Adhesion between nanobubbles and fine cassiterite particles

Adhesion is an important process of particle-bubble interaction in fine particle(-10μm)flotation.This paper studied the adhesion process and mechanism between nanobubbles and fine cassiterite particles by using a high-speed camera,atomic force microscope(AFM),adsorption capacity tests,and induction time tests.After being pretreated with nanobubbles(NBs)water,fine cassiterite particles flotation tests were carried out using caprylhydroxamic acid(CHA)as a collector.The results showed that NBs can improve the recovery and flotation rate of fine cassiterite while decreasing the collector dosage.The adsorption capacity test indicated that the cassiterite treated with NBs had lower demand for collector concentration.The AFM imaging results further demonstrate that NBs could reduce the adsorption of CHA on the surface of minerals.Since NBs played a part of the role of collector,it can improve the flotation effect while reducing the amount of collector.The induction time test and the high-speed camera observation test showed that NBs promoted the attachment between bubbles and cassiterite particles.On the other hand,NBs agglomerate cassiterite particles,increasing the probability of particles colliding with bubbles.

Formation of nanobubbles generated by hydrate decomposition:A molecular dynamics study

Natural gas hydrate is estimated to have huge reserves. Its exploitation can solve the global oil and gas shortage problem. Hydrates decompose into water and methane, and methane molecules are supersaturated to form nanobubbles.Methane nanobubbles can affect the decomposition efficiency of hydrates. They can provide abundant methane sources for the re-nucleation of hydrates. Molecular dynamics is employed in this study to investigate the decomposition process of type I methane hydrate and the formation of methane nanobubbles generated during decomposition under different methane mole fraction, pressures, and temperatures. The results indicate that external pressure inhibits the diffusion of methane molecules, thereby preventing the formation of nanobubbles. A higher mole fraction of methane molecules in the system requires a higher external pressure to generate stable nanobubbles after the decomposition of the hydrate structure.At 330 K, it is easy to form a nanobubble structure. Results of this study can help provide ideas for the study of efficient extraction and secondary nucleation of hydrates.

Inhibition mechanism of air nanobubbles on brass corrosion in circulating cooling water systems

Air nanobubbles(A-NBs)were used to inhibit the brass corrosion in circulating cooling water for the first time in the study.The results of mass loss method and electrochemical method showed that A-NBs had the obvious corrosion inhibition effect.The inhibition rate reached 52%at 35℃.The impedance and surface characterization results of corrosion samples indicated that the corrosion inhibition mechanisms of A-NBs mainly included adsorption of corrosion ions,promoting the formation of the passivation film on metal surface and the formation of the bubble layer and scale film on metal surface.A-NBs are potential excellent corrosion inhibitors.

Effect of temperature on the morphology of nanobubbles at mica/water interface

The great implication of nanobubbles at a solid/water interface has drawn wide attention of the scientific community and industries. However, the fundamental properties of nanobubbles remain unknown as yet. In this paper, the temperature effects on the morphology of nanobubbles at the mica/water interface are explored through the combination of AFM direct image with the temperature control. The results demonstrate that the apparent height of nanobubbles in AFM images is kept almost constant with the increase of temperature, whilst the lateral size of nanobubbles changes significantly. As the temperature increases from 28℃ to 42℃, the lateral size of nanobubbles increases, reaching a maximum at about 37℃, and then decreases at a higher temperature. The possible explanation for the size change of nanobubbles with temperature is suggested.

Interfacial nanobubbles produced by long-time preserved cold water

Interfacial gaseous nanobubbles which have remarkable properties such as unexpectedly long lifetime and significant potential applications, are drawing more and more attention. However, the recent dispute about the contamination or gas inside the nanobubbles causes a large confusion due to the lack of simple and clean method to produce gas nanobubbles. Here we report a convenient and clean method to effectively produce interfacial nanobubbles based on a pure water system. By adding the cold water cooled at 4 ℃ for more than 48 h onto highly oriented pyrolytic graphite （HOPG） surface, we find that the average density and total volume of nanobubbles are increased to a high level and mainly dominated by the concentrations of the dissolved gases in cold water. Our findings and methods are crucial and helpful for settling the newly arisen debates on gas nanobubbles.

