Bulk Nanobubbles Research Articles

Dynamic viscosity of water containing large-sized nanobubbles simulated with coarse-grained molecular dynamics model

Nanobubbles have received widespread interest due to prospective applications in various disciplines. A crucial approach to investigating nanobubbles is molecular dynamics simulation. However, investigating large-sized bulk nanobubbles requires expensive computer resources. By coarsening the water molecules, the computational complexity can be significantly reduced, making the investigation of large-sized nanobubbles possible. This article presents simulations and analysis of the microscopic characteristics of water containing large-sized nanobubbles using coarse-grained molecular dynamics, especially, the Green-Kubo formula is used to calculate the dynamic viscosity. We first observe the effects of temperature, pressure and diameter on the viscosity of nanobubble water. The results indicate that higher temperature leads to lower viscosity of nanobubble water while higher pressure leads to higher viscosity of nanobubble water. Additionally, it is observed that the viscosity of nanobubble water decreases gradually as the diameter of the nanobubble increases. It is a remarkable fact that the viscosity of nanobubble water is lower than that of water at the same temperature and pressure, which means the viscosity of water can be reduced with the addition of nanobubble. At last, a viscosity correction model for nanobubble water at room temperature is successfully developed.

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.

Nanobubbles in Ultrapure Water Can Self-Propel.

Nanobubbles are sub- micron-sized gas entities that find applications in a wide range of scientific fields. Typically, they are thought to diffuse according to Brownian motion. We report the existence of self-propelled motion of oxygen bulk nanobubbles in ultrapure water at body temperature. Their motion, to a large extent, is self-affine; there are different scaling exponents along the x- and y-axes as well as for the lateral displacement. We use fractal analysis, and we calculate the structure function, the normalised velocity autocorrelation function, the skewness, and the kurtosis. All descriptors attest the existence of a quasi-Gaussian stochastic process, which is classified as fractional Brownian motion. More than 50 \% of the trajectories along the x-axis follow superdiffusion, while this amount drops to 30 \% for motion along the y-axis as a result of the asymmetry of the field of view.

How bulk nanobubbles respond to elevated external pressures

Bulk nanobubbles, nanoscopic gaseous domains in aqueous solutions, exhibit surprising long-term stability and unique properties under varying environmental conditions. This study investigates the effects of external pressure on nanobubble stability and behavior through three experimental setups: pressurization at room temperature, pressurization at elevated temperatures, and constant pressure loading. Our findings reveal that increasing external pressure reduces nanobubble concentration and reshapes the bubble size distribution. Larger nanobubbles either disappeared or transformed into microbubbles, while smaller ones expanded, significantly narrowing the size distribution. These changes were found to be irreversible. Additionally, nanobubble stability is influenced by both the magnitude and duration of the applied pressure. Elevated temperatures further narrowed the size distribution at atmospheric pressure, and subsequent pressurization caused these nanobubbles to shrink, showing different response characteristics compared to room temperature. This research highlights the complex interplay between pressure, temperature, and nanobubble stability, offering valuable insight for practical applications in fields such as drug delivery, water treatment, and nanomaterial synthesis.

Polarizing Perspectives: Ion- and Dipole-Induced Dipole Interactions Dictate Bulk Nanobubble Stability.

The origin of the stability of bulk Nanobubbles (NBs) has been the object of scrutiny in recent years. The interplay between the surface charge on the NBs and the Laplace pressure resulting from the surface tension at the solvent-NB interface has often been evoked to explain the stability of the dispersed NBs. While the Laplace pressure is well understood in the community, the nature of the surface charge on the NBs has remained obscure. In this work, we aim to show that the solvent and the present ions can effectively polarize the NB surface by inducing a dipole moment, which in turn controls the NB stability. We show that the polarizability of the dispersed gas and the polarity of the dispersing solvent control the dipole-induced dipole interactions between the solvent and the NBs, and that, in turn, determines their stability in solution.

