Nanobubbles and the nanobubble bridging capillary force

Ozone nanobubble technology as a novel AOPs for pollutants degradation under high salinity conditions

Conventional water treatment systems frequently exhibit diminished efficiency at high salinity - a significant issue especially for real industrial effluents - mostly due to the creation of intricate structures between pollutants and salts. One of the primary obstacles associated with high salinity conditions is the generation of by-products that pose additional hurdles for treatment. In this work, we have investigated the novel advanced oxidation process a so-called ozone nanobubble technology for degradation of the pollutants at high salinity conditions. The mass transfer is often the rate-limiting step in gas-liquid process and the poor rate of mass transfer diminishes the overall efficacy. One of the primary disadvantages associated with ozone is its restricted solubility and instability when dissolved in an aqueous solution. These characteristics impose limitations on its potential applications and need the use of specialized systems to facilitate gas-liquid interaction. In this work, we have also suggested enhancing the ozonation process through the utilization of ozone nanobubbles. The findings of our experiment and subsequent analysis indicate that the presence of nanobubbles enhances the process of ozonation through three key mechanisms: (i) an increased mass transfer coefficient, (ii) a higher rate of reactive oxygen species (ROS) generation attributed to the charged interface, and (iii) the nanobubble interface serving as an active surface for chemical reactions. The predicted mass transfer coefficients were found to range from 3 to 3.5 min-1, a value that is notably greater than that seen for microbubbles. The study showcased the degradation of methylene blue dye through the utilization of ozone nanobubbles, which exhibited a much higher rate of dye degradation compared to ozone microbubbles. The confirmation of the radical degradation mechanism was achieved by the utilization of electron spin resonance (ESR) measurements. The developed process has high potential for application in industrial scale textile wastewater treatment.

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.

Investigating the Potential of CO2 Nanobubble Systems for Enhanced Oil Recovery in Extra-Low-Permeability Reservoirs

Carbon Capture, Utilization, and Storage (CCUS) stands as one of the effective means to reduce carbon emissions and serves as a crucial technical pillar for achieving experimental carbon neutrality. CO2-enhanced oil recovery (CO2-EOR) represents the foremost method for CO2 utilization. CO2-EOR represents a favorable technical means of efficiently developing extra-low-permeability reservoirs. Nevertheless, the process known as the direct injection of CO2 is highly susceptible to gas scrambling, which reduces the exposure time and contact area between CO2 and the extra-low-permeability oil matrix, making it challenging to utilize CO2 molecular diffusion effectively. In this paper, a comprehensive study involving the application of a CO2 nanobubble system in extra-low-permeability reservoirs is presented. A modified nano-SiO2 particle with pro-CO2 properties was designed using the Pickering emulsion template method and employed as a CO2 nanobubble stabilizer. The suitability of the CO2 nanobubbles for use in extra-low-permeability reservoirs was evaluated in terms of their temperature resistance, oil resistance, dimensional stability, interfacial properties, and wetting-reversal properties. The enhanced oil recovery (EOR) effect of the CO2 nanobubble system was evaluated through core experiments. The results indicate that the CO2 nanobubble system can suppress the phenomena of channeling and gravity overlap in the formation. Additionally, the system can alter the wettability, thereby improving interfacial activity. Furthermore, the system can reduce the interfacial tension, thus expanding the wave efficiency of the repellent phase fluids. The system can also improve the ability of CO2 to displace the crude oil or water in the pore space. The CO2 nanobubble system can take advantage of its size and high mass transfer efficiency, among other advantages. Injection of the gas into the extra-low-permeability reservoir can be used to block high-gas-capacity channels. The injected gas is forced to enter the low-permeability layer or matrix, with the results of core simulation experiments indicating a recovery rate of 66.28%. Nanobubble technology, the subject of this paper, has significant practical implications for enhancing the efficiency of CO2-EOR and geologic sequestration, as well as providing an environmentally friendly method as part of larger CCUS-EOR.

