A History of Nanobubbles - Langmuir (ACS Publications)

Invited Historical Article pubs.acs.org/Langmuir

A History of Nanobubbles Muidh Alheshibri, Jing Qian, Marie Jehannin, and Vincent S. J. Craig* Department of Applied Mathematics, Research School of Physics and Engineering, Australian National University, Canberra, ACT 2600, Australia ABSTRACT: We follow the history of nanobubbles from the earliest experiments pointing to their existence to recent years. We cover the effect of Laplace pressure on the thermodynamic stability of nanobubbles and why this implies that nanobubbles are thermodynamically never stable. Therefore, understanding bubble stability becomes a consideration of the rate of bubble dissolution, so the dominant approach to understanding this is discussed. Bulk nanobubbles (or fine bubbles) are treated separately from surface nanobubbles as this reflects their separate histories. For each class of nanobubbles, we look at the early evidence for their existence, methods for the production and characterization of nanobubbles, evidence that they are indeed gaseous, or otherwise, and theories for their stability. We also look at applications of both surface and bulk nanobubbles.

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INTRODUCTION For this special issue on nanobubbles, we have been asked to contribute with a summary of the history of nanobubble research, a task we agreed to perform with some trepidation. This is clearly a difficult task as it is only recently that an identifiable community of researchers working on nanobubbles has developed, hence the published research, particularly the early work, is scattered in the literature. Indeed, much of the research relating to nanobubbles in the early days was incidental to the main aims and motivations of the researchers. Additionally, it is inevitable that not all of the work can be given the attention it deserves and we have no doubt made some significant, though unintentional, omissions, and for this we apologize wholeheartedly. We also have not attempted to do justice to the recent literature, which is expanding rapidly and cannot adequately or appropriately be fully covered in a historical view of the subject. The contributions to our present understanding of nanobubbles are many, and tracking progress is troublesome, as research does not proceed linearly or in isolation. However, there are some landmark papers in nanobubbles research that have had a significant impact on the field. A selection of these is shown on the timeline in Figure 1.

is ultrafine bubbles. The international standards organization is currently evaluating standards for ultrafine bubbles (ISO/ TC281). A motivation for using the term ultrafine bubbles over nanobubbles in commercial applications is that the term “nano” has negative health associations in some cultures and in the field of nanoparticles the term nano is generally applied only to particles that are less than 100 nm in size. We use the term nanobubbles when we wish to talk about nanobubbles in general (including both bulk and surface nanobubbles). We make the distinction between gas-filled bubbles and vapor-filled bubbles. Vapor-filled bubbles may arise when a significant amount of energy is deposited into a small volume in a short time, such that the solvent vaporizes. This includes rectified diffusion due to sound waves or localized heating, an example of which is the heating of nanoparticles due to light absorption. Vapor bubbles will tend to collapse shortly after the energy supply ceases. There may be circumstances where a vapor bubble accumulates gas and becomes a nanobubble. We have not included vapor-filled bubbles in this review. An immobile three-phase line is defined as being pinned, and a mobile three-phase line is unpinned as described by Brutin.13

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NOMENCLATURE Here we use the term surface nanobubbles to describe gas-filled pockets on a surface that are in the form a spherical cap. The height of the nanobubbles is generally more than 10 nm and less than 100 nm. The radius of the contact line (three-phase line) is generally between 50 and 500 nm. The term bulk nanobubbles is used to describe gas-filled spherical bubbles that have a diameter of less than 1000 nm. An alternative and equivalent term also being used in the literature © 2016 American Chemical Society

NANOBUBBLE PREHISTORY

Bubbles are unstable. When a bubble is formed in solution, an interface is created. The energy cost of the interface per unit area is given by the surface tension, γ. Should the bubble rise to Special Issue: Nanobubbles Received: July 6, 2016 Revised: September 1, 2016 Published: September 4, 2016 11086