Intrinsic CO2 nanobubbles in alkaline aqueous solutions

Nanobubbles (NBs) and their derivative, bulk nanobubbles (BNBs), have been widely studied due to their potential wide range of applications. However, the stable existence of intrinsic NBs and BNBs in solutions is still controversial. This study investigates the existence and behaviour of unknown nano-entities in dilute alkaline solutions using Field-Flow Fractionation Multi-Angle Light Scattering (FFF-MALS), resulting in the first report of the presence, size distribution, and size distribution changes under different treatments. The results suggest that neutral and acidic pure solutions of salts at concentrations below the solubility product do not contain detectable nano entities. Alkaline solutions, on the other hand, such as carbonate buffer and sodium hydroxide, do contain nano scatterers in the range of ∼100 nm diameter that cannot be removed by boiling or filtration. Reducing the pH of the solutions or freezing reduces their number densities. The freeze-thaw procedure strongly suggests that these nano entities are BNBs. Zeta Nanoparticle Tracking Analysis (ZNTA) shows the zeta potential and the concentration in agreement with previous literature. It can be concluded that intrinsic BNBs in alkaline solutions are an integral part of the total nano-entity population. This study highlights the importance of considering intrinsic BNBs when investigating nanoparticles in alkaline solutions with analytical methods such as Static Light Scattering (SLS), which can detect low number densities in the original liquid state that may not be plausible by other detection methods due to their detection thresholds or acquisition of sample preparation methods of drying or freezing that do not allow real-time observations.

Stochastic computer experiments of the thermodynamic irreversibility of bulk nanobubbles in supersaturated and weak gas-liquid solutions.

Spontaneous gas-bubble nucleation in weak gas-liquid solutions has been a challenging topic in theory, experimentation, and computer simulations. In analogy with recent advances in crystallization and droplet formation studies, the diffusive-shielding stabilization and thermodynamic irreversibility of bulk nanobubble (bNB) mechanisms are revisited and deployed to characterize nucleation processes in a stochastic framework of computer experiments using the large-scale atomic/molecular massively parallel simulator code. Theoretical bases, assumptions, and limitations underlying the irreversibility hypothesis of bNBs, and their computational counterparts, are extensively described and illustrated. In essence, it is established that the irreversibility hypothesis can be numerically investigated by converging the system volume (due to the finiteness of interatomic forces) and the initial dissolved-gas concentration in the solution (due to the single-bNB limitation). Helium nucleation in liquid Pb17Li alloy is selected as a representative case study, where it exhibits typical characteristics of noble-gas/liquid-metal systems. The proposed framework lays down the bases on which the stability of gas-bNBs in weak and supersaturated gas-liquid solutions can be inferred and explained from a novel perspective. In essence, it stochastically marches toward a unique irreversible state along out-of-equilibrium nucleation/growth trajectories. Moreover, it does not attempt to characterize the interface or any interface-related properties, neither theoretically nor computationally. It was concluded that bNBs of a few tens of He-atoms are irreversible when dissolved-He concentrations in the weak gas-liquid solution are at least ∼50 and ∼105 molm-3 at 600 and 1000K (and ∼80 MPa), respectively, whereas classical molecular dynamics -estimated solubilities are at least two orders of magnitude smaller.

Nanobubble-assisted formation of non-gaseous nanoparticles in water

For the past decade, there has been an intense debate over the existence of stable bulk nanobubbles in water. To distinguish gaseous particles from solid particles and liquid droplets, a key quantity to be measured is the mass density of the nanoparticles. Here, the mass density of the nanoparticles formed in the process of nanobubble generation was determined by the observation of sedimentation equilibrium and Brownian motion of the nanoparticles. The diameter was about 450nm, whereas the measured mass density was 1.06 to 1.07g/cm3 which was larger than that of water. Thus, the detected nanoparticles in our experiments, which were often called stable bulk nanobubbles, were not gaseous. A comparison between the results of treated and untreated solutions demonstrated that the generation of nanobubbles assists in the formation of non-gaseous nanoparticles, and they sediment in water under gravity.

Surface Cleaning of Oil Contaminants Using Bulk Nanobubbles.

The removal of oil from solid surfaces, such as textiles and plates, remains a challenge due to the strong binding affinity of the oil. Conventional methods for surface cleaning often require surfactants and mechanical abrasion to enhance the cleaning process. However, in excess, these can pose adverse effects on the environment and to the material. This study investigated how bulk nanobubble water can clean oil microdroplets deposited on surfaces like glass coverslips and dishes. Microscopy imaging and further image analysis clearly revealed that these microdroplets detached from both hydrophobic and hydrophilic surfaces when washed with bulk nanobubble water within a fluidic microchannel. Oil contaminant cleaning was also conducted in water as mobile phase to mimic the circumstances that occur in a dishwasher and washing machine. Cleaning on a larger scale also proved very successful in the removal of oil from a porcelain bowl. These results indicate that nanobubble water can easily remove oil contaminants from glass and porcelain surfaces without the assistance of surfactants. This is in stark contrast to negligible results obtained with a control solution without nanobubbles. This study indicates that nanobubble technology is an innovative, low-cost, eco-friendly approach for oil removal, demonstrating its potential for broad practical applications.