Application and development of foam extraction technology in wastewater treatment: A review

With the advancement of technology, wastewater treatment has become a significant challenge limiting the clean and sustainable development of chemical and metallurgical industries. Foam extraction, based on interfacial separation and mineral flotation, has garnered considerable attention as a wastewater treatment technology due to its unique physicochemical properties. Although considerable excellent accomplishments were reported, there still lacks a comprehensive summary of process features and contaminant removal mechanisms via foam extraction. According to the latest research progresses, the principles and characteristics of foam extraction technology, the classification and application of flotation reagents are systematically summarized in this work. Then comprehensively commented on the application fields and prospects of iterative flotation technology such as ion flotation, adsorption flotation and floating-extraction. The shortcomings and limitations of the current foam extraction technologies were discussed, and the feasible process intensification techniques were highlighted. This review aims to enchance the understanding of the foam extraction mechanism, and provides guidance for the selection appropriate reagents and foam extraction technologies in wastewater treatment.

Molecular Dynamics Study of the Microscopic Mechanical Balance at the Three-Phase Contact Line of Interfacial Nanobubble

This study reveals the microscopic mechanical balance at the three-phase contact line (TPCL) of an interfacial nanobubble on a substrate with a wettability pattern using molecular dynamics simulations. The apparent contact angle was compared to that evaluated using Young's equation, in which the interfacial tensions were computed using a mechanical route. The comparison was conducted by changing the wettability of the substrate from hydrophilic to neutral while maintaining a hydrophobic region in the center of the substrate. When the wettability pattern pins the TPCL at the wettability boundary, the contact angle computed by Young's equation is larger than the apparent contact angle because a pinning force exists in the inward direction of the nanobubble. Conversely, on the surfaces where the wettability pattern does not pin the TPCL, the contact angle computed by Young's equation agrees with the apparent contact angle because the pinning force disappears. The distribution of principal stresses around the TPCL, which was visualized for the first time in this study, indicates that large compressive principal stresses exist between the liquid phase and the solid substrate interface, which pin the TPCL at the surface wettability boundary, and that the maximum principal stress occurs in the inward direction of the nanobubbles at the TPCL. The normalized pinning force estimated from the maximum principal stress is equivalent to that measured experimentally.

Understanding the Role of Surface Charge on Nanobubble Capillary Bridging during Particle-Particle Interaction

The interactions between particles due to long-range hydrophobic forces have been extensively investigated. The hydrophobic force is likely a capillary force that arises from the formation of capillary bridges due to the merging of nanobubbles. In this study, we aim to investigate the impact of the nanobubble surface charge on the capillary bridge and, subsequently, the interaction between particles. The surface charge of the nanobubbles was altered in the presence of various surfactants (cationic, anionic, and nonionic) and salts (mono-, di-, and trivalent). The particle-particle interaction was quantified by measuring the aggregate size of the hydrophobized glass particles. Both experimental and theoretical findings confirm that the interaction between particles was enhanced when the surface potential of the nanobubble was around the neutral regime. This is probably because, when the surface potential was close to neutral, the interaction between two surface-deposited nanobubbles dominated over electrostatic repulsion, which was more conducive to the formation of the nanobubble capillary bridge. The estimation of the constrained Gibbs potential also showed the capillary bridge to be more stable when surface charge density along the bridge gas-liquid interface was minimal.

Monitoring effects of hydrodynamic cavitation pretreatment of sodium oleate on the aggregation of fine diaspore particles through small-angle laser scattering

Hydrodynamic cavitation (HC) enhanced fine particle aggregation could be largely due to the generation of tiny bubbles and their role in bridging particles. However, the lack of adequate characterizations of aggregates severally limits our further understanding of the associated aggregation behaviors. In this study, the aggregation of fine diaspore particles was comparatively investigated in sodium oleate (NaOl) solutions with and without HC pretreatment through the small-angle laser scattering (SALS) technique in a shear-induced aggregation (SIA) system. Results showed that HC pretreatment caused the formation of bulk nanobubbles (BNBs), which significantly modified the particle interactions and thereby modified the size and mass fractal dimension (Df) of aggregates under different SIA conditions. Although HC pretreatment did not noticeably alter the gradual change trend of aggregate size and structure characteristics under specific variables, BNBs bridging facilitated the aggregation process towards the diffusion-limited cluster aggregation model, resulting in the formation of larger but looser aggregates. This effect was more pronounced under relatively high NaOl concentrations. Apart from BNBs, the aggregation was also affected by cavitation bubbles formed during shear cavitation, which was more significant under high stirring intensity conditions (i.e., 1800 rpm) than the low stirring intensity conditions (i.e., 600 rpm).

