Stable Air Nanobubbles in Water: the Importance of Organic Contaminants

Characterisation of Aqueous Ultra-high Homeopathic Potencies: Nanoparticle Tracking Analysis

BACKGROUND AND OBJECTIVES

 Over the past decade, research using various methods has claimed the material nature, including nanoparticles (NPs), of high homeopathic potencies. The current study aims to verify these findings using NP tracking analysis (NTA).

METHODS

 Six independent serial dilutions of commonly used homeopathic medicines-either soluble (Gelsemium, Pyrogenium, Kalium mur) or insoluble (Cuprum, Argentum, Silicea)-were prepared according to European Pharmacopoeia standards. We compared the homeopathic dynamisations (DYNs) in pure water with their potentised controls and with simple dilutions (DIL) up to 30cH/10-60. We also tested the influence of the container (glass or PET) on the solvent controls.

RESULTS

 We observed the presence of particles from 20 to 300-400 nm in all DYNs, DILs and controls, except in pure unstirred water. The sizes and size distributions of NPs in high homeopathic potencies were smaller than those in controls for soluble sources and larger for insoluble sources, even above 11cH. The opposite behaviour was observed in the number of NPs. When comparing DYN and DIL, the number, size, presence of aggregates or chains and brightness of NPs increased with DYNs, which was also observed above 11cH. Many NPs scattered light of low intensity, indicating the presence of material particles. The container had a significant effect on the number and size of NPs, indicating the involvement of the atmosphere and leaching processes.

CONCLUSION

 Homeopathic medicines contain NPs with specific properties, even when diluted beyond Avogadro's number. Homeopathic potentisation is not a simple dilution. The starting material, the solvent used, the type of container and the manufacturing method influence the characteristics of these NPs. The nature of these NPs is not known, but most likely they are a mixture of nanobubbles and elements from the atmosphere and container, including insoluble ones.

Hydroxide and Hydronium Ions Modulate the Dynamic Evolution of Nitrogen Nanobubbles in Water

It has been widely recognized that the pH environment influences the nanobubble dynamics and hydroxide ions adsorbed on the surface may be responsible for the long-term survival of the nanobubbles. However, understanding the distribution of hydronium and hydroxide ions in the vicinity of a bulk nanobubble surface at a microscopic scale and the consequent impact of these ions on the nanobubble behavior remains a challenging endeavor. In this study, we carried out deep potential molecular dynamics simulations to explore the behavior of a nitrogen nanobubble under neutral, acidic, and alkaline conditions and the inherent mechanism, and we also conducted a theoretical thermodynamic and dynamic analysis to address constraints related to simulation duration. Our simulations and theoretical analyses demonstrate a trend of nanobubble dissolution similar to that observed experimentally, emphasizing the limited dissolution of bulk nanobubbles in alkaline conditions, where hydroxide ions tend to reside slightly farther from the nanobubble surface than hydronium ions, forming more stable hydrogen bond networks that shield the nanobubble from dissolution. In acidic conditions, the hydronium ions preferentially accumulating at the nanobubble surface in an orderly manner drive nanobubble dissolution to increase the entropy of the system, and the dissolved nitrogen molecules further strengthen the hydrogen bond networks of systems by providing a hydrophobic environment for hydronium ions, suggesting both entropy and enthalpy effects contribute to the instability of nanobubbles under acidic conditions. These results offer fresh insights into the double-layer distribution of hydroxide and hydronium near the nitrogen-water interface that influences the dynamic behavior of bulk nanobubbles.

Nanobubbles improve peroxymonosulfate-based advanced oxidation: High efficiency, low toxicity/cost, and novel collaborative mechanism