DOI: 10.1021/acs.langmuir.6b02489 Langmuir 2016, 32, 11086−11100

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Invited Historical Article

Figure 1. Timeline of a brief selection of significant publications in the field of surface and bulk nanobubbles. 1950: Development of the Epstein− Plesset theory, which is used to predict the lifetime of a single bubble as a function of the bubble radius and saturation. According to this theory, a nanobubble in a saturated solution should dissolve within a few milliseconds.1 1994: Surface nanobubbles were proposed to account for very long range attractive forces measured between hydrophobic surfaces. Illustration: Forces measured between two hydrophobic FSCl spheres as a function of the separation distance.2 1997: The role of surface nanobubbles in the very long range attractive forces measured between hydrophobic surfaces claimed above2 was refuted by the short expected calculated lifetimes of surface nanobubbles.3 2000: The first images of surface nanobubbles recorded using atomic force microscopy (AFM) were published.4,5 These studies demonstrated that surface nanobubbles were long-lived and exhibited anomalous contact angles. Illustration: surface nanobubbles on mica, in water, imaged in tapping mode by AFM, reprinted from ref 5. 2003: The use of bulk nanobubbles as ultrasound contrast agents was reported.6 Illustration: Fluorescence microscopy image of cells incubated with nanobubbles reprinted from ref 7. 2006: The influence of salt and surfactants on the shape and size of surface nanobubbles was shown to be negligible, thus indicating that the stabilization of surface nanobubbles was unlikely because of contaminants. Illustration: AFM images of surface nanobubbles on a HOPG surface in water (left) and in 0.86 CMC SDS (middle) and 0.5 CMC CTAB (right) solutions; images extracted from ref 8. 2010: Replicas of bulk nanobubbles were imaged by cryo-scanning electron microscopy.9 2014: The relative mass density of nanoparticles compared to the solvent was measured using a microresonator. The results indicate that the density of the particles believed to be bulk nanobubbles corresponds to the particles being gaseous.10 Illustration: Sketch of a microresonator sensor reprinted from the Archimedes user manual, Malvern Instuments. 2015: Seo et al. directly showed that surface nanobubbles are gaseous using a concurrent combination of fluorescence microscopy and other imaging techniques.11 This work built on a previous study, published in 2007, in which CO2 surface nanobubbles were indirectly shown to be gas-filled using infrared spectroscopy.12 Illustration: Right: Fluorescent image of surface nanobubbles. Left: Merged image at the identical location recorded with bright-field imaging, reflection interference contrast microscopy, and fluorescence microscopy12

reservoir of solution surrounding it, and then consider a small perturbation such that the bubble becomes marginally larger. This will reduce the Laplace pressure and the solubility of the gas in the solution surrounding the bubble. In response, more gas will diffuse into the bubble. However, this increases the size of the bubble, which further lowers the Laplace pressure and solubility of the gas in the solution surrounding the bubble. A positive feedback loop is established in which the bubble grows and can never obtain equilibrium with the reservoir. Conversely, if we choose an initial perturbation from equilibrium such that the bubble becomes marginally smaller, then the solubility of the gas in the solution surrounding the bubble increases. As more gas is lost, the bubble becomes even smaller and again a positive feedback loop is established in which the bubble continuously shrinks and never obtains equilibrium with the reservoir. Thus, even if equilibrium is initially established between a bubble and the reservoir, a small perturbation will destroy the equilibrium and cause the bubble either to grow very large and be removed from the solution through buoyancy or to shrink from existence. We will call this the Laplace pressure bubble catastrophe (LPBC). The LPBC has, and continues to have, a major impact on nanobubble research as it implies that bubbles and particularly small bubbles cannot be thermodynamically stable. Theory of Bubble Growth and Dissolution. As no thermodynamic stability is possible, the kinetics of the processes described in the LPBC become very important in any consideration of nanobubble stability. This has been dealt with in a landmark paper from 1950 by Epstein and Plesset1 in which they state “a gas bubble in a liquid-gas solution will grow or shrink by diffusion according as the solution is oversaturated or undersaturated”. Epstein and Plesset developed a theory of

the surface and burst, then the interfacial area is reduced by the surface area of the bubble. In this case, the change in the Gibbs free energy, ΔG, of the system is ΔGT , P , n = γ ΔA