Flotation kinetics performance of different coal size fractions with nanobubbles

The flotation kinetics of different size fractions of conventional and nanobubble(NB) flotation were compared to investigate the effect of NBs on the flotation performance of various coal particle sizes. Six flotation kinetics models were selected to fit the flotation data, and NBs were observed on a hydrophobic surface under hydrodynamic cavitation by atomic force microscope scanning. Flotation results indicated that the best flotation performance of size fraction at-0.125+0.074 mm can be obtained either in conventional or NB flotation. NBs increase the combustible recovery of almost all the size fractions, but they increase the product ash content of-0.25+0.074 mm and reduce the product ash content of-0.045 mm at the same time. The first-order models can be used to fit the flotation data in conventional and NB flotation, and the classical first-order model is the most suitable one. NBs considerably enhance flotation rate on coarse size fraction(-0.5+0.25 mm) but decrease the flotation rate of the medium size(-0.25+0.074 mm). The improvement of flotation speed on fine coal particles(-0.074 mm) is probably the reason for the improved performance of raw sample flotation.

Surface nanobubbles on the hydrophobic surface and their implication to flotation

Nanobubbles play a potential role in the application of the flotation of fine particles.In this work,the identification of nanoentities was performed with a contact mode atomic force microscope(AFM).Moreover,the influences of setpoint ratio and amplitude of the cantilever and the responses of the formed surface nanobubbles to the fluctuation of pH,salt concentration,and surfactant concentration in the slurry were respectively studied.Nanobubbles were reported on the highly oriented pyrolytic graphite(HOPG)surface as the HOPG was immersed in de-ionized water under ambient temperature.The coalescence of nanobubbles occurred under contact mode,which provides strong evidence of the gaseous nature of these nanostructures on HOPG.The measuring height of the surface nanobubbles decreased with the setpoint ratio.The changes in the pH and concentration of methyl isobutyl carbinol(MIBC)show a negligible influence on the lateral size and height of the preex-isting surface nanobubbles.The addition of LiCl results in a negligible change of the lateral size;however,an obvious change is noticed in the height of surface nanobubbles.The results are expected to provide a valuable reference in understanding the properties of surface nanobubbles and in the design of nanobubble-assisted flotation processes.

The properties of surface nanobubbles formed on different substrates

The properties and stability of the reported surface nanobubbles are related to the substrate used and the generation method. Here, we design a series of experiments to study the influence of the hydrophobicity of the substrate and the produc- tion method on the formation and properties of nanobubbles. We choose three different substrates, dodecyltrichlorosilane （DTS） modified silicon, octadecyltrichlorosilane （OTS） modified silicon, and highly oriented pyrolytic graphite （HOPG） as nanohubble substrates, and two methods of ethanol-water exchange and 4-℃ cold water to produce nanobubbles. It is found that using ethanol-water exchange method could produce more and larger nanobubbles than the 4-℃ cold water method. The contact angle of nanobubbles produced by ethanol-water exchange depends on the hydrophobicity of sub- strates, and decreases with the increase of the hydrophobicity of substrates. More interestingly, nanoscopic contact angle approaches the macroscopic contact angle as the hydrophobicity of substrates increases. It is believed that these results would be very useful to understand the stability of surface nanobubbles.

