet absorbance at 254 nm (SUVA254 = 4.1 L/mg·m) < pH 7 < pH 9, while the effects of carbonate and temperature were insignificant. Ozone NBs were relatively stable in waters for a long period (e.g., ≥ 60 days) except for high hardness or low pH conditions. Higher levels of hydroxyl radicals were produced from ozone NB solutions as compared to conventional ozonation.

From Nanovesicles to Nanobubbles Based on Repeated Compression Method

Nanobubbles have been increasingly applied in biomedicine, which is attributed to their ability to work as ultrasound imaging contrast agents and powerful gene/drug carriers. Different production techniques or approaches have been developed to generate uniform and stable shelled nanobubbles. However, these shelled nanobubbles are usually prepared based on disordered shell materials, such as free phospholipids and polymers. In recent years, the continuous repeated compression method for a gas-liquid mixture has been developed to produce free and lipid-shelled nanobubbles. In this study, to explore the response of well-organized nanostructures to this method, the repeated compression method was used to treat preprepared liposomes and polymeric nanovesicles. Size distribution, morphologies, and ultrasound image contrast enhancement of these nanovesicles were determined before and after repeated compression. Results demonstrate that the presence of a phospholipid bilayer is vital to form liposome-based nanobubbles. And the low elastic modulus of the polymeric membrane is key to encapsulate gases into polymeric nanovesicles. Overall, it demonstrated the advantages of well-organized nanostructures to produce nanobubble structures, giving new insights into the preparation and understanding of nanobubbles.

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

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

Nanobubble Reactivity: Evaluating Hydroxyl Radical Generation (or Lack Thereof) under Ambient Conditions

Nanobubble (NB) generation of reactive oxygen species (ROS), especially hydroxyl radical (·OH), has been controversial. In this work, we extensively characterize NBs in solution, with a focus on ROS generation (as ·OH), through a number of methods including degradation of ·OH-specific target compounds, electron paramagnetic resonance (EPR), and a fluorescence-based indicator. Generated NBs exhibit consistent physical characteristics (size, surface potential, and concentration) when compared with previous studies. For conditions described, which are considered as high O2 NB concentrations, no degradation of benzoic acid (BA), a well-studied ·OH scavenger, was observed in the presence of NBs (over 24 h) and no EPR signal for ·OH was detected. While a positive fluorescence response was measured when using a fluorescence probe for ·OH, aminophenyl fluorescein (APF), we provide an alternate explanation for the result. Gas/liquid interfacial characterization indicates that the surface of a NB is proton-rich and capable of inducing acid-catalyzed hydrolysis of APF, which results in a false (positive) fluorescence response. Given these negative results, we conclude that NB-induced ·OH generation is minimal, if at all, for conditions evaluated.

Evidence of laser-induced nanobubble formation mechanism in water

Principles of laser-induced nanobubble formation in water are studied and presented. Nanobubbles were generated by laser light at intensities below threshold for laser-induced breakdown and subsequently expanded by a rarefaction wave to facilitate their observation and analysis. Different methods were used to study nanobubble formation and characteristics. Firstly, probability of nanobubble formation as a function of water sample purity was examined. Secondly, relation between laser fluence at different wavelengths and the number of generated nanobubbles was investigated. Thirdly, measurements of nanobubble lifetime were conducted indicating a contradiction to the Epstein-Plesset equation-based prediction of free bubble dissociation. Accumulated evidence suggests that the presence of physical impurities is a prerequisite for nanobubble formation. Consequently, a lack of impurities results in the absence of nanobubbles in contrast to assumptions by existing studies. The findings presented in this paper provide new insights into the fundamental properties of laser-induced nanobubbles in water.

Synergism of Surfactant Mixture in Lowering Vapor-Liquid Interfacial Tension

Through employing molecular dynamics, in this work, we study how a two-component surfactant mixture cooperatively reduces the interfacial tension of a flat vapor-liquid interface. Our simulation results show that in the presence of a given insoluble surfactant, adding a secondary surfactant would either further reduce interfacial tension, indicating a positive synergistic effect, or increase the interfacial tension instead, indicating a negative synergistic effect. The synergism of the surfactant mixture in lowering surface tension is found to depend strongly on the structure complementary effect between different surfactant components. The synergistic mechanisms are then interpreted with minimization of the bending free energy of the composite surfactant monolayer via cooperatively changing the monolayer spontaneous curvature. By roughly describing the monolayer spontaneous curvature with the balanced distribution of surfactant heads and tails, we confirm that the positive synergistic effect in lowering surface tension is featured with the increasingly symmetric head-tail distributions, while the negative synergistic effect is featured with the increasingly asymmetric head-tail distributions. Furthermore, our simulation results indicate that minimal interfacial tension can only be observed when the spontaneous curvature is nearly zero.

