Aging dynamics of bulk nanobubbles under pressure oscillations.

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.

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

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

The effect of ultrasound on bulk and surface nanobubbles: A review of the current status

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.

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.

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

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

Stability of soluble bulk nanobubbles: Many-body dissipative particle dynamics analysis

Bulk nanobubbles have unique properties and find potential applications in many important processes. However, their stability or long lifetime still needs to be understood and has attracted much attention from researchers. Bulk nanobubbles are generated based on ethanol-water exchange, a method that is generally used in the study of surface nanobubbles. Their formation and stability is further studied by using a new type of dynamic light scattering known as NanoSight. The results show that the concentration of the bulk nanobubbles produced by this method is about five times greater than that in the degassed group, which indicates the existence of bulk gas nanobubbles. The effects of ethanol/water ratios and temperature on the stability of the bulk nanobubbles have also been studied and their numbers reach a maximum at a ratio of about 1:10 (v/v).

In our previous work [2022 Phys. Chem. Chem. Phys. 24 9685], we used molecular dynamics simulations to show that bulk nanobubbles can be stabilized by forming a compressed amphiphile monolayer at bubble interfaces. This observation closely matches the origin of stability of microemulsions and inspired us to propose here that, in certain cases, stable bulk nanobubbles can be regarded as gaseous analogues of microemulsions: the nanobubble phase and the bubble-containing solution phase coexist with the external gas phase. This three-phase coexistence is then validated by molecular dynamics simulations. The stability mechanism for bulk nanobubbles is thus given: the formation of a compressed amphiphilic monolayer because of microbubble shrinking leads to a vanishing surface tension, and consequently the curvature energy of the monolayer dominates the thermodynamic stability of bulk nanobubbles. With the monolayer model, we further interpret several strange behaviors of bulk nanobubbles: gas supersaturation is not a prerequisite for nanobubble stability because of the vanishing surface tension, and the typical nanobubble size of 100 nm can be explained through the small bending constant of the monolayer. Finally, through analyzing the compressed amphiphile monolayer model we propose that bulk nanobubbles can exist ubiquitously in aqueous solutions.

The mechanism of the decrease in the surface tension of water containing bulk nanobubbles (ultrafine bubbles) is studied theoretically by numerical simulations of the adsorption of bulk nanobubbles at the liquid's surface based on the dynamic equilibrium model for the stability of a bulk nanobubble under the conditions of the Tuziuti experiment (Tuziuti, T., et al., Langmuir, 2023, 39, 5771-5778). It is predicted that the concentration of bulk nanobubbles in the bulk liquid decreases considerably with time, as many bulk nanobubbles are gradually adsorbed at the liquid's surface. A part of the decrease in surface tension is due to the Janus-like structure of a bulk nanobubble that could partly break the hydrogen bond network of water molecules at the liquid's surface because more than 50% of the bubble's surface is covered with hydrophobic impurities, according to the dynamic equilibrium model. The theoretically estimated decrease in surface tension due to the Janus-like structure of a bulk nanobubble agrees with the experimental data of the decrease in surface tension solely by bulk nanobubbles obtained by the comparison of before and after the elimination of bulk nanobubbles by the freeze-thaw process. This effect cannot be explained by the electric charge stabilization model widely discussed for the stability of a bulk nanobubble, although the present model is only applicable to the solution containing hydrophobic impurities. Another part of the decrease in surface tension should be due to impurities produced from a nanobubble generator, such as a mechanical seal, which was partly confirmed by the TOC measurements.

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.

This paper elucidates parts of the mystery behind the interfacial and colloidal stability of the novel bubble system of bulk nanobubbles. Stable bulk nanobubble suspensions have been generated in pure water using hydrodynamic cavitation in a high-pressure microfluidic device. The effects of pH adjustment, addition of different types of surfactant molecules and salts on the nanobubble suspensions have been studied. Results show that nanobubble interfaces in pure water are negatively charged, suggesting the formation of an electric double layer around the nanobubbles. It is presumed that the external electrostatic pressure created by the charged nanobubble interface, balances the internal Laplace pressure; therefore, no net diffusion of gas occurs at equilibrium and the nanobubbles are stable. Such stability increases with increasing alkalinity of the suspending medium. The addition of mono- and multi-valent salts leads to the screening of the electric double layer, hence, destabilizing the nanobubbles. Different surfactant molecules (non-ionic, anionic, cationic) affect the stability of bulk nanobubbles in different ways. Calculations based on the DLVO theory predict a stable colloidal system for bulk nanobubbles in pure water and this could be a further reason for their observed longevity. All in all, in pure water, the long-term stability of bulk nanobubbles seems to be caused by a combination of ion-stabilisation of their interface against dissolution and colloidal stability of the suspension.

Nanobubbles (NBs) hold significant promise in the fields of water treatment and environmental remediation due to their remarkable stability and longevity. Despite evidence of the stability of bulk nanobubbles (BNBs) in water, the underlying mechanisms of their stability remain elusive, with a notable gap in understanding the role of surface electronegativity in NBs' stability. In this work, an all-atom (AA) molecular dynamics (MD) simulation has been used to investigate the stability characteristics of individual BNBs and the aggregation behavior of double BNB clusters, incorporating the influence of the electrical double layer (EDL). The stability of individual BNBs is evaluated through analysis of the gas-liquid interface's high-density layer, the structure of the EDL, and the hydrogen bond (HB) network. A stabilization mechanism is proposed based on the surface electronegativity of BNBs. Meanwhile, the simultaneous construction of a double BNBs stability model reveals that nanobubble aggregation is the result of a competitive mechanism of van der Waals gravity and electrostatic repulsion. The validity of the proposed model is also verified by comparing the particle size and zeta tests of BNB solutions prepared with two gases with the nanobubble diameters and electrostatic energy obtained from the simulation model. A critical distance of 10 Å is determined, beyond which BNBs are less likely to coalesce. It is observed that the majority of BNBs are influenced by significantly greater electrostatic forces compared to the van der Waals force, which is hypothesized to be the main contributor to their stability.