Multiplicity of thermodynamic states of van der Waals gas in nanobubbles

The gas-containing nanobubbles have attracted extensive attention due to their remarkable properties and extensive application potential.However,a number of fundamental aspects of nanobubbles,including thermodynamic states for the confined gas,remain still unclear.Here we theoretically demonstrate that the van der Waals(vd W)gases confined in nanobubbles exhibit a unique thermodynamic state of remarkably deviating from the bulk gas phase,and the state transition behavior due to the sizedependent Laplace pressure.In general,the vd W gas inside nanobubbles present multiple stable or transient states,where 0–2 states are for supercritical gas and 0–4 for subcritical gas.Our further analysis based on Rayleigh–Plesset equation and free energy determination indicates that the gas states in nanobubbles exhibits different levels of stability,from which the coexistence of multiple bubble states and microphase equilibrium between droplets and bubbles are predicted.This work provides insight to understand the thermodynamic states appeared for gas in nanobubbles.

Lactoferrin-Conjugated Polylactic Acid Nanobubbles Encapsulated Perfluoropentane as a Contrast Agent for Ultrasound/Magnetic Resonance Dual-Modality Imaging

The development of contrast agents that can be activated by multiple modes is of great significance for tumor diagnosis.In this study,the lactoferrin(Lf)-conjugated polylactic acid(PLLA)nanobubbles(Lf-PLLA NBs)were used to encapsulate liquid perfluoropentane(PFP)with the double emulsion method,creating PFP loaded(PFP/Lf-PLLA)NBs for the ultrasound/magnetic resonance dual-modality imaging of subcutaneous tumor.The parti-cle diameter and stability of nanobubbles were investigated by photon correlation spectroscopy.The biocompat-ibility of nanobubbles was preliminarily evaluated by cell proliferation and migration assay,hemolysis rate,and blood biochemistry analysis.A B-mode clinical ultrasound real-time imaging system was used to perform ultra-sonic imaging in vivo.Magnetic resonance imaging in vivo was applied with a clinical 3.0 T magnetic resonance imaging(MRI)scanner system.The mean particle diameter of PFP/Lf-PLLA NBs was 320.2±4.1 nm with a low polydispersity index(PDI,0.145±0.025),and the NBs were negatively charged(−11.4±0.4 mV).The transmis-sion electron microscopy(TEM)results showed that PFP/Lf-PLLA NBs exhibited highly monodispersed and pos-sessed an obvious spherical structure of nanocapsules.Nanobubbles had good stability at 4°C.Different concentrations of the PFP/Lf-PLLA NBs solution had no effect on the cell in cytotoxicity and cell migration,and the results of hemolysis rate and blood biochemistry assay also indicated the good biocompatibility of NBs.On the ultrasound/magnetic resonance imaging of tumor-bearing mice,PFP/Lf-PLLA NBs showed signifi-cantly enhanced contrast ability of tumor tissue.Therefore,PFP/Lf-PLLA NBs had great potential to be a contrast agent for tumor dual-modality imaging in vivo.

Enrichment of microplastic pollution by micro-nanobubbles

Microplastic pollution has become a global environmental concern.It has been reported that microplastics are easily accessible to a wide range of aquatic organisms and ultimately enter the human body along the food chain.They pose a severe threat to ecosystems,organisms and even human health due to their durability and persistence.However,how to reduce microplastic pollution still remains a challenge in terms of scientific techniques and policy-making.There is currently still a lack of effective methods for microplastic recycling and removal.Luckily,a new technique,micro-nanobubbles(MNBs),may provide a possible and highly effective method to enrich microplastic pollution:their great advantages[1]include a high specific surface area,long lifetime and ability to adsorb microplastics of the same size and hydrophobicity.Then they further adsorb on larger bubbles such as microbubbles or millimeter bubbles and float to the water surface together.In this study,we present a new method using MNBs to enrich microplastic pollution with high efficiency.Two types of microplastics,millimeter-scale plastic fragments and microplastic particles,were chosen as the model microplastic pollution systems to study the enrichment efficiency of MNBs on microplastics.Results showed that MNBs can efficiently enrich these microplastics.The enrichment efficiency increases with flotation time until a maximum value is reached.It is proved that MNBs not only collect the microplastic pollution but also reduce detergent use in domestic laundry sewage.This is because detergent,as a surfactant,is easily absorbed on the surface of MNBs and can be collected together with the microplastic pollution.Our research has demonstrated that the MNB technique could be promising for use in microplastic recycling and reducing detergent pollution in daily life.