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.

Long-range hydrophobic force enhanced interfacial photocatalysis for the submerged surface anti-biofouling

Developing anti-biofouling and anti-biofilm techniques is of great importance for protecting water-contact surfaces. In this study, we developed a novel double-layer system consisting of a bottom immobilized TiO2 nanoflower arrays (TNFs) unit and an upper superhydrophobic (SHB) coating along with the assistance of nanobubbles (NBs), which can significantly elevate the interfacial oxygen level by establishing the long-range hydrophobic force between NBs and SHB and effectively maximize the photocatalytic reaction brought by the bottom TNFs. The developed NBs-SHB/TNFs system demonstrated the highest bulk chemical oxygen demand (COD) reduction efficiency at approximately 80% and achieved significant E. coli and Chlorella sp. inhibition efficiencies of 5.38 and 1.99 logs. Meanwhile, the system showed a sevenfold higher resistance to biofilm formation when testing in a wastewater matrix using a wildly collected biofilm seeding solution. These findings provide insights for implementing nanobubble-integrated techniques for submerged surface protection.

Nanobubble-Induced Aggregation of Ultrafine Particles: A Molecular Dynamics Study

Nanobubble-induced aggregation (NBIA) of fine and ultrafine particles in liquid is a promising method for enhancing floatation rates in mineral processing, cleaning contaminants from water, and reviving marine ecosystems. Although the current experimental techniques can measure the nanobubble capillary force between two surfaces with controlled approach speed, they are not capable of imaging NBIA dynamics of fine/ultrafine particles by real-time observation with nanoscale spatial resolution. In this work, we use molecular dynamics (MD) simulations to study dynamics of NBIA of Ag particles in a Lennard-Jones fluid system. The molecular-level modeling allows us to study microscopic details of NBIA dynamics that are inaccessible by current experimental means. Using MD simulations, we investigated the effects of NB size, surface wettability, surface roughness, and contact line pinning on NBIA dynamics. Our modeling results show that both concave NB bridges between two hydrophobic surfaces and convex NB bridges between two hydrophilic surfaces can result in an attractive nanobubble capillary force (NBCF) that causes the aggregation of Ag particles in liquids. The equilibrium separation between two fully aggregated particles can be well predicted by the improved capillary force model. We also observe that the change of contact angle after the contact line pinning occurs at the sharp edge of a particle, which slows the aggregation process. Our thermodynamics analysis shows that there is a critical contact angle below which the merged surface NBs will detach from the surface instead of causing aggregation. The prediction of the critical contact angle is corroborated by our MD simulation results.

Nanobubbles: a promising efficient tool for therapeutic delivery of antibacterial agents for the Staphylococcus aureus infections

The current research is focused to address the implementation of nanobubbles technology to antibacterial agents against Staphylococcus aureus infections. Nanobubbles technology is a novel, latest research employed in many medical fields including drug discovery. In this present work, supramolecular nanoliquid formulation of potential antiseptic agent chloroxylenol-based Dettol and its enhanced antibacterial activity, biocompatibility assessment was studied. Nanobubble technology was adopted to prepare nanoformulation (NB-D) using a household hand mixer under thermostatically controlled conditions. A high-stability nanoformulation with high potential antibacterial activity against human pathogenic strains of Pseudomonas aeruginosa and Staphylococcus aureus was produced by the nanobubbles created in the antiseptic solution. The overall vitality of both strains was significantly reduced in all dose tests on NB-D treatment as a result of the antibacterial activity as assessed by the well-diffusion assay, turbidometric microdilution assay, biofilm inhibition assay, and total count reduction assay. Biocompatibility of the NB-D formulation was studied by the determination of cytotoxicity against HaCaT-human keratinocytes and hemocytes. NB-D treatment did not induce any notable cytotoxic effect on HaCaT cells by showing none of the changes in cell morphology and architecture. No toxic effect on the hematocytes was observed in NB-D treatment. The enhanced antibacterial activity and best biocompatibility of NB-D result shows that the nanobubble technology could be used as an effective strategy for the formulation of antiseptics or disinfectants against high health risk infectious organisms. The novelty of the work is the formation of supramolecular nanoformulation on antiseptic agent which promised the results enhanced than the raw antiseptic agent.