Cl- activated peroxymonosulfate (PMS) oxidation technology can effectively degrade pollutants, but the generation of chlorinated disinfection byproducts (DBPs) limits the application of this technology in water treatment. In this study, a method of nanobubbles (NBs) synergistic Cl-/PMS system was designed to try to improve this technology. The results showed the synergistic effects of NBs/Cl-/PMS were significant and universal while its upgrade rate was from 12.89% to 34.97%. Moreover, the synergistic effects can be further improved by increasing the concentration and Zeta potential of NBs. The main synergistic effects of NBs/Cl-/PMS system were due to the electrostatic attraction of negatively charged NBs to Na+ from NaCl, K+ from PMS, and H+ from phenol, which acted as a "bridge" between Cl- and HSO5- as well as phenol and Cl-/HSO5-, increasing active substance concentration. In addition, the addition of NBs completely changed the oxidation system of Cl-/PMS from one that increases environmental toxicity to one that reduces it. The reason was that the electrostatic attraction of NBs changed the active sites and degradation pathway of phenol, greatly reducing the production of highly toxic DBPs. This study developed a novel environmentally friendly oxidation technology, which provides an effective strategy to reduce the generation of DBPs in the Cl-/PMS system.

Fluorocarbon Nanodroplets: Their Formation and Stability in Complex Solution Systems

Perfluorocarbon (PFC) nanodroplets (NDs) are expanding in a wide range of applications in biotechnology and nanotechnology. Their efficacy in biological systems is significantly influenced by their size uniformity and stability within bioelectrolyte contexts. Presently, methods for creating monodisperse, highly concentrated, and well-stabilized PFC NDs under harsh conditions using low energy consumption methods have not been thoroughly developed, and their stability has not been sufficiently explored. This gap restricts their applicability for advanced medical interventions in tissues with high pH levels and various electrolytic conditions. To tackle these challenges and to circumvent potential toxicity from surface stabilizers, we have conducted an in-depth investigation into the formation and stability of uncoated perfluorohexane (PFH) NDs, which were synthesized by using a low-energy consumption solvent exchange technique, across complex electrolyte compositions or a broad spectrum of pH levels. The results indicated that low concentrations of low-valent electrolyte ions facilitate the nucleation of NDs and consistently accelerate Ostwald ripening over an extended period. Conversely, high concentrations of highly valent electrolyte ions inhibit nucleation and decelerate the ripening process over time. Given the similarities between the properties of NDs and nanobubbles, we propose a potential stabilization mechanism. Electrolytes influence the Ostwald ripening of NDs by adjusting the adsorption and distribution of ions on the NDs' surface, modifying the thickness of the electric double layer, and fine-tuning the energy barrier between droplets. These insights enable precise control over the stability of PFC NDs through the meticulous adjustment of the surrounding electrolyte composition. This offers an effective preparation method and a theoretical foundation for employing bare PFC NDs in physiological settings.

Do Nanobubbles Exist in Pure Alcohol?

The existence of nanobubbles in pure water has been extensively debated in recent years, and it is speculated that nanobubbles may be ion-stabilized. However, nanobubbles in the alcohol-water mixture and pure alcohols are still controversial due to the lack of ions present in the alcohol system. This work tested the hypothesis that stable nanobubbles exist in pure alcohol. The ultrasound and oscillatory pressure fields are used to generate nanobubbles in pure alcohol. The size distribution, concentration, diameter, and scattering intensity of the nanobubbles were measured by nanoparticle tracking analysis. The light scattering method measures the zeta potential. The Mie scattering theory and electromagnetic wave simulation are utilized to estimate the refractive index (RI) of nanobubbles from the experimentally measured scattering light intensity. The average RI of the nanobubbles in pure alcohols produced by ultrasound and oscillating pressure fields was estimated to be 1.17 ± 0.03. Degassing the nanobubble sample reduces its concentration and increases its size. The average zeta potential of the nanobubbles in pure alcohol was measured to be -5 ± 0.9 mV. The mechanical stability model, which depends on force balance around a single nanobubble, also predicts the presence of nanobubbles in pure alcohol. The nanobubbles in higher-order alcohols were found to be marginally colloidally stable. In summary, both experimental and theoretical results suggest the existence of nanobubbles in pure alcohol.

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.

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.