Controllable Nucleation of Nanobubbles at a Modified Graphene Surface

The properties of nanoscale gas bubbles at the solid/water interface have been investigated for more than 20 years. However, the stability of nanobubbles remains far from being understood. How to control the formation of nanobubbles is the key issue for understanding their long lifetime. In this work, using molecular dynamics simulations we modify the substrate （graphene） with charge dipoles in which the local properties of the surface could be changed. Nanobubbles could be stabilized on the local hydrophobic area and modified area with the hydrophilic boundary where gas nuclei are deposited beforehand. Those results provide two methods to control the nucleation of gas nanobubbles and fix them on a target area.

An experimental study on size distribution and zeta potential of bulk cavitation nanobubbles

Nanobubble flotation technology is an important research topic in the field of fine mineral particle separation.The basic characteristics of nanobubbles,including their size,concentration,surface zeta potential,and stability have a significant impact on the nanobubble flotation performance.In this paper,bulk nanobubbles generated based on the principle of hydrodynamic cavitation were investigated to determine the effects of different parameters(e.g.,surfactant(frother)dosage,air flow,air pressure,liquid flow rate,and solution pH value)on their size distribution and zeta potential,as measured using a nanoparticle analyzer.The results demonstrated that the nanobubble size decreased with increasing pH value,surfactant concentration,and cavitation-tube liquid flow rate but increased with increasing air pressure and increasing air flow rate.The magnitude of the negative surface charge of the nanobubbles was positively correlated with the pH value,and a certain relationship was observed between the zeta potential of the nanobubbles and their size.The structural parameters of the cavitation tube also strongly affected the characteristics of the nanobubbles.The results of this study offer certain guidance for optimizing the nanobubble flotation technology.

Formation and stability of ultrasonic generated bulk nanobubbles

Although various and unique properties of bulk nanobubbles have drawn researchers＇ attention over the last few years,their formation and stabilization mechanism has remained unsolved. In this paper, we use ultrasonic methods to produce bulk nanobubbles in the pure water and give a comprehensive study on the bulk nanobubbles properties and generation. The ultrasonic wave gives rise to constant oscillation in water where positive and negative pressure appears alternately. With the induced cavitation and presence of dissolved air, the bulk nanobubbles formed. ＂Nanosight＂（which is a special instrument that combines dynamic light scattering with nanoparticle tracking analysis） was used to analyze the track and concentration of nanobubbles. Our results show that in our experiment, sufficient bulk nanobubbles were generated and we have proven they are not contaminations. We also found nanobubbles in the ultrasonic water change in both size and concentration with ultrasonic time.

Nanobubbles produced by hydraulic air compression technique

The anoxia of coastal water has already been a serious problem all over the word.Nanobubbles are proved to have great applications in water remediation because they could effectively increase the oxygen content and degrade organic matters in water.But the existing methods to produce nanobubbles are complicated and high cost to operate,especially in deep sea.In this paper,we presented a low-cost method,hydraulic air compression(HAC),to produce a large number of nanobubbles and proved that nanoscale gas bubbles could be produced by HAC for the first time.Nanoparticle tracking analysis was used to measure the size and concentration of produced nanobubbles.It indicated that the concentration of nanobubbles would increase as the downpipe height increases.Degassed measurements proved that produced“nanoparticles”are gas nanobubbles indeed.More dissolved oxygen in water would provide the source for larger number of nanobubble formation.Those results are expected to be very helpful for water remediation in ocean in the future.