Microfluidic nanobubbles: observations of a sudden contraction of microbubbles into nanobubbles

Microfluidic devices are often utilized to generate uniform-size microbubbles. In most microfluidic bubble generation experiments, once the bubbles are formed the gas inside the bubbles begin to dissolve into the surrounding aqueous environment. The bubbles shrink until they attain an equilibrium size dictated by the concentration and type of amphiphilic molecules stabilizing the gas-liquid interface. Here, we exploit this shrinkage mechanism, and control the solution lipid concentration and microfluidic geometry, to make monodisperse bulk nanobubbles. Interestingly, we make the surprising observation of a critical microbubble diameter above and below which the scale of bubble shrinkage dramatically changes. Namely, microbubbles generated with an initial diameter larger than the critical diameter shrinks to a stable diameter that is consistent with previous literature. However, microbubbles that are initially smaller than the critical diameter experience a sudden contraction into nanobubbles whose size is at least an order-of-magnitude below expectations. We apply electron microscopy and resonance mass measurement methods to quantify the size and uniformity of the nanobubbles, and probe the dependence of the critical bubble diameter on the lipid concentration. We anticipate that further analysis of this unexpected microbubble sudden contraction regime can lead to more robust technologies for making monodisperse nanobubbles.

Lipid-Coated Nanobubbles in Plants

One of the more surprising occurrences of bulk nanobubbles is in the sap inside the vascular transport system of flowering plants, the xylem. In plants, nanobubbles are subjected to negative pressure in the water and to large pressure fluctuations, sometimes encompassing pressure changes of several MPa over the course of a single day, as well as wide temperature fluctuations. Here, we review the evidence for nanobubbles in plants and for polar lipids that coat them, allowing nanobubbles to persist in this dynamic environment. The review addresses how the dynamic surface tension of polar lipid monolayers allows nanobubbles to avoid dissolution or unstable expansion under negative liquid pressure. In addition, we discuss theoretical considerations about the formation of lipid-coated nanobubbles in plants from gas-filled spaces in the xylem and the role of mesoporous fibrous pit membranes between xylem conduits in creating the bubbles, driven by the pressure gradient between the gas and liquid phase. We discuss the role of surface charges in preventing nanobubble coalescence, and conclude by addressing a number of open questions about nanobubbles in plants.

Molecular Approach for Understanding the Stability, Collision, and Coalescence of Bulk Nanobubbles

Though long-lived nanobubbles (NBs) have been reported by multiple researchers, the underlying reason behind their stability is still obscure. Some of the conjectured reasons include diffusive shielding, the presence of surface charges, and stability due to contamination. Still, the stability of NBs against coalescence and Ostwald ripening is not confirmed. Using molecular dynamics simulations, the present study aims to understand the stabilization effects due to diffusive shielding and the presence of an electrical double layer at the surface of NBs. Accumulation of charges on NBs for different concentrations of ions is discussed. Also, the collision of equal-sized NBs with different approach velocities and offset distances is simulated. A regime map is predicted on the basis of initial approach velocity and offset distance. The transition in regime obtained upon increasing the offset distance is discussed, which differs from the collision characteristics of macroscopic bubbles and drops. The merging of NBs is initiated through the bridge formation, for which the temporal evolution rate along with the scaling argument is presented. The stress terms involved and the corresponding regimes are predicted based on the fluid properties. For all the cases where merging is observed, the estimated probability is observed to be low, which suggests the stability of NBs against coalescence.

On Some Aspects of Nanobubble-Containing Systems

Theoretical studies are reviewed for bulk nanobubbles (ultrafine bubbles (UFBs)), which are gas bubbles smaller than 1 μm in diameter. The dynamic equilibrium model is discussed as a promising model for the stability of a UFB against dissolution; more than half of the surface of a UFB should be covered with hydrophobic material (impurity). OH radicals are produced during hydrodynamic or acoustic cavitation to produce UFBs. After stopping cavitation, OH radicals are generated through chemical reactions of H2O2 and O3 in the liquid water. The possibility of radical generation during the bubble dissolution is also discussed based on numerical simulations. UFBs are concentrated on the liquid surface according to the dynamic equilibrium model. As a result, rupture of liquid film is accelerated by the presence of UFBs, which results in a reduction in “surface tension”, measured by the du Noüy ring method. Finally, the interaction of UFBs with a solid surface is discussed.