On Nanobubble Dynamics under an Oscillating Pressure Field during Salting-out Effects and Its DLVO Potential

We have investigated the origin, stability, and nanobubble dynamics under an oscillating pressure field followed by the salting-out effects. The higher solubility ratio (salting-out parameter) of the dissolved gases and pure solvent nucleates nanobubbles during the salting-out effect, and the oscillating pressure field enhances the nanobubble density further as solubility varies linearly with gas pressure by Henry's law. A novel method for refractive index estimation is developed to differentiate nanobubbles and nanoparticles based on the scattering intensity of light. The electromagnetic wave equations have been numerically solved and compared with the Mie scattering theory. The scattering cross-section of the nanobubbles was estimated to be smaller than the nanoparticles. The DLVO potentials of the nanobubbles predict the stable colloidal system. The zeta potential of nanobubbles varied by generating nanobubbles in different salt solutions, and it is characterized by particle tracking, dynamic light scattering, and cryo-TEM. The size of nanobubbles in salt solutions was reported to be higher than that in pure water. The novel mechanical stability model is proposed by considering both ionic cloud and electrostatic pressure at the charged interface. The ionic cloud pressure is derived by electric flux balance, and it is found to be twice the electrostatic pressure. The mechanical stability model for a single nanobubble predicts the existence of stable nanobubbles in the stability map.

Clean production and characterization of nanobubbles using laser energy deposition

We have demonstrated the production of laser bulk nanobubbles (BNB) with ambient radii typically below 500 nm. The gaseous nature of the nanometric objects was confirmed by a focused acoustic pulse that expands the gas cavities to a size that can be visualized with optical microscopy. The BNBs were produced on demand by a collimated high-energy laser pulse in a "clean" way, meaning that no solid particles or drops were introduced in the sample by the generation method. This is a clear advantage relative to the other standard BNB production techniques. Accordingly, the role of nanometric particles in laser bubble production is discussed. The characteristics of the nanobubbles were evaluated with two alternative methods. The first one measures the response of the BNBs to acoustic pulses of increasing amplitude to estimate their rest radius through the calculation of the dynamics Blake threshold. The second one is based on the bubble dissolution dynamics and the correlation of the bubble's lifetime with its initial size. The high reproducibility of the present system in combination with automated data acquisition and analysis constitutes a sound tool for studying the effects of the liquid and gas properties on the stability of the BNBs solution.

Ostwald ripening of aqueous microbubble solutions

Bubble solutions are of growing interest because of their various technological applications in surface cleaning, water treatment, and agriculture. However, their physicochemical properties, such as the stability and interfacial charge of bubbles, are not fully understood yet. In this study, the kinetics of radii in aqueous microbubble solutions are experimentally investigated, and the results are discussed in the context of Ostwald ripening. The obtained distributions of bubble radii scaled by mean radius and total number were found to be time-independent during the observation period. Image analysis of radii kinetics revealed that the average growth and shrinkage speed of each bubble is governed by diffusion-limited Ostwald ripening, and the kinetic coefficient calculated using the available physicochemical constants in the literature quantitatively agrees with the experimental data. Furthermore, the cube of mean radius and mean volume exhibit a linear time evolution in agreement with the Lifshitz-Slezov-Wagner (LSW) theory. The coefficients are slightly larger than those predicted using the LSW theory, which can be qualitatively explained by the effect of finite volume fraction. Finally, the slowdown and pinning of radius in the shrinkage dynamics of small microbubbles are discussed in detail.

The fate of bulk nanobubbles under gas dissolution

Artificially added or undesired organic and inorganic contaminants in solution that are interfacially active always tend to be adsorbed at the gas-liquid interface of micro- and nano-bubbles, affecting the stability of the tiny bubbles. In this work, by using molecular dynamics simulations we study how the adsorbed surfactant-like molecules, with their amphiphilic character, affect the dissolution of the existing bulk nanobubbles under low gas supersaturation environments. We find that, depending on the concentration of the dissolved gas and the molecular structure of surfactants, two fates of bulk nanobubbles whose interfaces are saturated by surfactants are found: either remaining stable or being completely dissolved. With gas dissolution, the bubble shrinks and the insoluble surfactants form a monolayer with an increasing areal density until an extremely low (close to 0) surface tension is reached. In the limit of vanishing surface tension, the chemical structure of surfactants crucially affects the bubble stability by changing the monolayer elastic energy. Two basic conditions for stable nanobubbles at low gas saturation are identified: vanishing surface tension due to bubble dissolution and positive spontaneous curvature of the surfactant monolayer. Based on this observation, we discuss the similarity in the stability mechanism of bulk nanobubbles and that of microemulsions.