(1)

where ΔA is the change in interfacial area. Similarly, when two bubbles coalesce the overall interfacial area is reduced and the energy of the system is reduced. Laplace Pressure Bubble Catastrophe (LPBC). Bubbles are also unstable with respect to their size. Only when the gas within a bubble is at equilibrium with the dissolved gas in solution do we expect to see a bubble maintain a constant size. We will see that these conditions are generally not met and nearly impossible to maintain due to the Laplace pressure. The Laplace pressure, ΔP, for a spherical object of radius r is given by

ΔP =

2γ r

(2)

where γ is the interfacial tension of the bubble interface and ΔP describes the increase in pressure within the bubble with respect to the immediate surroundings. However, the solubility of gas in solution increases with pressure, thus the Laplace pressure increases the solubility and the smaller the bubble, the greater the solubility of the gas. Because of the Laplace pressure, the gas within a bubble can be in equilibrium only with the dissolved gas in solution if the solution is supersaturated with respect to the external pressure. Thus, a bulk bubble can be at equilibrium only if the solution surrounding it is not at equilibrium; it is not possible that both are in equilibrium at the same time. Regardless, consider a single bubble that is in equilibrium with an infinitely large 11087


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no consensus as to why surface nanobubbles last much longer than the Epstein and Plesset theory predicts. Therefore, at this point it is not clear whether the Epstein and Plesset theory in providing a fundamental understanding of the dissolution and growth of bubbles has provided a solid platform for the investigation of nanobubbles or has somehow clouded our understanding of nanobubbles.

the diffusion process around a bubble. To do so, a number of simplifications were made. A single isolated bubble was assumed to be stationary (not rising due to buoyancy or translating with Brownian motion). Note that this assumption leads to an underestimation of the diffusion rates. The motion of the bubble boundary as it grows or shrinks was also ignored. This approximation is valid as the gas concentration within the bubble is very much greater than in the surrounding liquid and the region surrounding the bubble through which the diffusion takes place is very much larger than the bubble itself. Additionally, it was assumed that at time zero the bubble had already established a steady state. To solve for the diffusion, it was necessary to establish the concentration of gas dissolved in solution adjacent to the bubble and very far from the bubble. The latter is the level of the gas in solution, and the former is implicitly determined by applying Henry’s law where the total pressure is the sum of the external pressure and the Laplace pressure. The type of gas enters through the parameters for the coefficient of diffusivity, the saturation concentration, and the gas density in the bubble. Importantly, this theory predicts that in saturated solutions small bubbles will very rapidly shrink and disappear. The lifetime of bubbles smaller than 1000 nm predicted by the Epstein and Plesset theory is less than 0.02 s; therefore, such bubbles should dissolve and disappear before they can be detected or measured (Figure 2). This conclusion has had a

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EARLY NANOBUBBLE RESEARCH Dichotomy between Surface Nanobubbles and Bulk Nanobubbles. To a significant extent surface nanobubbles and bulk nanobubbles have had distinct histories, with different researchers, different techniques and different ideas being employed in their investigation. This dichotomy is still largely present today with only a few researchers actively working on both topics, and different ideas, interests, and applications are being explored in each field. Therefore, we trace the trajectories of research on surface and bulk nanobubbles separately. Early Research on Bulk Nanobubbles. Reports of bulk nanobubbles preceded reports of surface nanobubbles; therefore, they will take precedence here. Sette and Wanderlingh14 in 1962 demonstrated that high-energy neutrons present in cosmic rays, or introduced artificially, reduced the sound energy required to initiate the cavitation of bulk water. They argued that oxygen recoil nuclei deposit energy that results in the formation of cavitation nuclei that are stabilized by contaminants. Furthermore, they showed that by shielding water from neutrons the cavitation threshold energy increased over a period of ∼5 h, indicating that the microcavities persisted for at least this long. Later, Hemmingsen15 studied cavitation in solutions supersaturated with gas and found that the supersaturation threshold for cavitation could be increased by prior application of very high pressures. This was attributed to the removal of cavitation nuclei in both the bulk and at surfaces by forcing the dissolution of gas making up the cavities.15

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Figure 2. Calculated nanobubble radius versus time using the Epstein and Plesset theory1 (eq 16) for a nitrogen-filled nanobubble of initial radius 1000 nm in a solution that is saturated with dissolved nitrogen gas. (T = 300 K, γ = 0.072 J m−2, D = 2.0 × 10−9 m2 s−1, Csat = 0.6379 mol m−3, ρ1 atm = 40.6921 mol m−3).

