Long-Term Stability of Surface Nanobubbles in Undersaturated Aqueous Solution

Ionic adsorption on bulk nanobubble interfaces and its uncertain role in diffusive stability

HYPOTHESIS

Bulk nanobubbles have been proposed to improve gas exchange in a variety of applications, such as in water treatment, theragnostics, and microfluidic surface cleaning. However, there is currently no consensus regarding the mechanism responsible for their reportedly long lifetimes, which contradicts classical understanding of diffusive bubble dynamics. Recently, there has been increasing support for an electrostatic stability mechanism, following from experiments that observe negatively charged zeta potentials around nanobubbles.

SIMULATIONS

We use high-fidelity Molecular Dynamics simulations to model bulk nanobubbles under mechanical equilibrium in a sodium iodide electrolyte solution, to investigate ionic adsorption on the liquid-gas interface, and resulting zeta potential. We critically examine the hypothesised electrostatic stress underpinning this previously suggested stability mechanism, which is theorised to stabilise the nanobubbles against dissolution by counteracting the otherwise dominant effects of surface tension, however, has been too difficult to directly measure in experiments.

FINDINGS

Ions adsorb onto the liquid-gas interface, confirming an Electric Double Layer (EDL) distribution around the nanobubble with an estimated zeta potential, in accordance with experiments. However, we find no significant electrostatic stress exerted on the nanobubble surface, as any ion charge density in the EDL is completely neutralised by the rearrangement of the water molecules. As a result, the internal gas pressure is still well predicted by the standard Laplace pressure equation (with a fitted Tolman length correction ), challenging an essential assumption underlying the previously proposed theories, and we instead speculate on alternative mechanisms for electrostatic-based stability.

Role of Capillary Fluctuations in Stabilizing Bulk Nanobubbles

The existence of bulk nanobubbles (BNBs) has long been questioned, primarily due to the limitations of experimental techniques and the widespread assumption that spherical bubbles cannot achieve a stable equilibrium. In this study, we develop a model that describes the stability of BNBs based on experimental observations, revealing that the intensity N of thermal capillary waves, which affect BNB through localized thermal fluctuations inducing deformations and altering pressure distribution, significantly influences the stability of these BNBs in different saturated environments. Our computational results corroborate three frequently reported but controversial characteristics of BNBs: their typical size distribution ranges from 100 to 200 nm around Rstable = 107 nm with N = 10,000; they maintain stability in undersaturated conditions; and their size distributions show minimal fluctuations across different saturation levels. Our results provide a possible mechanism for understanding BNB stability and align well with experimental observations, thereby significantly enhancing the potential for their application in the field of soft matter science.

Theoretical model of dynamics and stability of nanobubbles on heterogeneous surfaces

Surface nanobubbles have revealed a new mechanism of gas-liquid-solid interaction at the nanoscale; however, the nanobubble evolution on real substrates is still veiled, because the experimental observation of contact line motions at the nanoscale is too difficult.

HYPOTHESIS

This study proposes a theoretical model to describe the dynamics and stability of nanobubbles on heterogeneous substrates. It simultaneously considers the diffusive equilibrium of the liquid-gas interface and the mechanical equilibrium at the contact line, and introduces a surface energy function to express the substrate's heterogeneity.

VALIDATION

The present model unifies the nanoscale stability and the microscale instability of surface bubbles. The theoretical predictions are highly consistent to the nanobubble morphology on heterogeneous surfaces observed in experiments. As the nanobubbles grow, a lower Laplace pressure leads to weaker gas adsorption, and the mechanical equilibrium can eventually revert to the classical Young-Laplace equation above microscale.

FINDINGS

The analysis results indicate that both the decrease in substrate surface energy and the increase in gas oversaturation are more conducive to the nucleation and growth of surface nanobubbles, leading to larger stable sizes. The larger surface energy barriers result in the stronger pinning, which is beneficial for achieving stability of the pinned bubbles. The present model is able to reproduce the continual behaviors of the three-phase contact line during the nanobubble evolution, e.g., "pinning, depinning, slipping and jumping" induced by the nanoscale defects.