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.

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.

Inhibited nanobubble transport in a saturated porous medium: Effects of deposited colloidal particles

Nanobubbles have recently attracted much interest for their practical use and potential applications in environmental issues. The pre-existence of deposited colloidal particles in porous media commonly occurs when nanobubbles applied to porous media interact with deposited colloidal particles. However, the current understanding of the effect of the interactions with pre-deposited colloidal particles on nanobubble transport in saturated porous media remains incomplete, and the effects are often overlooked. Therefore, we performed 1D column experiments with sequential injections of colloidal and nanobubble suspensions to study the effect of pre-deposited materials on the retention and release of colloids and nanobubbles in packed glass beads. In this study, we used resonant mass measurements to measure the number concentrations of colloids and nanobubbles during transport experiments for the first time to distinguish between coexisting solid colloidal particles and nanobubbles with different buoyancies in mixed effluent during transport. The nanobubble retention increased because of the pre-existence of deposited colloidal particles, indicating that the deposited colloidal particles act as additional deposition sites and physical obstacles for nanobubbles through physicochemical (including hydrophobic) interactions. This study also provides a future reference for the applicability of resonant mass measurement to cotransport experiments of different buoyant particles, including colloids, nanobubbles, and oil droplets.

Dynamics of aqueous peptide solutions in folded and disordered states examined by dynamic light scattering and dielectric spectroscopy

Characterizing the segmental dynamics of proteins, and intrinsically disordered proteins in particular, is a challenge in biophysics. In this study, by combining data from broadband dielectric spectroscopy (BDS) and both depolarized (DDLS) and polarized (PDLS) dynamic light scattering, we were able to determine the dynamics of a small peptide [ε-poly(lysine)] in water solutions in two different conformations (pure β-sheet at pH = 10 and a more disordered conformation at pH = 7). We found that the segmental (α-) relaxation, as probed by DDLS, is faster in the disordered state than in the folded conformation. The water dynamics, as detected by BDS, is also faster in the disordered state. In addition, the combination of BDS and DDLS results allows us to confirm the molecular origin of water-related processes observed by BDS. Finally, we discuss the origin of two slow processes (A and B processes) detected by DDLS and PDLS in both conformations and usually observed in other types of water solutions. For fully homogeneous ε-PLL solutions at pH = 10, the A-DLS process is assigned to the diffusion of individual β-sheets. The combination of both techniques opens a route for understanding the dynamics of peptides and other biological solutions.

Free-Radical Generation from Bulk Nanobubbles in Aqueous Electrolyte Solutions: ESR Spin-Trap Observation of Microbubble-Treated Water

Microbubbles are very fine bubbles that shrink and collapse underwater within several minutes, leading to the generation of free radicals. Electron spin resonance spectroscopy (ESR) confirmed the generation of hydroxyl radicals under strongly acidic conditions. The drastic environmental change caused by the collapse of the microbubbles may trigger radical generation via the dispersion of the elevated chemical potential that had accumulated around the gas-water interface. The present study also confirmed the generation of ESR signals from the microbubble-treated waters even after several months had elapsed following the dispersion of the microbubbles. Bulk nanobubbles were expected to be the source of the spin-adducts of hydroxyl radicals. Such microbubble stabilization and conversion might be caused by the formation of solid microbubble shells generated by iron ions in the condensed ionic cloud around the microbubble. Therefore, the addition of a strong acid might cause drastic changes in the environment and destroy the stabilized condition. This would restart the collapsing process, leading to hydroxyl radical generation.