EVIDENCE OF BULK NANOBUBBLES

Possibly the earliest direct evidence of bulk nanobubbles with diameters of less than 1 μm was reported by Johnson and Cooke in 1981.16 They reported that bubbles produced by shear in seawater were observed to be stable for long periods (>22 h) as a result of the formation of surface films formed from naturally present surfactants. They demonstrated that such encased bubbles were gas-filled, as they expanded when put under tension (negative pressure) and contracted under an applied pressure (Figure 3); moreover, some could be destroyed by the application of positive pressure. As salt water inhibits bubble coalescence,17−19 the breaking of waves in seawater readily produces large numbers of small bubbles. This work indicates that these bubbles can shrink to form nanobubbles that are stable for up to 24 h, indicating that oceanic populations of nanobubbles are likely to be substantial.16 Similarly, in fresh water with substantial levels of organic material we might expect that turbulence due to rapids or waterfalls may also produce significant populations of nanobubbles. Interestingly, fossilized nanobubbles that were coated with organic material have been preserved in Lake Kivu in the rift valley of east Africa (Figure 4).20 It has been proposed that such coated spheres are precursors to primordial cells and therefore may be significant in the development of early life.20

profound influence on nanobubble research, as it has led to reports of long-lived surface and bulk nanobubbles being treated with great caution and dubiosity.3 Indeed many researchers now active in the field brought with them strong skepticism over the existence of nanobubbles because of these predictions. The observation of nanobubbles, particularly surface nanobubbles, and their extended stability has led to the close examination of the applicability of the theory of Epstein and Plesset. A number of aspects have been examined. If the nanobubble surface is contaminated, then this can lead both to a reduction in the surface tension and to a diffusion barrier for molecules crossing the interface. Furthermore, as discussed later, the conditions under which nanobubbles are produced require supersaturation of the liquid phase, and supersaturation of the medium increases the expected bubble lifetimes. Finally, nanobubbles are not single isolated bubbles but rather exist in numbers, and as such, the swarm effect must be considered. In the case of surface nanobubbles, other effects that confer stability, such as pinning, have been proposed. Currently, the stability of bulk nanobubbles remains unexplained, and there is 11088


