Stability Of Bulk Nanobubbles Research Articles

Many-body dissipative particle dynamics analysis: Generation and stability of bulk nanobubbles under the influence of pressure

Bulk nanobubbles can be generated through various methods such as ethanol-water exchange, vacuum method, and pressure method. However, the mechanisms and stability of nanobubble generation under pressure are unknown. In this study, a new pressure control method is proposed by combining the pressure control advantages of dissipative particle dynamics (DPD) with molecular dynamics (MD). The effectiveness and rationality of the new pressure control method are validated by using system temperature and pressure as indicators. Furthermore, based on the Ferralo experiment, the experimental process is decomposed and abstracted into specific simulation processes to investigate the effect of pressure on the stability of bulk nanobubbles. The results show that the pressure changes in the two-phase flow can be well controlled by the pressure plate and the pressure regulator based on DPD. A large amount of bulk nanobubble generation occurs during depressurization in a confined space, while pressurization leads to the dissolution of unstable bulk nanobubbles, while stable bulk nanobubbles persist.

The Effect of the Structure of an Electric Double Layer on the Stability of Bulk Nanobubbles

The Effect of the Structure of an Electric Double Layer on the Stability of Bulk Nanobubbles

The generation and stability of bulk nanobubbles by compression-decompression method: The role of dissolved gas

Nanoscale gas bubbles are paid more and more attention due to their significant applications in different fields including the environmental remediation, plant and animal growth as well as medical diagnosis, etc. As reported, the local gas saturation plays an important role for the formation of surface nanobubbles (NBs) but is less importance for their stability. As for bulk NBs, few researches focused on the influence of dissolved gas on their generation and stability because it is thought generally that a limited amount of gas could dissolve into water. Herein, we reported for the first time the relationship of dissolved gases (Kr, O2 and N2) and the formation and stability of bulk NBs. Firstly, we developed a compression-decompression method to produce the water with super-high concentration of dissolved gas. About 60 mg/L oxygen dissolved gas was created to promote the formation of bulk NBs. It was showed that high bulk NB concentrations were produced using the compression-decompression method by controlling the loading pressure and time at the same time. The evolution process showed that the concentration of dissolved gas would decrease quickly with the deposited time. However, the concentration of formed bulk NBs did not follow the same way as dissolved gas concentration. It exhibited a complicated change over the time. Typically, first sharp increase to one order higher concentration than at the beginning and then decreased with a fluctuation within 72 h. More interestingly, the time of this sharp increase in nanobubble concentration depended on the type of gas, the krypton (Kr) gas system took longer time to reach the highest concentration and the oxygen (O2) as well as nitrogen (N2) gas system reached the highest concentration at about 4 h generally. The change of zeta potential of those NBs followed the same fluctuation as their concentration. Finally, we presumed a theoretical model to explain the evolution mechanism of bulk NBs. It indicated that there is a competition of different bubble behaviors (nucleation, clustering and coalescence) in different time periods. This study provides a new technique to produce high concentration of bulk NBs and dissolved gas in solution. Those results are very significance for further understanding the mechanism of formation and stability of bulk NBs under a super-high concentration of dissolved gas and may be used in some chemical reactions related with gas to promote the reaction efficiency.

On the role of surface charge and surface tension tuned by surfactant in stabilizing bulk nanobubbles

The abnormal stability of bulk nanobubbles has attracted substantial attention from academia and industry since classical Epstein–Plesset theory predicts that gas bubbles cannot keep stable thermodynamic equilibrium spontaneously. The dual nature of ionic surfactants, acting simultaneously as charge carriers and attenuators of surface tension at the gas–liquid interface, enables them to play a unique role in bulk nanobubble stabilization. Herein the stability of bulk nanobubbles in anionic, cationic and nonionic surfactant solutions over a wide range of concentration is studied. Experimental results show that different kinds of surfactant molecules are involved in the nucleation and stabilization of bulk nanobubbles in different ways. We demonstrate that the accumulation of net charges carried by surfactant ions at the interface is predominantly responsible for stabilizing nanobubbles, instead of the reduction of surface tension. With theoretical calculations, we further quantify the coupling action of interfacial tension and surface charge on the stable equilibrium state of bulk nanobubbles. The result illustrates that the interfacial tension and the degree of gas saturation together determine the upper limit of the bubble size that can exist stably. Our study identifies a route to explore the intrinsic mechanism of bulk nanobubble’s stability.