Separation of emulsified crude oil from produced water by gas flotation: A review

The development and production of oil and gas fields would eventually result in a considerable amount of oily generated water, posing serious risks to humans and the environment. Nowadays, the oil concentration in the drainage stream of the produced water is strictly regulated, and many countries have established strict emission standards. As an indispensable oily wastewater treatment technology, flotation technology has attracted much attention because of its maturity, economy, practicality, and relative efficiency. Firstly, this paper summarizes and compares flotation techniques, such as dissolved gas flotation, induced gas flotation, electroflotation, and compact flotation units widely used in produced water treatment offshore in recent years. Considering the complexity of the mechanism of oil removal by air flotation, the mechanism of the oil droplet-bubble interaction is further discussed. The effects of flocculant, PH, and salinity on the oil droplet-bubble interaction in the flotation process were summarized from the perspective of the microscopic colloidal interface, which has a specific guiding role in improving the oil removal efficiency in the gas flotation process. Finally, the research status of produced water treatment by air flotation is summarized, and the feasible research direction is put forward.

Manipulating Trapped Nanobubbles Moving and Coalescing with Surface Nanobubbles

Trapped nanobubbles are observed nucleating at nanopits on a pitted substrate, while surface nanobubbles are usually formed on the smooth solid surface in water. In this work, trapped nanobubbles and surface nanobubbles were captured by a tapping-mode atomic force microscope (AFM) on a nanopitted substrate based on the temperature difference method. A single trapped nanobubble was manipulated to change into a surface nanobubble, then to change into the trapped nanobubble again. At the same time, surface nanobubbles can be moved to merge into a trapped nanobubble. Our results show that the scan load and the size of the scan area were the main factors that significantly affect the mobility of surface/trapped nanobubbles. The coalescence and mutual transformation of the two kinds of nanobubbles indicate that trapped nanobubbles and surface nanobubbles have the same chemical nature, which also provides vital experimental proof of the existence of nanobubbles in the course of contact line depinning. Our results are of great significance for understanding nanobubble stability and providing guidelines in some industrial applications.

The Influence of Air Nanobubbles on Controlling the Synthesis of Calcium Carbonate Crystals

Numerous approaches have been developed to control the crystalline and morphology of calcium carbonate. In this paper, nanobubbles were studied as a novel aid for the structure transition from vaterite to calcite. The vaterite particles turned into calcite (100%) in deionized water containing nanobubbles generated by high-speed shearing after 4 h, in comparison to a mixture of vaterite (33.6%) and calcite (66.3%) by the reaction in the deionized water in the absence of nanobubbles. The nanobubbles can coagulate with calcite based on the potential energy calculated and confirmed by the extended DLVO (Derjaguin-Landau-Verwey-Overbeek) theory. According to the nanobubble bridging capillary force, nanobubbles were identified as the binder in strengthening the coagulation between calcite and vaterite and accelerated the transformation from vaterite to calcite.

A Study of Mechanisms of Nanobubble-Enhanced Flotation of Graphite

This study was conducted to investigate the mechanisms of enhanced microfine flake graphite (MFG) flotation by nanobubbles generated based on the principle of hydrodynamic cavitation. The effects of nanobubbles on graphite surface properties were characterized in terms of the flotation kinetics, collector adsorption behavior, Zeta potential, IR spectra, contact angle, etc. The results show that the surface nanobubbles increased the hydrophobic attraction and reduced the electrostatic repulsion between the graphite particles and collector molecules, significantly improving the flotation selectivity and the kinetic flotation rate and promoting the agglomeration of MFG.