Hydroxide and Hydronium Ions Modulate the Dynamic Evolution of Nitrogen Nanobubbles in Water.

It has been widely recognized that the pH environment influences the nanobubble dynamics and hydroxide ions adsorbed on the surface may be responsible for the long-term survival of the nanobubbles. However, understanding the distribution of hydronium and hydroxide ions in the vicinity of a bulk nanobubble surface at a microscopic scale and the consequent impact of these ions on the nanobubble behavior remains a challenging endeavor. In this study, we carried out deep potential molecular dynamics simulations to explore the behavior of a nitrogen nanobubble under neutral, acidic, and alkaline conditions and the inherent mechanism, and we also conducted a theoretical thermodynamic and dynamic analysis to address constraints related to simulation duration. Our simulations and theoretical analyses demonstrate a trend of nanobubble dissolution similar to that observed experimentally, emphasizing the limited dissolution of bulk nanobubbles in alkaline conditions, where hydroxide ions tend to reside slightly farther from the nanobubble surface than hydronium ions, forming more stable hydrogen bond networks that shield the nanobubble from dissolution. In acidic conditions, the hydronium ions preferentially accumulating at the nanobubble surface in an orderly manner drive nanobubble dissolution to increase the entropy of the system, and the dissolved nitrogen molecules further strengthen the hydrogen bond networks of systems by providing a hydrophobic environment for hydronium ions, suggesting both entropy and enthalpy effects contribute to the instability of nanobubbles under acidic conditions. These results offer fresh insights into the double-layer distribution of hydroxide and hydronium near the nitrogen-water interface that influences the dynamic behavior of bulk nanobubbles.

Bulk Nanobubbles through Gas Supersaturation Originated by Hot and Cold Solvent Mixing.

The nucleation mechanism of bulk nanobubbles remains unclear despite the considerable attention they have received in recent years. We propose two hypotheses: (i) The gas supersaturation in the bulk liquid is the primary factor for nanobubble nucleation, and (ii) the mixing of the same solvent at varying gas solubilities should produce nanobubbles, provided that the first hypothesis is correct. To test this hypothesis, we performed extensive experiments on nanobubble nucleation in both water and organic solvents. The temperature difference between hot and cold samples ranged from 10 to 80 °C in pure solvents such as water, methanol, ethanol, propanol, and butanol prepared and mixed in equal proportions. To the best of our knowledge, we report bulk nanobubble nucleation by mixing hot and cold solvents for the first time. The refractive index value calculations using Mie scattering theory confirmed the existence of nanobubbles. When surface tension dominates over surface charge, the critical work for nanobubble formation is ΔFc ∝ 1/ξ2, and when surface charge dominates over surface tension, the critical work is ΔFc ∝ ξ1/4. Our experimental results verify such dependency by measuring nanobubbles nucleated with varying degrees of gas supersaturation.

Effects of micro- and nano-bubbles on the flotation separation of unburned carbon from coal fly ash

ABSTRACT Nanobubble technology has found extensive use in enhancing the flotation efficiency of various minerals. The utilization of nanobubbles in flotation encompasses bulk nanobubbles, surface nanobubbles, and microbubbles; however, a comprehensive comparison of their respective roles is lacking. Furthermore, there is a dearth of studies on employing nanobubble systems in the flotation of coal fly ash. The coal fly ash characters were initially studied to reveal its ultrafine size (d50: 24.80 μm) and porous structure that includes a multitude of hydrophilic functional groups. These attributes contribute to the inadequate hydrophobicity of unburned carbon within the ash. Subsequently, the effects of different collector types, frother types, and air flow rates on the efficiency of carbon removal using conventional flotation methods were examined. Comparative analysis showed that conventional flotation using diesel oil outperformed that which employed kerosene as the collector. Additionally, higher carbon removal efficiency was achieved using MIBC, with a longer carbon chain length, as the frother, as opposed to sec-Octyl alcohol. Furthermore, a direct relationship between air flow rate and coal fly ash flotation performance was observed – higher air flow rates corresponded to improved flotation outcomes. Finally, a comparison between conventional flotation and nanobubble-assisted flotation (utilizing bulk nanobubbles, surface nanobubbles, and micro-nanobubbles) was conducted. Remarkably, micro-nanobubble flotation yielded the most promising results, with the lowest loss-on-ignition in flotation tailings (3.15%) and the highest removal efficiency of unburned carbon (85.01%). These findings satisfied the loss-on-ignition requirements of Grade I fly ash standards.