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Production and Characterization of Bulk Nanobubbles. The new millennium brought a burst of publications on bulk nanobubbles, and for the first time, attention was focused on measuring the physical properties and concentration of bulk nanobubbles. Kim et al.24 in 2000 reported the generation of bulk nanoparticles by sonication (42 kHz, 70 W) in the presence of a palladium-coated surface, directly in a cuvette placed in a dynamic light scattering apparatus. In purified water, they produced a bimodal distribution of nanoparticles with peaks around 80 and 350 nm. Particles created at pH 3 were slightly larger than particles created at pH 12. The particles were attributed to nanobubbles. The measured zeta potential for the nanoparticles was consistent with reports of the zeta potential measured on larger bubbles.25 The nanoparticles were stable for at least 60 min. In this work, it was not demonstrated that the particles being measured were actually nanobubbles; therefore, we must consider the possibility that surface cavitation due to ultrasound caused the formation of palladium nanoparticles. However, when the zeta potential reported is compared to that of palladium nanoparticles26 it is found to be inconsistent with the particles being palladium. Later work by the same corresponding author using a different ultrasound frequency (20 kHz) and generator but an otherwise similar method found that the nanoparticle size increased with ultrasound power and time of application.27 Particles of 700 nm and larger were produced in this manner. Between 2001 and 2009, Kikuchi et al. studied the generation of nanobubbles by electrolysis.28−32 Electrolysis evolves gas and supersaturates the solution with hydrogen gas at the cathode and oxygen gas at the anode. In the cathodic solution, particles 10−600 nm in diameter were detected by dynamic light scattering (DLS) and were shown to be stable for at least 4 h.30 In the anodic solution, particles 30 nm in diameter were produced by electrolysis and measured over 3 days, after which they had increased in size to 250 nm. After 5 days, particles were no longer detected. According to the authors, the particles in the anodic and cathodic solutions are oxygen and hydrogen nanobubbles, respectively. This work also showed that oxygen contained within nanobubbles is not detected when using a standard dissolved oxygen meter or the Winkler oxygen titration method. However, if the pH of the solution is reduced by the addition of acid, then the Winkler method detects an increase in oxygen concentration, which was attributed to the oxygen liberated from the nanobubbles. This study and the observed lifetimes suggest that nanobubbles are not rapidly equilibrating with the surrounding solution, as we would expect from the Epstein and Plesset theory. Jin et al.33−35 in a series of publications examined aqueous solutions of tetrahydrofuran, ethanol, urea, sugars, surfactants, and α-cyclodextrin by dynamic laser light scattering. These measurements as well as those of other researchers reveal a slow mode that corresponds to structures ∼100 nm in diameter. It was found that the slow mode could not be removed by simple filtration through a 20-nm-pore-sized filter but was gradually removed by numerous repeated filtrations, regardless of the type of solute present. Moreover, the slow mode was restored by the injection of particle-free air. They concluded that the slow mode was due to nanobubbles stabilized by the surface activity of the organic molecules. However, Habich et al.36 and Sedlak et al.37 performed similar experiments using degassed solutions and found that the level of light scattering remained significant. They attributed the

Figure 3. Panel A shows the distribution of bubbles in seawater (shaded region) compared to those subjected to an additional pressure of ∼0.8 atm (unshaded region). The substantial reduction in bubble size indicates that the surface contamination had reduced the surface tension and therefore the Laplace pressure to a low level. Panel B shows how an initial bubble distribution (shaded region) shifted to smaller sizes after a period of 22 h (unshaded region). This figure was taken from Johnson and Cooke.16

Figure 4. Electron micrograph of hollow spherical particles of diameter 800−1000 nm. The 50-nm-thick walls predominantly consist of a polymeric resinous material incorporating ZnS that has formed around nanobubbles of methane as they form and rise in Lake Kivu, east Africa. This figure was reproduced from ref 20.

Following this work, little was published on bulk nanobubbles until the 1990s, when Bunkin et al.21−23 reported the existence of stable microbubbles in dilute solutions of electrolytes. These microbubbles were thought to be stabilized by repulsive interactions between ions adsorbed to the interface and provide nuclei for optical cavitation. 11089