Synergistic Treatment of Congo Red Dye with Heat Treated Low Rank Coal and Micro-Nano Bubbles

In this study, the adsorption method and micro-nano bubble (MNB) technology were combined to improve the efficiency of organic pollutant removal from dye wastewater. The adsorption properties of Congo red (CR) on raw coal and semi-coke (SC) with and without MNBs were studied. The mesoporosity of the coal strongly increased after the heat treatment, which was conducive to the adsorption of macromolecular organics, such as CR, and the specific surface area increased greatly from 2.787 m²/g to 80.512 m²/g. MNBs could improve the adsorption of both raw coal and SC under different pH levels, temperatures and dosages. With the use of MNBs, the adsorption capacity of SC reached 169.49 mg/g, which was much larger than that of the raw coal at 15.75 mg/g. The MNBs effectively reduced the adsorption time from 240 to 20 min. In addition, the MNBs could ensure the adsorbent maintained a good adsorption effect across a wide pH range. The removal rate was above 90% in an acidic environment and above 70% in an alkaline environment. MBs can effectively improve the rate of adsorption of pollutants by adsorbents. SC was obtained from low-rank coal through a rapid one-step heating treatment and was used as a kind of cheap adsorbent. The method is thus simple and easy to implement in the industrial context and has the potential for industrial promotion.

Influence of Air Solubility on the Flotation Performance of Low-Rank Coal

Low-rank coal (LRC) contains large amounts of harmful impurities that must be processed before utilization. Flotation is an effective means for separating fine particles, which can be influenced by air solubility in water. In this work, deaerated water (DW), ordinary water (OW), and pressurized water (PW) were prepared to research the underlying mechanism of the effect of air solubility on the flotation performance of LRC. The results show that PW dissolves the greatest amount of air in the three kinds of water (DW, OW, and PW). The flotation performance of LRC in different water types is directly proportional to air solubility in aqueous solutions. In addition, the induction time of LRC in PW (600 ms) is significantly shorter than those in OW (1200 ms) and DW (4000 ms). Atomic force microscopy (AFM) studies reveal that typical interfacial nanobubbles (NBs) only form on a highly oriented pyrolytic graphite (HOPG) surface in PW due to the supersaturated air in water. Furthermore, the interaction between LRC particles and HOPG in PW is significantly stronger than those in both OW and DW, which is attributed to the capillary force of rgw nanobubble bridge formed between particles. The hydrophobic interaction enhanced by NBs is critically important for the attachment of LRC particles to macrobubbles in flotation. Overall, air solubility in water has a great effect on LRC flotation performance, and PW flotation technology can be extended to LRC purification.

Ion adsorption stabilizes bulk nanobubbles

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

Stimuli-responsive nanobubbles for biomedical applications

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

Mechanical Strength and Hydration Characteristics of Cement Mixture with Highly Concentrated Hydrogen Nanobubble Water

In this study, highly concentrated hydrogen nanobubble water was utilized as the blending water for cement mortar to improve its compressive and flexural strengths. Highly concentrated nanobubbles can be obtained through osmosis. This concentration was maintained by sustaining the osmotic time. The mortar specimens were cured for 28 days, in which the nanobubble concentration was increased. This improved their flexural strength by 2.25–13.48% and compressive strength by 6.41–11.22%, as compared to those afforded by plain water. The nanobubbles were densified at high concentrations, which caused a decrease in their diameter. This increased the probability of collisions with the cement particles and accelerated the hydration and pozzolanic reactions, which facilitated an increase in the strength of cement. Thermogravimetric analysis and scanning electron microscopy were used to confirm the development of calcium silicate hydrate (C-S-H) and hydration products with an increase in the nanobubble concentration. Quantitative analysis of the hydration products and the degree of hydration were calculated by mineralogical analysis.