The effect of nanobubbles on Langmuir-Blodgett films

Nanobubbles (NBs) are classified in two distinct categories: surface and bulk. Surface NBs are readily observed using atomic force microscopy (AFM), while the existence of bulk NBs has been a subject of debate, conflicting with the diffusion theory's predictions. Current methodologies for identifying bulk NBs yield inconclusive results. In this study, Langmuir Blodgett (LB) technique and AFM, are utilized to visualize NB imprints on anionic, cationic and zwitterionic lipid films deposited on glass-slide substrates. Our analysis of Langmuir monolayers compression isotherms reveals the impact of bulk NBs on lipid monolayer development. AFM scans of the deposited lipid films consistently show NB imprints. Notably, cationic and zwitterionic film depositions exhibit NB formations from the 1st layer, whereas in anionic films, these formations are observed only after the 3rd layer. These results suggest that the origin of these imprinted formations may be attributed to bulk NBs.

A diffused double-layer model of bulk nanobubbles in aqueous NaCl solutions

Bulk nanobubbles, known for their extraordinary stability due to their electrically charged surface, still have an elusive mechanism. This research examines the surface electrical properties of oxygen nanobubbles in various NaCl solution concentrations, utilizing a diffused double-layer model. The process involves the creation of nanobubbles through the hydrodynamic cavitation technique, followed by an assessment of their size and zeta potential. To quantify their influence on nanobubble stability, the thickness of the double layer, the density of surface charge, electrostatic repulsion and attractive force, and interaction energy between bubbles are evaluated. The results show that adding NaCl to nanobubble solutions increases bubble size while decreasing their negative zeta potential, minimizing repulsive electrostatic interactions among the bubbles. These findings are consistent with the diffused double-layer theory. Specifically, higher NaCl concentrations in solutions lead to thinner double layers and decreased energy barriers. Observing nanobubble stability in a 10 mM NaCl solution over six months reveals a decrease in bubble size accompanied by an increase in zeta potential. This rise in negative zeta potential correlates with changes in Debye length and ionic strength, inversely proportional to the solution's conductivity, likely due to reduced NaCl concentration over time. Furthermore, the pressure differential at the bubble interface is insufficient to overcome the pressure caused by the liquid's surface tension, indicating potential limitations in applying the diffused double-layer model. This highlights the need for further investigation to establish a more comprehensive understanding of the mechanical stability of bulk nanobubbles.

Effect of Bulk Nanobubbles on the Flocculation and Filtration Characteristics of Kaolin Using Cationic Polyacrylamide

This study investigated the influence of bulk nanobubbles (NBs) on the flocculation and filtration behavior of kaolin suspensions treated with cationic polyacrylamide (CPAM). Traditionally, flocculation relies on bridging mechanisms by polymers like CPAM. The present work examines the possibility of combining NBs with CPAM to achieve more efficient kaolin separation. The settling behavior of kaolin suspensions with and without bulk nanobubbles was compared. The results with 2 mL CPAM and 300 s settling time revealed that bulk NBs significantly enhanced flocculation efficiency, with supernatant zone height reductions exceeding 50% compared to CPAM alone, indicating a faster settling rate resulting from bulk NBs. This improvement in the settling rate is attributed to NBs’ ability to reduce inter-particle repulsion (as evidenced by a shift in zeta potential from −20 mV to −10 mV) and bridge kaolin particles, complementing the action of CPAM. Additionally, the study demonstrated that bulk NBs improved dewatering characteristics by lowering the medium resistance and specific cake resistance during filtration. These findings pave the way for the utilization of bulk NBs as a novel and efficient strategy for kaolin separation in mineral processing, potentially leading to reduced processing times and lower operational costs.

Effect of bulk nanobubbles on flocculation of kaolin in the presence of cationic polyacrylamide

The study explores the impact of nanobubble flotation technology on fine mineral processes, focusing on its interaction with cationic polyacrylamide (CPAM) in kaolin flocculation. Nanobubbles influence particle size and promote aggregation. Experimental procedures involve bulk nanobubble preparation, kaolin suspension, and CPAM solutions, with analysis of sedimentation rates, turbidity, and zeta potential. Results show accelerated sedimentation and reduced turbidity with nanobubbles compared to traditional methods. Zeta potential measurements and DLVO theory support nanobubbles' role in reducing electrostatic interaction, facilitating flocculation. This research advances understanding in nanobubble-mediated mineral processing, highlighting eco-friendly flocculants and practical implications for optimization.