Generating Stable Nitrogen Bubble Layers on Poly(methyl methacrylate) Films by Photolysis of 2-Azidoanthracene

2-Azidoanthracene (2N3-AN) can act as a photochemical source of N2 gas when dissolved in an optically transparent polymer such as poly(methyl methacrylate) (PMMA). Irradiation at 365 or 405 nm of a 150 μm-thick polymer film submerged in water causes the rapid appearance of a surface layer of bubbles. The rapid appearance of surface bubbles cannot be explained by normal diffusion of N2 through the polymer and likely results from internal gas pressure buildup during the reaction. For an azide concentration of 0.1 M and a light intensity of 140 mW/cm2, the yield of gas bubbles is calculated to be approximately 40%. The dynamics of bubble growth depend on the surface morphology, light intensity, and 2N3-AN concentration. A combination of nanoscale surface roughness, high azide concentration, and high light intensity is required to attain the threshold N2 gas density necessary for rapid, high-yield bubble formation. The N2 bubbles adhered to the PMMA surface and survived for days under water. The ability to generate stable gas bubbles "on demand" using light permits the demonstration of photoinduced flotation and patterned bubble arrays.

Contact angle and stability of interfacial nanobubble supported by gas monolayer

Since solid-liquid interfacial nanobubbles (INBs) were first imaged, their long-term stability and large contact angle have been perplexing scientists. This study aimed to investigate the influence of internal gas density and external gas monolayers on the contact angle and stability of INB using molecular dynamics simulations. First, the contact angle of a water droplet was simulated at different nitrogen densities. The results showed that the contact angle increased sharply with an increase in nitrogen density, which was mainly caused by the decrease in solid-gas interfacial tension. However, when the nitrogen density reached 2.57 nm-3, an intervening gas monolayer (GML) was formed between the solid and water. After the formation of GML, the contact angle slightly increased with increasing gas density. The contact angle increased to 180° when the nitrogen density reached 11.38 nm-3, indicating that INBs transformed into a gas layer when they were too small. For substrates with different hydrophobicities, the contact angle after the formation of GML was always larger than 140° and it was weakly correlated with substrate hydrophobicity. The increase in contact angle with gas density represents the evolution of contact angle from macro- to nano-bubble, while the formation of GML may correspond to stable INBs. The potential of mean force curves demonstrated that the substrate with GML could attract gas molecules from a longer distance without the existence of a potential barrier compared with the bare substrate, indicating the potential of GML to act as a gas-collecting panel. Further research indicated that GML can function as a channel to transport gas molecules to INBs, which favors stability of INBs. This research may shed new light on the mechanisms underlying abnormal contact angle and long-term stability of INBs.

Oxidation of Congo red by Fenton coupled with micro and nanobubbles

Dye wastewater is a kind of refractory organic wastewater. Fenton coupled with micro-nano bubbles (MNBs+FT) was used for the degradation of Congo red (CR), aiming at simplifying the organic pollutants degradation process and reducing the cost of the process. The optimum condition of Fenton alone, the outlet pressure of the cavitation process and different combinations on the degradation of CR dye wastewater were discussed in this study. The results showed that the degradation of CR (100 mg/L) could reach 94.4% by using the MNBs+FT at the pH of 7, which was 72% higher than that using Fenton oxidation alone and 79% higher than that using MNBs alone. Based on the same degradation efficiency, the traditional Fenton process alone required 8 times the dose of oxidants of these combination systems, and the synergy coefficient of MNBs+FT was up to 2.44. ESR analysis indicated that ·OH was the predominant active species during the degradation of CR and MNBs+FT improved the utilization efficiency of H2O2 and produced more ·OH. Besides, the MNBs+FT could extend the pH range of the high-efficiency oxidation reaction, and it could also keep a high degradation rate under neutral conditions, which eliminated the process of adjusting the pH and reduced the anti-corrosion requirements of the equipment. According to the economic analysis results, the total cost of treatment for the MNBs/FT was about 13% of the cost of only the Fenton process. This study provides a reference for the application of MNBs+FT systems in full-scale dye wastewater treatment.

Interfacial Micropancakes: Gas or Contaminations?

Micropancake, a flat domain with micrometer-scale lateral size and a few nanometer thickness, is usually accompanied by the generation of interfacial nanobubbles at the liquid/solid surfaces. Unlike the nanobubbles, micropancakes are difficult to be produced efficiently, impeding further investigations of their mysterious properties. Very recently, An et al. even argued that the previously observed micropancakes were most likely the contaminate, not the gas layers. Herein, to reveal the nature of micropancakes with solid evidence, we presented the in situ characterization of micropancakes at a highly oriented pyrolytic graphite (HOPG) surface produced by the ethanol-water exchange or gas-supersaturated water. By washing with deeply degassed water (DW), the dissolution of those micropancakes was clearly observed, indicating that they may very well be composed of gas. In addition, the analysis of the force measurements showed the intrinsic differences between those gaseous micropancakes and the insoluble organic films. The data and results supported the interpretation that the real existence of gas micropancakes at liquid/solid surfaces.