Recent advances for understanding the role of nanobubbles in particles flotation

Traditional froth flotation is the primary method for the separation and upgrading of fine mineral particles. However, it is still difficult for micro-fine and low-quality minerals to effectively separate. It is generally believed that bubble miniaturization is of great significance to improve flotation efficiency. Due to their unique physical and chemical properties, the application of nanobubbles (NBs) in ore flotation and other fields has been widely investigated as an important means to solve the problems of fine particle separation. Therefore, a fundamental understanding of the effect of NBs on flotation is a prerequisite to adapt it for the treatment of fine and low-quality minerals for separation. In this paper, recent advances in the field of nanobubble (NB) formation, preparation and stability are reviewed. In particular, we highlight the latest progress in the role of NBs on particles flotation and focus in particular on the particle-particle and particle-bubble interaction. A discussion of the current knowledge gap and future directions is provided.

Hollow nanocapsules of NiFe hydroxides to enable doxorubicin delivery and combinational tumour therapy

In this study, fine hollow nanocapsules, consisting of NiFe hydroxides (denoted as H-NiFe(OH)_x), are designed and synthesized for the delivery of an anticancer drug (Doxorubicin, DOX) and tumour depletion. Owing to its fascinating characteristics of "Fe²⁺ preservation and regeneration", H-NiFe(OH)_x presents considerable Fenton activity for hydroxyl radical (˙OH) induction. Efficient delivery of DOX is ensured due to its hollow microstructure, and a typical pH-responsive drug release is enabled. More importantly, the intracellular DOX, in addition to its intrinsic antitumour properties, induces extra exogenous H₂O₂ which favors the production of ˙OH by H-NiFe(OH)_x in tumour cells. In consequence, remarkable in vitro and in vivo antitumour properties are successfully achieved. This drug delivery system is particularly inspirational to further studies in the exploration of intelligent therapeutic platforms for combinational tumour therapy.

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

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

Effect of ultrasonication on the flotation of fine graphite particles: Nanobubbles or not?

It has been reported that nanobubbles can be produced by ultrasonication. However, it remains unclear whether part of the contribution of ultrasonication on flotation performance can be attributed to the generation of nanobubbles. In this work, we systematically studied this point of ultrasonication by combining a series of techniques including flotation testing, AFM (atomic force microscope) measurement, and settling testing. AFM imaging showed that no surface nanobubbles were found at the HOPG-water interface before and after subjection to ultrasonication. Further, surface nanobubbles were generated with solution exchange before ultrasonciation and then they were subjected to ultrasonication. It was found that ultrasonication did not destroy the pre-existing surface nanobubbles at the HOPG (highly oriented pyrolytic graphite) -water interface. Settling tests and flotation tests indicate that ultrasonication has a negligible influence on the interaction between graphite particles and thus flotation performance. Nanobubbles were not one of the outcomes for ultrasonication.

The Effect of Dissolved Gases on the Short-Range Attractive Force between Hydrophobic Surfaces in the Absence of Nanobubble Bridging

The short-range attractive forces between hydrophobic surfaces are key factors in a wide range of areas such as protein folding, lipid self-assembly, and particle-bubble interaction such as in industrial flotation. Little is certain about the effect of dissolved (well-controlled) gases on the interaction forces, in particular in those systems where the formation of surface nanobubble bridges is suppressed. Here, we probe the short-range attractive force between hydrophobized silica surfaces in aqueous solutions with varying but well-controlled isotherms of gas solubility. The first contact approach force measurement method using AFM shows that decreasing gas solubility results in a decrease of the force magnitude as well as shortening of its range. The behavior was found to be consistent across all four aqueous systems and gas solubilities tested. Using numerical computations, we corroborate that attractive force can be adequately explained by a multilayer dispersion force model, which accounts for an interfacial gas enrichment (IGE), that results in the formation of a dense gas layer (DGL) adjacent to the hydrophobic surface. We found that the DGL on the hydrophobic surface is affected only by the concentration of dissolved gases and is independent of the salt type, used to control the gas solubility, which excludes the effect of electrical double-layer interactions on the hydrophobic force.