Investigating the stability mechanisms of single bulk Nanobubbles: A molecular dynamics perspective

In this study, we investigated the relationship between bubble stability and various parameters, including radial pressure, density, surface tension, charge distribution, and Brownian motion, using molecular dynamics simulations and classical bubble stability theory. Our research sheds light on the contrasting behavior of small and large nanobubbles in water, with larger bubbles demonstrating a prolonged lifespan and lower internal pressure. Specifically, nanobubbles with radii of 14 Å, 18 Å, and 22 Å were found to have pressures of 792 atm, 647 atm, and 518 atm, respectively. Notably, our calculations revealed that smaller bubbles possess a thicker liquid-gas interface compared to the larger bubbles. Additionally, our analysis of gas-liquid interactions and mean squared displacement demonstrated that smaller bubble gas molecules experience stronger liquid forces and exhibit enhanced diffusivity. Importantly, we validated the reliability of classical theories by confirming the consistency between the calculated solubility of nanobubbles using Henry's law and simulation results. Moreover, our findings aligned with the predictions of Young's equation regarding surface tension at the calculated plane, highlighting its applicability in understanding nanobubbles behavior. Notably, we observed that gas density has minimal impact on the surface tension of nanobubbles according to Young's equation. Overall, our research provides valuable insights into the generation of stable nanobubbles for diverse applications.

Electrochemically reactive colloidal nanobubbles by water splitting

Hypothesis: The existing literature reports have conflicting views on reactive oxygen species (ROS) generation by bulk nanobubbles. Consequently, we propose the hypothesis that (i) ROS may be generated during the process of nanobubble generation through water splitting, and (ii) bulk nanobubbles possess electrochemical reactivity, which could potentially lead to continuous ROS generation even after the cessation of nanobubble production.Experiments: A comprehensive set of experiments was conducted to generate nanobubbles in pure water using the water-splitting method. The primary aims of this study are as follows: (i) nanobubble generation by electrolysis and its characterization; (ii) to provide conclusive evidence that the nano-entities are indeed nanobubbles; (iii) to quantify the production of reactive oxygen species during the process of nanobubble generation and (iv) to establish evidence for the presence of electrochemically reactive nanobubbles.The findings of our experiment suggest that bulk nanobubbles possess the ability to generate reactive oxygen species (ROS) during the process of nanobubble nucleation. Additionally, our results indicate that bulk nanobubbles are electrochemically reactive after the cessation of nanobubble production. The electron spin spectroscopy (ESR) response and degradation of the dye compound over time confirm the electrochemical reactivity of bulk nanobubbles.

Size-dependent coalescence of nanobubbles in pure water

Previously, we experimentally demonstrated that similar-sized bulk nanobubbles (NBs) preferentially coalesce when the NB size is ∼100 nm. To explain this peculiar NB coalescence phenomenon, we performed a nanoparticle trajectory analysis to determine the size distribution profile and time-dependent concentration of the bulk NBs generated by pressurizing a porous alumina thin film with ordered straight nanoholes. Furthermore, we propose a physical model based on the hypothesis that the repulsive energy increases when two NBs establish contact, and that coalescence occurs when the kinetic energy of the NBs overcomes the repulsive energy. We simulated the observed NB-size distribution by using the proposed model. The simulation results explained the observed NB-size distribution and lifetime of the NBs. They further indicated that NB coalescence was suppressed for small NBs with diameters of approximately 100 nm or less, resulting in their long lifetimes.

Effect of ionic environment in aqueous solution on nucleation and stabilization of bulk nanobubbles

Bulk nanobubbles have attracted significant interest due to their growing demand for widespread applications. A thorough comprehension of the essential factors dictating nanobubble properties holds immense significance in industrial practices. This study endeavors to examine the characteristics of bulk nanobubbles formed in a complex ionic solution containing surfactants and sodium chloride. The experimental findings demonstrate that both ionic and nonionic surfactants can effectively promote the nucleation of nanobubbles. The introduction of salts has an opposing effect as it reduces both the number concentration and the Zeta potential of nanobubbles. There is a concentration threshold for NaCl above which the nanobubble properties are significantly affected. The salt effectively promotes the adsorption of surfactant ions, which in turn leads to a decrease in interfacial tension. The Zeta potential of nanobubbles is established through the competing interactions between the surface adsorption and ion shielding effect. Subsequently, the critical stable conditions for nanobubbles generated in solutions containing surfactants and salts are calculated, while considering both the kinetic stability of nanobubble populations and the thermodynamic stability of individual nanobubbles. These findings offer valuable insights into the intricate influence of complex aqueous ionic environment on nanobubbles, thereby setting the stage for the deliberate manipulation of them.