Dynamic interplay between interfacial nanobubbles: oversaturation promotes anisotropic depinning and bubble coalescence

Probing the dynamics of nanobubbles is essential to understand their longevity and behavior. Importantly, such an observation requires tools and techniques having high temporal resolutions to capture the intrinsic characteristics of the nanobubbles. In this work, we have used the in situ liquid-phase electron microscopy (LPEM) technique to gain insights into nanobubbles' behavior and their interfacial dynamics. Interestingly, we could observe a freely growing-shrinking nanobubble and a pinned nanobubble under the same experimental conditions, suggesting the possibility of multiple nanobubble stabilization theories and pathways. Remarkably, the study reveals that a freely growing-shrinking nanobubble induces anisotropic depinning in the three-phase contact line of a strongly pinned neighboring nanobubble. The anisotropic depinning is attributed to the differential local gas saturation levels, depending on the relative positioning of the freely growing-shrinking nanobubble. Furthermore, we also observed a unique pull-push phenomenon exhibited by the nanobubble's interfaces, which is attributed to the van der Waals interactions and the electric double layer collectively. The role of the electric double layer in suppressing and delaying the merging is also highlighted in this study. The present work aims to reveal the role of locally varying gas saturation in the depinning of nanobubbles, their longevity due to the electric double layer, and the consequent coalescence, which is crucial to understand the behavior of the nanobubbles. Our findings will essentially contribute to the understanding of these novel nanoscale gaseous domains and their dynamics.

From Bubbles to Nanobubbles

Nanobubbles are classified into surface and bulk. The main difference between them is that the former is immobile, whereas the latter is mobile. The existence of sNBs has already been proven by atomic force microscopy, but the existence of bNBs is still open to discussion; there are strong indications, however, of its existence. The longevity of NBs is a long-standing problem. Theories as to the stability of sNBs reside on their immobile nature, whereas for bNBs, the landscape is not clear at the moment. In this preliminary communication, we explore the possibility of stabilizing a bNB by Brownian motion. It is shown that a fractal walk under specific conditions may leave the size of the bubble invariant.

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

The generation, and stability of nanobubbles are of particular interest for fundamental research and have potential application in numerous fields. Several attempts were made in the literature to produce nanobubbles through acoustic cavitation. However, the generation and stability mechanisms of nanobubbles in the acoustic field are unclear. Here, we review the effect of ultrasound parameters on bulk nanobubbles and surface nanobubbles. On this basis, we discuss the proposed generation and stability mechanisms of nanobubbles from the perspective of transient and stable acoustic cavitation. Moreover, we propose some future research directions for a deeper understanding of the role of ultrasound in the generation and stability of nanobubbles.

Electrocatalytic generation and tuning of ultra-stable and ultra-dense nanometre bubbles: an in situ molecular dynamics study

Electrocatalytic generation of nanometre gas bubbles (nanobubbles) and their tuning are important for many energy and chemical processes. Studies have sought to use indirect or ex situ methods to investigate the dynamics and properties of nanobubbles, which are of fundamental interest. Alternatively, we present a molecular dynamics simulation method, which features in situ and high spatial resolution, to directly address these fundamentals. Particularly, our simulations can quantitatively reproduce the generation of ultra-stable and ultra-dense nanobubbles observed in electrochemical experiments. More importantly, our results demonstrate that the classical nucleation theory is still valid even for the scale down to several nanometres, to predict the dynamics and properties of nanobubbles. This provides general guidelines to design efficient nanocatalysts and nanoelectrodes. In our specific case, nanoelectrodes with wetting angles below 71° can suppress the generation of surface nanobubbles.

Recent advances for understanding the role of nanobubbles in particles flotation

Traditional froth flotation is the primary method for the separation and upgrading of fine mineral particles. However, it is still difficult for micro-fine and low-quality minerals to effectively separate. It is generally believed that bubble miniaturization is of great significance to improve flotation efficiency. Due to their unique physical and chemical properties, the application of nanobubbles (NBs) in ore flotation and other fields has been widely investigated as an important means to solve the problems of fine particle separation. Therefore, a fundamental understanding of the effect of NBs on flotation is a prerequisite to adapt it for the treatment of fine and low-quality minerals for separation. In this paper, recent advances in the field of nanobubble (NB) formation, preparation and stability are reviewed. In particular, we highlight the latest progress in the role of NBs on particles flotation and focus in particular on the particle-particle and particle-bubble interaction. A discussion of the current knowledge gap and future directions is provided.