Interaction Mechanisms and Application of Ozone Micro/Nanobubbles and Nanoparticles: A Review and Perspective

Ozone micro/nanobubbles with catalytic processes are widely used in the treatment of refractory organic wastewater. Micro/nanobubble technology overcomes the limitations of ozone mass transfer and ozone utilization in the application of ozone oxidation, and effectively improves the oxidation efficiency of ozone. The presence of micro/nanobubbles keeps the catalyst particles in a dynamic discrete state, which effectively increases the contact frequency between the catalyst and refractory organic matter and greatly improves the mineralization efficiency of refractory organic matter. This paper expounds on the characteristics and advantages of micro/nanobubble technology and summarizes the synergistic mechanism of microbubble nanoparticles and the mechanism of catalyst ozone micro/nanobubble systems in the treatment of refractory organics. An interaction mechanism of nanoparticles and ozone microbubbles is suggested, and the proposed theories on ozone microbubble systems are discussed with suggestions for future studies on systems of nanoparticles and ozone microbubbles.

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.

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.

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.

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.

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.

On-Demand Bulk Nanobubble Generation through Pulsed Laser Illumination

We demonstrate the temporally and spatially controlled nucleation of bulk nanobubbles in water through pulsed laser irradiation with a collimated beam. Transient bubbles appear within the light exposed region once a tension wave passes through. The correlation between illumination and cavitation nucleation provides evidence that gaseous nanobubbles are nucleated in the liquid by a laser pulse with an intensity above 58  MW/cm^{2}. We estimate the radius of the nanobubbles through microscopic high-speed imaging and by solving the diffusion equation to be below 420 nm for ∼80% of the bubble population. This technique may provide a novel approach to test theories on existence of stable bulk nanobubbles.

Collective Dynamics of Bulk Nanobubbles with Size-Dependent Surface Tension

It has been suggested that irreversible adsorption at the gas/liquid interface of bulk nanobubbles will reduce the Laplace pressure, leading to their stability. However, most previous studies have focused on the stability of individual nanobubbles. Bulk nanobubbles are polydispersed suspensions, and gas molecules can diffuse between bubbles, leading to their collective dynamics, which may be crucial to understanding their formation process and stability. In this study, we proposed a mean-field theory for computing the evolution of the size-distribution function of bulk nanobubbles with size-dependent surface tension. We applied this theory to investigate the evolution of bulk nanobubbles with insoluble surfactants pinned at their gas/water interface. The results show that Ostwald ripening can be suppressed when enough surfactants are adsorbed. Bulk nanobubbles can be produced by the shrinkage of microbubbles in an air-saturated solution. The mean stable size is controlled by the amount of surfactants and the initial microbubble concentration; these predictions are qualitatively consistent with the experimental results of micro/nanobubbles produced using the microfluidic method.

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.

Indocyanine Green Assembled Nanobubbles with Enhanced Fluorescence and Photostability

Indocyanine green (ICG) is a near-infrared (NIR) fluorescent dye for extensive biomedical application. However, its fluorescence intensity is limited by its poor aqueous stability and concentration-dependent aggregation. To overcome these limitations, ICG self-assembled nanobubbles (ICG-NBs) with an average size of 244.6 nm are fabricated. In the ICG-NB assembled structures, the ICG molecules are arrayed on the gas-liquid interface by the hydrophobic interaction with the gas core and hydrophilic heads with water. Results show that ICG-NBs exhibited good monodispersity and excellent fluorescence and size stability. Compared with ICG solution, the ICG-NBs indicate the enhanced quantum yield and fluorescence intensity. The surface-enhanced Raman scattering (SERS) spectra and fluorescence lifetime measurement demonstrate that the ICG molecule assembled NBs could result in the changes of molecular vibration and time-resolved intensity decays of ICG. Thus, the ICG-NBs could be more beneficial for optical imaging in clinical applications in the future.

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.

How Bulk Nanobubbles Might Survive

The existence of bulk nanobubbles has long been regarded with scepticism, due to the limitations of experimental techniques and the widespread assumption that spherical bubbles cannot achieve stable equilibrium. We develop a model for the stability of bulk nanobubbles based on the experimental observation that the zeta potential of spherical bubbles abruptly diverges from the planar value below 10  μm. Our calculations recover three persistently reported-but disputed-properties of bulk nanobubbles: that they stabilize at a typical radius of ∼100  nm, that this radius is bounded below 1  μm, and that it increases with ionic concentration.