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scattering to contaminants introduced with the organic solvents or from the vessels holding the solvents rather than nanobubbles. This has led to the work of Jin et al. generally being discounted. However, the origin of the slow relaxation mode remains unresolved as the contamination argument does not explain the reported inability of filters to remove the objects and cannot explain the observed changes in compressibility of the solution upon removal of the objects that cause the slow relaxation mode,34 thus the issue remains unresolved. Evidence That Bulk Nanobubbles Do Indeed Contain Gas. The above experiments are correctly criticized for a lack of direct evidence that the nanoparticles being observed actually consist of gas. However, a number of quite varied experiments provide more direct evidence of long-lived gas-filled bulk nanobubbles. Oeffinger and Wheatley in 2004 employed surfactantstabilized nanobubbles as ultrasound contrast agents.38 The initial population of surfactant-stabilized bubbles was produced by the sonication of a perfluorocarbon gas. This produced a population of bubbles with a mean diameter >1 μm. This sample was then centrifuged to promote creaming of the larger bubbles. In doing so, the mean diameter of the dispersion was reduced to ∼400 nm. Evidence that these objects are indeed nanobubbles is twofold. First, they were demonstrably less dense than water, as the larger particles creamed more effectively during centrifugation, and second, the ultrasound contrast enhancement observed is consistent with the particles being gas-filled nanobubbles as opposed to oil droplets. Najafi et al.39 produced nanobubbles in a closed cuvette for zeta potential measurement by increasing the temperature. This reduces the solubility of dissolved gases and precipitates nanobubbles, which had a mean size of 290 nm. The zeta potentials measured were consistent with those measured for larger bubbles. Moreover, before the temperature change, no scattering was detected and a temperature increase leads to an increase in solubility for most materials, particularly candidate contaminants such as hydrocarbons. Ohgaki et al.9 in 2010 using gas injection produced nanobubbles using N2, CH4, and Ar in water. They claim extremely high nanobubble concentrations of ∼1013 bubbles per mL that persisted for up to 2 weeks. Using a technique that is well established for biological samples, nanobubbles were preserved by rapid cryogenic freezing of a droplet, which was subsequently cleaved and coated to form a replica of the surface. The replica was imaged by scanning electron microscopy, revealing a population of nanobubbles. The images are reproduced in Figure 5. In complementary work, Uchida et al.40 produced nanobubble solutions by gas injection of ultrapure oxygen. Rapid cryogenic freezing was then used to prepare replicas of the surface of a fractured water droplet, and the resulting replica was imaged by transmission electron microscopy. Nanobubbles ∼100 nm in size were revealed (Figure 6). In later work, they showed that nanobubbles produced in the same manner increased in size from ∼400 to ∼750 nm over a period of a week when stored in a sealed bottle.41 Recently, cryo-SEM has been employed to directly image bulk nanobubbles embedded in amorphous ice produced by a nitrogen-evolving chemical reaction.42 However, sample freezing leads to unavoidable perturbation of the sample. Some could argue that the observed features are only defects induced by the freezing. Countering this is the extensive use of this technique in biological imaging

Figure 5. Panel A: Rapid cryogenic freezing and subsequent fracture of a solution of nanobubbles was used to obtain replicas of the surface, which were subsequently imaged by scanning electron microscopy to reveal nanobubbles ∼100 nm in diameter. Panel B: A highermagnification image of a single nanobubble. (Images were taken from ref 9.)

Figure 6. Replica transmission electron microscopy images showing spherical objects thought to be O2-filled nanobubbles. The image is taken from ref 40.

and the general acceptance that rapid freezing does not generally produce artifacts of this nature. Bunkin et al.43 using a modulation interference microscope to image nanobubbles in sodium chloride solutions. This technique can determine the refractive index of particles with respect to the surrounding medium. The results show that a latex particle registers a higher refractive index than the surrounding medium, whereas a nanobubble registers a significantly lower refractive index than the surrounding medium as depicted in Figure 7. Significantly, unlike most 11090


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Figure 7. Optical density images of a latex particle (left) and a nanobubble (right) demonstrating that the refractive index of the nanobubble is significantly lower than that of the surrounding solution. Images were taken from ref 43.

Figure 8. Instrument trace of the resonance frequency as particles pass through the microresonator of an Archimedes particle analyzer in which the positive peaks are not usually seen and represent particles with density lower than that of the solution (left). The size distribution obtained by the measurement of individual particles is shown on the right. (The figures are taken from ref 44, in which the left image has the colors inverted to improve the presentation.)

any information on the device or method used to produce bubbles other than to say it was a commercial device. There is a serious concern that many devices sold as nanobubble generators have little to no evidence to support their claims. In summary, evidence that nanobubbles are indeed gas-filled comes from a range of experiments. The particles have been shown to be less dense than the aqueous solution surrounding them, have a lower refractive index, provide a very strong reflection of sound waves, have surface charges consistent with the air−water interface, and have been imaged using a range of well-established cryogenic techniques. One may make a range of arguments to dismiss each of these studies. The experiment of Kobayashi,44 however, is very convincing, as the technique measures the density of the particles and this was found to be consistent with them being gas-filled. However, this result needs to be independently verified before it can be widely accepted. Explanations for the Stability of Bulk Nanobubbles. A number of explanations have been proposed for the long-time stability of bulk nanobubbles. In some cases, the solution is highly supersaturated and this will extend the lifetime, but this cannot be said for all circumstances. Using the Epstein and Plesset theory, we can calculate the influence of saturation levels on nanobubble lifetimes. This is shown in Figure 9. If the