Wetting of nanopores probed with pressure

Nanopores are both a tool to study single-molecule biophysics and nanoscale ion transport, but also a promising material for desalination or osmotic power generation. Understanding the physics underlying ion transport through nano-sized pores allows better design of porous membrane materials. Material surfaces can present hydrophobicity, a property which can make them prone to formation of surface nanobubbles. Nanobubbles can influence the electrical transport properties of such devices. We demonstrate an approach which uses hydraulic pressure to probe the electrical transport properties of solid state nanopores. We show how pressure can be used to wet pores, and how it allows control over bubbles or other contaminants in the nanometer scale range normally unachievable using only an electrical driving force. Molybdenum disulfide is then used as a typical example of a 2D material on which we demonstrate wetting and bubble induced nonlinear and linear conductance in the regimes typically used with these experiments. We show that by using pressure one can identify and evade wetting artifacts.

Effects of Gas Adsorption and Surface Conditions on Interfacial Nanobubbles

Gas aggregation and formation of interfacial nanobubbles (INBs) provide challenges and opportunities in the operation of micro-/nanofluidic devices. In the current study, we used molecular dynamics(MD) simulations to investigate the effects of hydrophobicity and various homogeneous surface conditions on gas aggregation and INB stability with a series of 3D argon-water-solid and water-solid systems. Among various signatures of surface hydrophobicity, the potential of mean force (PMF) minima exhibited the strongest correlation with the water molecular orientation at the liquid-solid interface, compared to the depletion layer width and the droplet contact angle. Our results indicated that argon aggregation on the substrate was a function of hydrophobicity as well as competition between gas-solid and water-solid PMFs. Thus, one precondition for gas aggregation on a surface is that the free energy minima of gas induced by the surface be much lower than that induced by water. We found that although the presence of gas molecules had little effect on the measures of wettability, it enhanced density fluctuations near liquid-solid interfaces. The PMF of gas along the surface tangential plane exhibited a small energy barrier between the epitaxial gas layer (EGL) in the bubble and the gas enrichment layer (GEL) in the liquid, which may benefit nanobubble stability. Much lower PMF in the EGL compared to that in the GEL indicated that gas molecules could migrate from the GEL to the nanobubble basement. However, density fluctuations enhanced by the GEL could reduce the energy barrier, thus reducing the stability of INBs.

Comment on "Bulk Nanobubbles or Not Nanobubbles: That is the Question"

In a recent article, Jadhav and Barigou ( Langmuir 2020, 36 (7), 1699-1708) investigated the question of the existence of stable bulk nanobubbles in water generated by hydrodynamic cavitation, ultrasound cavitation, and the addition of an organic compound (namely, ethanol) to water. They firmly conclude that these procedures result in stable bulk nanobubbles. However, a number of previous works documented that the nanoentities observed in water upon such procedures are not nanobubbles. Here, we analyze work of Jadhav and Barigou and show that conclusions regarding the nanobubble nature of the nanoentities are incorrect and are due to the choice of experimental techniques with weak sensitivity, methodical issues in the use of otherwise proper experimental techniques, and ambiguous outcomes of the rest of experiments.

The interplay among gas, liquid and solid interactions determines the stability of surface nanobubbles

Surface nanobubbles are gaseous domains found at immersed substrates, whose remarkable persistence is still not fully understood. Recently, it has been observed that the formation of nanobubbles is often associated with a local high gas oversaturation at the liquid-solid interface. Tan, An and Ohl have postulated the existence of an effective potential attracting the dissolved gas to the substrate and producing a local oversaturation within 1 nm from it that can stabilize nanobubbles by preventing outgassing in the region where gas flow would be maximum. It is this effective solid-gas potential - which is not the intrinsic, mechanical interaction between solid and gas atoms - its dependence on chemical and physical characteristics of the substrate, gas and liquid, that controls the stability and the other characteristics of surface nanobubbles. Here, we perform free energy atomistic calculations to determine, for the first time, the effective solid-gas interaction that allows us to identify the molecular origin of the stability and other properties of surface nanobubbles. By combining the Tan-An-Ohl model and the present results, we provide a comprehensive theoretical framework allowing, among others, the interpretation of recent unexplained experimental results, such as the stability of surface nanobubbles in degassed liquids, the very high gas concentration in the liquid surrounding nanobubbles, and nanobubble instability in organic solvents with high gas solubility.