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

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

Conditions of Nucleation and Stability of Bulk Nanobubbles

Conditions of Nucleation and Stability of Bulk Nanobubbles

Theoretical Analysis on the Stability of Single Bulk Nanobubble

As predicted by classical macroscopic theory, the lifetime for nanoscale gas bubbles is extremely short, which causes conflict when detecting stable bulk nanobubbles experimentally in recent years. In fact, the stability of bulk nanobubbles depends on the surrounding liquid environment. Also, the dynamic process of gas in water involves the dissolution, diffusion, release, and transportation of gas as well as the properties of nanobubbles inside. Here, based on previous reports, we introduce the gas transport parameter ℓ in the classical diffusion equation by considering the gas diffusion near the bulk nanobubble at different locations in a container and consider the MacLeod-Sugden relationship between the surface tension and densities of liquid and gas for computing the lifetime of single bulk nanobubbles in an open system. The results show that the single nanobubble lifetime depends on the inner density and gas transport length. It could reach the order of 10–100 s for a single nanobubble with an initial radius of 200 nm, and provides a new idea to prolong the lifetime of the single bulk nanobubble. Meanwhile, compared with the continuous influence of the inner density on the gas diffusion flux near the nanobubble, the range of the gas transport near the nanobubble on the gas diffusion flux is limited, which is affected by the dissolution time of the nanobubble. Our findings would be helpful to explore the storage conditions of nanobubbles and the mechanism of mass transfer at the gas-liquid interface at the micro-scale.

Effect of electrolytes and surfactants on generation and longevity of carbon dioxide nanobubbles

The objective of this research was to investigate the effect of individual additions of mono- and divalent electrolytes (NaCl and CaCl2), anionic (sodium dodecyl sulfate, SDS) and non-ionic surfactants (polysorbate 80, Tween 80) at varied concentrations on the generation and stability of bulk nanobubbles (NBs) from carbon dioxide (CO2) gas in aqueous system. Overall, NBs generated in the small-amount salt fluids exhibited significantly (p ≤ 0.05) lower size range (150–350 nm). Smaller diameter and higher zeta potential magnitudes (18–24 mV) of the NBs in SDS medium were also observed and related to the higher CO2 concentration (~1850 ppm) and lower surface tension (~64 mN/m) of the solution. However, the gas NBs were disappeared with the incorporation of Tween 80. The outcomes provide some more research-based details about the impact of potential nano-bubble stabilising agents on characteristics of NBs contributing to the green and sustainable NB-related applications in food sectors.

Formation and Stability of Bulk Nanobubbles by Vibration.

Vibration is a very common process in nature, industry, biology, etc. Thus, whether vibration could induce the formation of nanoscale bubbles in water or not is very important for some chemical or biological reactions. In this paper, we designed a control experiment to simulate the vibration process to explore the production and stability of bulk nanobubbles. Experimental results showed that the vibration could indeed induce the formation of a certain number of bulk nanobubbles in water. In addition, the formation of bulk nanobubbles depended on the frequency and time of vibration. The existence of gas-liquid interface played an important role for the bulk nanobubbles formation because that external air is a possible important gas source. Our findings would be helpful to explore the mystical behavior of nanobubbles in natural processes.

Proving and interpreting the spontaneous formation of bulk nanobubbles in aqueous organic solvent solutions: effects of solvent type and content.

We show that the mixing of organic solvents with pure water leads to the spontaneous formation of suspended nano-entities which exhibit long-term stability on the scale of months. A wide range of solvents representing different functional groups are studied: methanol, ethanol, propanol, acetone, DMSO and formamide. We use various physical and chemical analytical techniques to provide compounded evidence that the nano-entities observed in all these aqueous solvent solutions must be gas-filled nanobubbles as they cannot be attributed to solvent nanodroplets, impurities or contamination. The nanobubble suspensions are characterized in terms of their bubble size distribution, bubble number density and zeta potential. The bubble number density achieved is a function of the type of solvent. It increases sharply with solvent content, reaching a maximum at an intermediate solvent concentration, before falling off to zero. We show that, whilst bulk nanobubbles can exist in pure water, they cannot exist in pure organic solvents and they disappear at some organic solvent-water ratio depending on the type of solvent. The gas solubility of the solvent relative to water as well as the molecular structure of the solvent are determining factors in the formation and stability of bulk nanobubbles. These phenomena are discussed and interpreted in the light of the experimental results obtained.

Formation and Stability of Bulk Nanobubbles in Different Solutions.

