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

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.

The generation and stabilization of nanobubbles (NBs) are crucial concerns, considering their great potential for applications in various fields. Nonetheless, research on the stabilization of bulk nanobubbles (BNBs) generation across various systems under ultrasonic irradiation is relatively few. For example, how dissolved gases and different conditions affect the evolution of BNBs in the acoustic field remains unclear. Therefore, this study focused on generating and stabilizing BNBs over time under various conditions including ultrasonic frequency, power, and dissolved gases in both open and closed systems. First, for a given solution, the concentration of BNBs would increase with higher ultrasonic power and lower ultrasonic frequency. Furthermore, a considerably elevated concentration of BNBs was obtained in closed systems relative to open systems, which may be attributed to a closed system providing a more stable environment for nucleation growth, thus facilitating the generation and stabilization of BNBs. More surprisingly, by changing dissolved gas saturation, we found that in gas-saturated water, the concentration of BNBs becomes higher than in the other two saturations: supersaturated and undersaturated water. A detailed study also found that the concentration of formed BNBs differs based on the positions of vessel, and more BNBs will be formed at the bottom or upper side of the vessel, indicating bubbles easily nucleate near the vessel wall and at the gas-liquid interface. This study provides essential insights into the principles of the generation and stabilization of NBs under ultrasonic fields, potentially expanding application ranges and improving the efficiency of ultrasonic irradiation.

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

Generation and stability of bulk nanobubbles: A review and perspective

Nanobubbles (NBs), given its extraordinary properties, have drawn keen attention in the field of nanotechnology worldwide. However, compared to that of surface NBs, generation of stable bulk NBs remains an arduous task with the prevailing method. In this study, we developed a pressure-driven method to prepare bulk NBs by repeatedly compressing sulfur hexafluoride (SF6) gas into water. The results show that NBs with a mean diameter of 240 ± 9 nm and a polydispersity index of 0.25 were successfully prepared. The generated NBs had a high negative zeta potential (-40 ± 2 mV) with stability of more than 48 h. Under the condition of 600 times repeated compression, the NB concentration could reach about 1.92 × 1010 bubbles/mL. Furthermore, we examine the possible formation mechanism involved in NB generation by virtue of optical microscopy and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. The microscopic results showed that microbubbles about 10-50 μm formed first and then decreased to be nanoscale-sized. A stronger hydrogen bond was detected by ATR-FTIR spectroscopy during the shrinking of microbubbles into NBs. It is speculated that the disappearance of microbubbles contributes to the formation of NBs, and the strong hydrogen bond at the gas-water interface prompts the stability of NBs. Therefore, repeated compression of the gas in aqueous solution could be a new method to prepare stable nanosized bubbles for wide applications in the future.

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.

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.

Recently, nanobubbles (NBs) have garnered significant attention due to their unique properties. Surface-attached NBs are thermodynamically favored, and their stability is widely accepted. However, the mechanisms underlying the homogeneous nucleation and stability of bulk NBs remain not fully understood. In this work, we have investigated the dynamic evolution of supersaturated hydrogen in water using all-atom molecular dynamics (MD) simulations across varying hydrogen-to-water ratios (Nhydrogen/Nwater) and ambient temperature, focusing on the homogeneous nucleation and stability of bulk NBs. The results demonstrate that as the initial Nhydrogen/Nwater increases, distinct evolutionary states can be identified, including sustained supersaturation, stable nanobubbles, and gas–liquid phase separation. The correlations between Nhydrogen/Nwater, temperature, and dynamic evolution were quantified, showing that stable NBs appear only within an intermediate range. The nucleation was well explained theoretically by free energy analyses. These results show that the Nhydrogen/Nwater significantly affects driving energy, while ambient temperature impacts the energy requirements for bubble nucleation. Additionally, classical nucleation theory was applied to the nanobubbles but was found to overestimate the energy barrier, resulting in predicted nucleation rates several orders of magnitude lower than those obtained from MD simulations. Finally, the mechanisms governing the stability of bulk hydrogen NBs were explored. Both surface effects and supersaturation play critical roles in maintaining the mechanical equilibrium. During the nucleation stage, the diffusion rate of hydrogen molecules is reduced in NBs, yet the diffusion of hydrogen is dominated by Brownian motion, featuring unrestricted behaviors.

Bulk nanobubbles (NBs) are remarkably long-lived in liquids, yet the molecular mechanisms underpinning their stability remain unresolved. In this work, 50 ns all-atom molecular dynamics simulations were performed to investigate how gas identity (O2, N2, and air with N2:O2 = 4:1), initial gas loading, alkalinity (pH 7 and 13), and organic additives (acetic acid/acetate, ethanol/ethoxide, and hexane) influence the stability of 5 nm NBs in water. Stability was evaluated by the percentage of gas atoms retained in the bubble, density profiles, hydrogen-bond statistics, and radial distribution functions. Higher initial gas density markedly enhanced stability, and N2-NBs consistently outperformed O2-NBs, consistent with the lower solubility of N2. Alkaline conditions exerted only a minor stabilizing effect, most pronounced for air-NBs. Organic additives affected stability according to their hydrophobicity: hydrophobic hexane substantially increased gas retention, especially at low gas loading, by promoting gas clustering and re-adsorption at the NB interface, whereas hydrophilic solutes had negligible influence. RDF analyses revealed that this stabilization correlates with weakened gas–water hydrogen bonding and enhanced gas–gas and gas–hexane interactions. These results elucidate the molecular determinants of NB persistence and offer design guidelines for tuning bubble longevity in environmental and industrial systems.

The influence of storage conditions and container materials on the long term stability of bulk nanobubbles — Consideration from a perspective of interactions between bubbles and surroundings

Dynamic tracking of bulk nanobubbles from microbubbles shrinkage to collapse

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

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.

The generation of nanobubbles (NBs) by replacing different dissolved gas solutions has been widely adopted. Recently, we have found that mixing solutions with different gas contents can also produce a large number of NBs. However, the mechanism of the formation of NBs during mixing has not been well explored. Here, we designed a series of experiments to investigate the influence of mixing of different solutions on the concentration and size contribution of formed NBs via the help of nanoparticle track analysis. The effect of nanosolids was also investigated. The pressurization and depressurization were used to produce NBs. The results indicated that NBs can be influenced by the gas contents and nanosolids. The addition of nanosolids is beneficial to produce more NBs. Both the nanosolids and gas contents together are expected to substantially increase the concentration of NBs. These results will be very helpful to understand the formation and stability of NBs.

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