other studies of bulk nanobubbles there is no reason to suppose that these solutions were supersaturated with dissolved gas. Perhaps the most direct evidence that bulk nanobubbles consist of gas was presented by Kobayashi et al. in 2014.44 An instrument called the Archimedes was used to detect the mass density relative to the solvent of individual nanoparticles as they passed one by one through a microresonator.45 As particles pass through the resonator, the resonance frequency changes, decreasing for particles denser than water and increasing for particles less dense than water.45 Moreover, the size of the particles can also be measured. Measurements on air-filled bubbles generated by gas injection revealed a population of nanoparticles with a modal size of 112 nm with positive buoyancy and a very low mass density (Figure 8). The size determined by this method agreed with that determined by other techniques such as dynamic light scattering and single particle tracking. Leroy et al.46 have examined solutions treated with a bubble generator for evidence of bulk nanobubbles using ultrasound and found no evidence of bulk nanobubbles. This technique is of interest as it is sensitive to the presence of gas. The sensitivity of this technique is currently insufficient for the detection of bulk nanobubbles at the concentrations typically reported (108/mL). Unfortunately, the authors did not provide 11091


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that potentially contain bulk nanobubbles and determine if they are indeed gaseous. Uses and Potential Applications of Bulk Nanobubbles. Despite the limited amount of research into bulk nanobubbles, the level of industry activity and interest in the field is growing rapidly. This is reflected in the efforts to set an ISO standard (ISO/TC281) for fine bubbles, which includes ultrafine bubbles, the growing membership of the Fine Bubble Industry Association (FBIA), and the increasing number of patents in the area. Below we provide a brief summary of the currently reported and potential applications of bulk nanobubbles. The terminal rise velocity of a spherical bubble of radius r, due to buoyancy, UT, is dependent on the boundary condition and for a no-slip boundary condition is given by

Figure 9. Bulk nanobubble lifetimes calculated using the Epstein and Plesset theory1 (eq 16) for an N2-gas-filled nanobubble of initial radius 1000 nm as a function of the saturation level of N2 gas in solution. When the saturation level is matched to that required for the bubble to be stable, then the lifetime is effectively infinite. However, a very small deviation in the saturation level below that required for equilibrium leads to very rapid dissolution of the bubble. Similarly, a very small deviation of saturation above the level required for equilibrium would lead to rapid growth of the bubble (not shown) (T = 300 K, γ = 0.072 J m−2, D = 2.0 × 10−9 m2 s−1, Csat = 0.6379 mol m−3, ρ1 atm = 40.6921 mol m−3) .

UT =

2r 2Δρg 9μ

(3)

where Δρ is the difference in density between the bubble and the solution, g is the acceleration due to gravity, and μ is the viscosity of the liquid. If a slip boundary condition is employed, then the terminal velocity is 1.5UT, though studies show that a no-slip boundary condition is appropriate in nearly all cases because of minute amounts of contamination.50,51 The calculated no-slip terminal rise velocity for a nanobubble of radius 50 nm is 2.7 nm s−1, and that for a nanobubble of radius 500 nm is 272 nm s−1. These extremely low rise velocities mean that the effect of buoyancy on nanobubbles is insignificant compared to Brownian motion. A number of applications make use of this, particularly those that require water oxygenation as smaller bubbles have longer residence times and therefore have more time to deliver gas to solution and a larger surface area for a given volume. Moreover, smaller bubbles have higher Laplace pressures and consequently increase the solution saturation concentration of the gas surrounding a bubble. Ebina et al. have reported substantial positive effects of water infused with oxygen nanobubbles, less than 200 nm in diameter, on the growth of plants, fish, and mice.52 If these effects are verified and can be applied more broadly to farming, then very substantial benefits are expected. Oxygenation using nanobubbles is also being applied to the bioremediation of groundwater pollution53 and water treatment.53,54 Bulk nanobubbles are finding biological and medical applications. Bubbles are effective ultrasound contrast agents because of their extremely low density and their nonlinear response to ultrasound.6,55−59 Nanosized bubbles are preferred in some applications as they can be used to target the capillaries outside the pulmonary bed.38 Nanobubbles or related entities have also been implicated in oxygenated medical saline for the treatment of asthma and other autoimmune diseases with remarkable efficacy.60−67 Hydrogen nanobubbles are also present in health drinks that employ a cathodic stream from electrolysis.68,69 Nanobubbles have also been implicated in the disruption of water transport due to hydraulic failure in the xylem of trees.70 Nanobubbles are being assessed for application to mineral separations using froth flotation.71 Another possible application is in depletion flocculation72 for the dewatering of mineral tailings. Depletion flocculation usually employs large quantities of high-molecular-weight nonadsorbing polymers, which impart an attraction between colloidal particles. If this could be carried out using nanobubbles, then it would be both economically and environmentally advantageous and would eliminate the need to