The existence of bulk nanobubbles is still controversial in spite of their significance in a large range of applications. Here, we developed a new method of compression-decompression to produce controllably bulk nanobubbles. Then, we further investigated the generation of bulk nanobubbles in pure water, acid, alkaline, and salt solutions using nanoparticle tracking analysis. The results indicated that the concentration of bulk nanobubbles depends on the decompression time and would reach a maximum value when the decompression time is about 30 min for the pure water system. More importantly, we gave a relatively direct evidence of the existence of bulk nanobubbles by measuring the X-ray fluorescence intensity of Kr in acid, alkaline, and salt solutions. It is shown that the decrease tendency in intensity of Kr in alkaline solution is similar to that in the concentration of bulk nanobubbles with the deposited time, indicating that the bulk nanobubbles produced indeed have gas inside. Furthermore, the concentration and stability of bulk nanobubbles in an alkaline solution are greatest compared with other two solutions regardless of gas types. The concentration of bulk nanobubbles will decrease in the order alkaline > acid/pure water > salt solutions. We believe that our results should be very helpful in understanding the formation and stability of bulk nanobubbles in different solutions.

Bulk Nanobubbles from Acoustically Cavitated Aqueous Organic Solvent Mixtures

We investigate the existence and stability of bulk nanobubbles in various aqueous organic solvent mixtures. Bulk nanobubble suspensions generated via acoustic cavitation are characterized in terms of their bubble size distribution, bubble number density, and zeta potential. We show that bulk nanobubbles exist in pure water but do not exist in pure organic solvents, and they disappear at some organic solvent-water ratio. We monitor the nanobubble suspensions over a period of a few months and propose interpretations for the differences behind their long-term stability in pure water versus their long-term stability in aqueous organic solvent solutions. Bulk nanobubbles in pure water are stabilized by their substantial surface charge arising from the adsorption of hydroxyl ions produced by self-ionization of water. Pure organic solvents do not autoionize, and therefore, nanobubbles cannot exist in concentrated aqueous organic solvent solutions. Because of preferential adsorption of organic solvent molecules at the nanobubble interfaces, the surface charge of the nanobubbles decreases with the solvent content, but the strong hydrogen bonding near their interfaces ensures their stability. The mean bubble size increases monotonically with the solvent content, whereas the surface tension of the mixture is sharply reduced. This is in agreement with literature results on macro- and microbubbles in aqueous organic solutions, but it stands in stark contrast to the behavior of macro- and microbubbles in aqueous surfactant solutions.

Stable Air Nanobubbles in Water: the Importance of Organic Contaminants.

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

On the Existence and Stability of Bulk Nanobubbles.

Bulk nanobubbles are a novel type of nanoscale bubble system. Because of their extraordinary behavior, however, their existence is not widely accepted. In this paper, we shed light on the hypothesis that bulk nanobubbles do exist, they are filled with gas, and they survive for long periods of time, challenging present theories. An acoustic cavitation technique has been used to produce bulk nanobubbles in pure water in relatively large numbers approaching 109 bubble·mL-1 with a typical diameter of 100-120 nm. We provide multiple evidence that the nanoentities observed in suspension are nanobubbles given that they disappear after freezing and thawing of the suspensions, their nucleation rate depends strongly on the amount of air dissolved in water, and they gradually disappear over time. The bulk nanobubble suspensions were stable over periods of many months during which time the mean diameter remained unchanged, suggesting the absence of significant bubble coalescence, bubble breakage, or Ostwald ripening effects. Measurements suggest that these nanobubbles are negatively charged and their zeta potential does not vary over time. The presence of such a constant charge on the nanobubble surfaces is probably responsible for their stability. The effects of pH, salt, and surfactant addition on their colloidal stability are similar to those reported in the literature for solid nanoparticle suspensions, that is, nanobubbles are more stable in an alkaline medium than in an acidic one; the addition of salt to a nanobubble suspension drives the negative zeta potential toward zero, thus reducing the repulsive electrostatic forces between nanobubbles; and the addition of an anionic surfactant increases the magnitude of the negative zeta potential, thus improving nanobubble electrostatic stabilization.

Generation and Stability of Bulk Nanobubbles.

Recently, extremely small bubbles, referred to as nanobubbles, have drawn increased attention due to their novel properties and great potential for various applications. In this study, a novel method for the generation of bulk nanobubbles (BNBs) was introduced, and stability of fabricated BNBs was investigated. BNBs were created from CO2 gas with a mixing method; the chemical identity and phase state of these bubbles can be determined via infrared spectroscopy. The presence of BNBs was observed with a nanoparticle tracking analysis (NTA). The ATR-FTIR spectra of BNBs indicate that the BNBs were filled with CO2 gas. Furthermore, the BNB concentration and its ζ-potential were about 2.94 × 108 particles/mL and -20 mV, respectively (24 h after BNB generation with a mixing time of 120 min). This indicates the continued existence and stability of BNBs in water for an extended period of time.