Study of Interactions between Interfacial Nanobubbles and Probes of Different Hydrophobicities

In this study, hydrophilic, medium hydrophobic, and strong hydrophobic probes are obtained via treatment with plasma and octadecyl trichlorosilane. The interaction between the probes and interfacial nanobubbles (INBs) is examined using atomic force microscopy. The results show that a hydrophilic probe can scan the true shape of the INBs, and the distance between the first inflection point and the zero point of the approach force curve is equal to the vertical height of the nanobubble. The medium hydrophobic probe caused severe deformation of INB morphologies in the horizontal direction during scanning; nevertheless, the complete shape of the INB is obtained using this probe by lowering the scanning parameters. However, the characteristic of the approach force curve proves that the size of the nanobubbles is underestimated. The strong hydrophobic probe deforms INB morphologies severely, whose size cannot be obtained. The maximum attractive force in the approach force curve and the adhesive force in the retract force curve obtained using the strong hydrophobic probe are approximately 6 and 12 nN, respectively, which are both higher than those of the hydrophilic and medium hydrophobic probes. It is reasoned that the liquid film is maintained between the hydrophilic probe and the INBs, the medium hydrophobic probe pierces the INBs slightly, while the strong hydrophobic probe punctures the liquid film and demonstrates a pinning effect.

Towards Understanding the Role of Surface Gas Nanostructures: Effect of Temperature Difference Pretreatment on Wetting and Flotation of Sulfide Minerals and Pb-Zn Ore

Surface nanobubbles at hydrophobic interfaces now attract much attention in various fields but their role in wetting-related phenomena is still unclear. Herein, we report the effect of a preliminary contact of "hot" solids with cold water previously proposed for generation of surface nanobubbles, on wettability of compact materials and flotation of particulate galena (PbS), sphalerite (ZnS), and Pb-Zn sulfide ore. Atomic force microscopy was applied to visualize the nanobubbles at galena crystals heated in air and contacted with cold water; X-ray photoelectron spectroscopy was used to characterize the surface composition of minerals. Contact angles measured with the sessile drop of cold water were found to increase when enhancing the support temperature from 0 to 80 °C for sphalerite and silica, and to pass a maximum at 40-60 °C for galena and pyrite (FeS2) probably due to oxidation of sulfides. The temperature pretreatment depressed the recovery of sulfides in collectorless schemes and improved the potassium butyl xanthate-assisted flotation both for single minerals and Gorevskoye Pb-Zn ore. The results suggest that the surface nanobubbles prepared using the temperature difference promote flotation if minerals are rather hydrophobic and insignificantly oxidized, so the addition of collector and activator (for sphalerite) is necessary.

New Insights into the Role of Surface Nanobubbles in Bubble-Particle Detachment

It is well recognized that an improved flotation recovery can be achieved by introducing nanobubbles to common flotation practice due to the increased capture efficiency between bubbles and particles. However, the specific role of nanobubbles in bubble-particle interactions (collision, attachment, and detachment) is not well understood. In the present study, we explore the role of surface nanobubbles in bubble-particle detachment. Surface nanobubbles were introduced via ethanol-water exchange and their presence was confirmed using laser scanning confocal microscopy (LSCM). The effect of surface nanobubbles on bubble-particle detachment behavior was then investigated using an oscillating bubble apparatus. Bubble-particle aggregate stability was evaluated using critical detachment amplitude. Further, bubble-particle detachment forces in the absence and presence of nanobubbles were measured directly using a micro-nano mechanical testing system. Using LSCM, numerous surface nanobubbles were observed on a glass surface after ethanol-water exchange, regardless of wettability. The number and lateral dimensions of generated nanobubbles on the hydrophilic surface were significantly smaller than that on the hydrophobic surface. Surface nanobubbles increased the stability of bubble-particle aggregates. Macroscopic air bubbles coalesce with the nanobubbles on the particle surface, increasing the pinning effect of the three-phase contact line and advancing contact angle. As a result, the capillary force between bubbles and particles increased in the presence of surface nanobubbles.