Acoustic Characteristics of Biosynthetic Bubbles for Ultrasound Contrast Imaging

Biosynthetic bubbles produced by floating microorganisms, such as bacteria and algae, have recently attracted wide attention as novel ultrasound contrast agents owing to their significant potential in ultrasound imaging and acoustic reporter gene-based imaging. However, the acoustics properties of these bubbles are unclear. In this study, we developed a finite-element model to describe the oscillation of nonspherical biosynthetic bubbles composed of a gas core encapsulated in a protein shell. In this model, the elastic properties of the bubble shells were characterized in terms of the density, thickness, Young's modulus, and Poisson's ratio. Theoretical calculations were performed for a single bubble and an assembly of randomly oriented bubbles. Our results demonstrate that (1) there are many types of surface oscillation modes for nonspherical biosynthetic bubbles, and a systematic relationship exists between the surface modes and the resonance frequencies; (2) the bubble shell shape has a significant effect on the acoustic behavior; (3) the resonance frequency of an ellipsoidal bubble decreases with the decrease in its polar radius-to-equatorial axis ratio; and (4) the acoustic scattering of a randomly oriented suspension is isotropic at and below the first resonance frequency. Our findings provide physical insight into the biomedical applications of biosynthetic bubbles and can be used to optimize the acoustics properties of such bubbles.

Effect of Sodium Oleate on the Adsorption Morphology and Mechanism of Nanobubbles on the Mica Surface

Nanobubbles promote the flotation of fine-grained minerals. In the associated mechanism, the aggregation of fine particles is first promoted, which increases the probability of collision between particles and bubbles. However, the interaction between nanobubbles and mineral particles is often neglected, especially when the surface properties of the nanobubbles are modified by flotation collectors. In this study, the interaction mechanism between nanobubbles and the mica surface is investigated by nanoparticle tracking analysis, zeta potential measurement, and atomic force microscopy. The results reveal that the hydrophobic group of sodium oleate points toward the inside of the nanobubble and the hydrophilic group faces outward after the interaction of sodium oleate molecules and nanobubbles. A surfactant micelle with nanobubbles as the core is formed, thus considerably reducing the concentration of sodium oleate to form micelles. The adsorption of the modified nanobubbles on the mineral surface is carried out by the specific adsorption of the exposed hydrophilic group and the mineral surface. This adsorption method is superior to the hydrophobic interaction between the nanobubbles and the hydrophobic mineral surface. Further, the nanobubbles are highly selective for the activation sites on the mineral surface in the adsorption mode. This study will help understand the interaction between nanobubbles and collectors to further apply nanobubbles to treat fine-grained mineral particles.

Stability of Surface Nanobubbles without Contact Line Pinning

Although the stability of most surface nanobubbles observed can be well interpreted by contact line pinning and supersaturation theory, there is increasing evidence that at least for certain situations, contact line pinning is not required for nanobubble stability. This raises a significant question of what is the stability mechanism for those sessile nanobubbles. Through theoretical analysis and molecular dynamics simulations, in this work, we report two mechanisms for stabilizing surface nanobubbles on flat and homogeneous substrates. One is attributed to constant adsorption of trace impurities on the nanobubble gas?liquid interface, through which nanobubble growing or shrinking causes the increase and decrease of interfacial tension, acting as a restoring force to bring the nanobubble to its equilibrium size. The other is attributed to the deformation of a soft substrate induced by the formed nanobubble, which in turn stabilizes the nanobubble via impeding the contact line motion, similar to self-pinning of microdroplets on soft substrates. Both mechanisms can interpret, depending on the specified conditions, how surface nanobubbles can remain stable in the absence of contact line pinning.

Armoured nanobubbles; ultrasound contrast agents under pressure

HYPOTHESIS

Robust methods for differentiating long-lived nanobubbles from other nanoparticles are required. Evaluation of the density and compressibility of nanoparticles should enable nanobubbles to be differentiated from other nanoparticles, although the response of nanobubbles to pressure can be strongly influenced by a coating of insoluble surfactant. Here we evaluate the response of nanobubbles armoured with a coating of insoluble surfactants in order to determine if they can be differentiated from other nanoparticles.

EXPERIMENTS

Dynamic light scattering was used to size candidate nanoparticles under the influence of external pressure and resonant mass measurements were employed to assess the density of candidate nanoparticles.

FINDINGS

The resonant mass measurement revealed a significant population of lipid-coated gas nanobubbles. These nanobubbles are proven to be gas entities, by their response to application of pressure. The pressure at which the gas within the nanobubbles condenses is shifted to higher pressure due to the mechanical resistance of the lipid shell, which shields the bubble contents from up to ∼0.8 atm. of the external pressure The presence of lipids of low solubility at the nanobubble-solution interface effectively results in a negative Laplace pressure, which stabilizes these nanobubbles against dissolution.

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.