Study on the Formation and Properties of Trapped Nanobubbles and Surface Nanobubbles by Spontaneous and Temperature Difference Methods

Trapped nanobubbles are gas domains trapped at nanopits on the solid-liquid interface. This is different from surface nanobubbles that usually form at the smooth surface. Herein, both trapped nanobubbles and surface nanobubbles formed on the nanopitted polystyrene film were studied by a spontaneous formation method and a temperature difference method. Trapped nanobubbles behave more flexibly than surface nanobubbles under different scanning loads. The nanopits under trapped nanobubbles appear after being subjected to large force scanning, and both trapped nanobubbles and surface nanobubbles can recover after reducing the scan load. The contact angles of the two kinds of nanobubbles were calculated and were found to be approximately constant. Configurations of trapped nanobubbles including under the pit mouth, protruding out but pinning at the pit mouth, and protruding out and extending around the pit mouth were experimentally observed. Gas oversaturation in the liquid after replacing the low-temperature water with high-temperature water was evaluated and was found to be a key factor for nanobubble formation and led to trapped nanobubbles protruding out and extending. Our study should be helpful in understanding the formation mechanism and properties of trapped nanobubbles and surface nanobubbles, and it will also be useful for further research on the control of nanobubble distribution.

(Non)Existence of Bulk Nanobubbles: The Role of Ultrasonic Cavitation and Organic Solutes in Water

A drawback of studies on bulk nanobubbles is the absence of direct proof that the nano-objects reported are really nanobubbles. The aim of our work was to provide such a proof or disproof. We focused on two effects (processes) commonly considered in research on nanobubbles: ultrasonic cavitation and addition of organic compounds to water, which could create in principle a barrier at the gas/water interface contributing to the stability of nanobubbles. We found that both of these processes lead to the generation of nano-objects, which are, however, not bulk nanobubbles. Ultrasonication leads to the formation of fine metal nanoparticles originating from the disintegration of the surface of the metal ultrasonic probe. Addition of organic solutes to water leads to the formation of a population of nanoparticles/nanodroplets originating from the so-called mesoscale solubilization of hydrophobic compounds present in the added solute as molecularly dissolved impurities. Subsequent ultrasonication of such mixtures adds metal nanoparticles and only slightly modifies the size distribution of mesoscale particles. While our results do not dismiss existence of nanobubbles in general, described effects must be seriously taken into account, especially in the case of biomedical applications where they can result in serious side effects.

Stability of Surface Nanobubbles without Contact Line Pinning

Although the stability of most surface nanobubbles observed can be well interpreted by contact line pinning and supersaturation theory, there is increasing evidence that at least for certain situations, contact line pinning is not required for nanobubble stability. This raises a significant question of what is the stability mechanism for those sessile nanobubbles. Through theoretical analysis and molecular dynamics simulations, in this work, we report two mechanisms for stabilizing surface nanobubbles on flat and homogeneous substrates. One is attributed to constant adsorption of trace impurities on the nanobubble gas?liquid interface, through which nanobubble growing or shrinking causes the increase and decrease of interfacial tension, acting as a restoring force to bring the nanobubble to its equilibrium size. The other is attributed to the deformation of a soft substrate induced by the formed nanobubble, which in turn stabilizes the nanobubble via impeding the contact line motion, similar to self-pinning of microdroplets on soft substrates. Both mechanisms can interpret, depending on the specified conditions, how surface nanobubbles can remain stable in the absence of contact line pinning.

Stability, Dynamics, and Tolerance to Undersaturation of Surface Nanobubbles

The theoretical understanding of surface nanobubbles-nanoscale gaseous domains on immersed substrates-revolves around two contrasting perspectives. One perspective, which considers gas transport in the nanobubbles' vicinity, explains numerous stability-related properties but systematically underestimates the dynamical response timescale by orders of magnitude. The other perspective, which considers gas transport as the bulk liquid equilibrates with the external environment, recovers the experimentally observed dynamical timescale but incorrectly predicts that nanobubbles progressively shrink until dissolution. We propose a model that couples both perspectives, which is capable of explaining the stability, dynamics, and unexpected tolerance of surface nanobubbles to undersaturated environments.