Stochastic induction time of attachment due to the formation of transient holes in the intervening water films between air bubbles and solid surfaces

HYPOTHESIS

Bubble attachment to hydrophobic solid surfaces is influenced by the liquid film instability. Inclusion of transiently formed holes within the film rather than the so-called hydrophobic force in the theory is expected to better describe and explain film rupture and triple contact line formation in the bubble-surface attachment process. The significance of surface hydrophobicity and hole formation renders the stochastic nature of the induction time of attachment.

EXPERIMENTS

A combination of high-speed video microscopy and theoretical analysis was applied to investigate the induction time of attachment and critical film thickness of air bubbles rising freely perpendicularly to silica surfaces of different hydrophobicities.

FINDINGS

Film rupture occurred statistically for shorter induction times and thicker films on the more hydrophobic surface, rejecting the conjecture of hydrophobic force. Computed results of the critical base radius of the transient holes causing film rupture were merged together nicely, independently of surface hydrophobicity. The paper sheds light on the significance of hydrophobicity on the attachment process by means of a novel and easily implemented methodology, without relying on the debatable hydrophobic force.

On importance of external conditions and properties of the interacting phases in formation and stability of symmetrical and unsymmetrical liquid films

Importance of external conditions and properties of phases creating liquid films, in outcome of the air bubble collisions with liquid/air and liquid/solids interfaces in clean water and in liquid solutions, is critically reviewed. The review is focussed on initial stages of the liquid films formation by bubbles colliding with interfaces, as well as, on analysis of the most important factors responsible for the collision's outcome, that is, either the rapid bubble bouncing or formation of the symmetrical or unsymmetrical liquid films and their thinning to the critical rupture thicknesses. Data on formation of liquid films under dynamic conditions, both in pure liquids and solutions of electrolytes and various surface-active substances, are reviewed and importance of hydrodynamic boundary conditions at interacting interfaces for energy balance in the system is discussed. It is shown that the liquid films stability, which in stagnant systems are directly determined by properties of the liquid/gas and liquid/solid interfaces, can be quite different in dynamic environment. A mechanism of the bubble bouncing from various interfaces in terms of interplay between energy exchange and kinetics of liquid film drainage is analyzed. It is shown that this mechanism is universal and irrelevant on the nature of interacting phases. Moreover, mechanisms responsible for wetting (unsymmetrical) film stability under dynamic conditions are discussed in light of the most recent studies, showing a crucial role of electrolyte, kind and concentration of surface-active substances, electrical surface charge, hydrophilic/hydrophobic properties of solids and presence of air entrapped (nano- and/or micro-bubbles) at surfaces of highly hydrophobic solids in the liquid films rupture.

Probing the "Gas Tunnel" between Neighboring Nanobubbles

Surface nanobubbles are the main gaseous domains forming at solid-liquid interfaces, and their abnormally long lifetime (stability) is still an open question. A hypothesis "gas tunnel" was presented in a recent simulation study [ACS Nano 2018, 12 (3), 2603-2609], which was thought to connect two neighboring nanobubbles and make the nanobubbles remain stable. Herein, we aim to experimentally investigate the existence of gas tunnel and its role in governing nanobubble dynamics. By using an atomic force microscope, mutual effects between different gaseous domains including nanobubbles, nanopancakes, and nanobubble-pancake composite on a PS substrate undergoing violent tip perturbation and their effects on the undisturbed neighbors were investigated. The pancake between two nanobubbles can behave as a visible gas tunnel under the tip-bubble interaction. Based on statistical analysis of volume change in the different gas domains, the concept of a generalized gas tunnel is presented and experimentally verified. Nanobubbles are surrounded by a water depletion layer which will act as a channel along solid/liquid surfaces for adjacent nanobubbles to communicate with each other. Moreover, the change in contact angle of nanobubbles with the concentration of local gas oversaturation was studied, and the equilibrium contact angle of nanobubbles is further verified experimentally.