A review of recent theoretical and computational studies on pinned surface nanobubbles

The observations of long-lived surface nanobubbles in various experiments have presented a theoretical challenge, as they were supposed to be dissolved in microseconds owing to the high Laplace pressure. However, an increasing number of studies suggest that contact line pinning, together with certain levels of oversaturation, is responsible for the anomalous stability of surface nanobubbles. This mechanism can interpret most characteristics of surface nanobubbles. Here, we summarize recent theoretical and computational work to explain how the surface nanobubbles become stable with contact line pinning. Other related work devoted to understanding the unusual behaviors of pinned surface nanobubbles is also reviewed here.

Concentration of nitrogen molecules needed by nitrogen nanobubbles existing in bulk water

This paper investigates the stability of nitrogen nanobubbles under dif~ ferent concentrations of nitrogen molecules by molecular dynamics simulations. It is found that the stability of nanobubbles is very sensitive to the concentration of nitrogen molecules in water. A sharp transition between disperse states and assemble states of nitrogen molecules is observed when the concentration of nitrogen molecules is changed. The relevant critical concentration of nitrogen molecules needed by the existing nitrogen nanobubbles is analyzed.

Role of substrate softness in stabilizing surface nanobubbles

The contact line pinning and supersaturation theory for the nanobubble stability has attracted extensive concerns from experimental investigators,and some experimenters argue that the contact line pinning is unnecessary.To interpret the experimental observations,we have proposed previously through molecular dynamics simulations that the deformation of soft substrates caused by surface nanobubbles may play an important role in stabilizing surface nanobubbles,while yet no quantitative theory is available for explanation of this mechanism.Here,the detailed mechanism of self-pinning-induced stability of surface nanobubbles is investigated through theoretical analysis.By manipulating substrate softness,we find that the formation of surface nanobubbles may create a deformation ridge nearby their contact lines which leads to the self-pinning effect.Theoretical analysis shows that the formation of nanobubbles on sufficiently rigid substrates or on liquid-liquid interfaces corresponds to a local free energy maximum,while that on the substrates with intermediate softness corresponds to a local minimum.Thus,the substrate softness could regulate the surface nanobubble stability.The critical condition for the self-pinning effect is determined based on contact line depinning,and the effect of gas supersaturation is explored.Finally,the approximate stability range for the surface nanobubbles is also predicted.

Theoretical study on the stability of nanobubbles and its verification in molecular dynamics simulation

The stability of vapor nanobubbles in bulk liquid was investigated theoretically and the critical bubble size was derived from macroscale thermodynamic equations,below which the system destabilizes with sharp drop in pressure.This critical size was quantitatively verified in molecular dynamic simulation using the Lennard-Jones model of argon,where stronger attraction between the molecules at lower density is found to contribute most to the drop of system pressure and,as the Laplace pressure on the curved bubble interface fails to balance the pressure difference across the interface,the bubbles become unstable.The theoretical model could be extended to other systems where reliable equations of state and interfacial tension are available.

Enrichment of surface charge contributes to the stability of surface nanobubbles

Surface nanobubbles are spontaneously formed at the interface between hydrophobic surfaces and aqueous solutions,which show extraordinarily longer lifetime than that was predicted by the classical thermodynamics model.In the present work,by using a surface plasmon resonance microscopy(SPRM)to quantitatively measure the dissolution kinetics of individual surface nanobubbles in real time,we explored the effects of ionic strength and pH value on the dissolution rates(lifetime)of nanobubbles.The results revealed that nanobubbles could exist stably for a long time in low-concentration electrolyte solutions or high-concentration non-electrolyte solutions,while they dissolved quickly in highconcentration electrolyte solutions.With the increase of ionic strength,the dissolution rates were accelerated by 2-3 orders of magnitude,and thus the lifespan of these surface nanobubbles was significantly shortened.In addition to ionic strength,it was further found that,with the increase of acidity or alkalinity of the solution,the dissolution rates of the surface nanobubbles were faster than that in neutral solution.These results demonstrated that the interfacial charge enrichment significantly contributed to the extraordinary stability of the surface nanobubbles.