solution is saturated such that it is in equilibrium with the nanobubble, then the predicted bubble lifetime is infinite. However, a very small deviation from the equilibrium condition dramatically influences the stability, such that a deviation of only 0.0001% below the saturation level would see the nanobubble dissolve in 2 s. Thus, supersaturation can confer stability only under very narrow circumstances, which are never likely to be maintained for any significant time under anything but the most contrived and controlled circumstances. The effect of very high concentrations of nanobubbles has also been considered. Under these circumstances, the diffusion of gas out of a nanobubble is slowed by the effective increase in dissolved gas concentration due to surrounding bubbles.47 This is likely to be significant only at very high volume fractions. In the work of Johnson and Cooke,16 the formation of a skin of contaminant molecules was clearly implicated in the stability of the bubbles they observed. However, in other nanobubble preparations the level of contamination will be much lower, so this is unlikely to be a universal stabilizing mechanism. Bunkin22 proposed that ions at the air−water interface repel each other, effectively reducing the surface tension. However, this implies that the air−water interface cannot charge regulate and that the concentration of ions at the interface would not be determined by the chemical potential set by the concentration in the bulk. A recent explanation proposed by Yasui et al.48 is an adaptation of the dynamic equilibrium model proposed by Brenner and Lohse49 to explain the stability of surface nanobubbles. In this case, solid particles adsorbed to the nanobubble surface are thought to provide a reservoir of gas, as the substrate does in the dynamic equilibrium model. This model has already been abandoned for surface nanobubbles. The adaptation here requires that a reasonable fraction of the nanobubble surfaces are covered with hydrophobic particles, which is not supported by the experimental evidence. In summary, there is currently no accepted explanation for the long-term stability of bulk nanobubbles; therefore, reports of bulk nanobubbles are generally treated with skepticism. In response, new techniques are being adapted to study systems 11092


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remove the flocculant for downstream processing. Nanobubbles are appealing for use in cleaning applications as a dispersion of nanobubbles presents a significant surface area of high interfacial tension that can attract contaminants as a result of favorable energetics and thereby prevent their deposition onto surfaces. Bulk nanobubbles have been demonstrated to effectively clean proteins from surfaces.73 The absence of added surfactants means that chemical residues are not an issue and that the cleaning process is likely to be more environmentally friendly.

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SURFACE NANOBUBBLES Surface Forces and Long-Range Hydrophobic Attraction. The first hint of the existence of surface nanobubbles arose from surface force measurements between hydrophobic surfaces immersed in aqueous solutions. Israelachvili and Pashley in 1982 reported that the interaction forces measured using a surface forces apparatus between mica surfaces made hydrophobic by surfactant adsorption from solution were more attractive than expected from electrostatic or dispersion forces.74 The additional attraction they reported that extended to a separation of ∼10 nm became known as the long-range hydrophobic attraction or interaction. It is interesting that the mechanism of this attraction (as distinct from much longer ranged, more extreme, attractive forces also reported as the long-range hydrophobic attraction) is yet to be resolved.75 A particular advantage of the surface forces apparatus is that the optical fringes used to determine the separation of the surfaces also reveals the refractive index of the material between the surfaces. The technique has a resolution of ∼0.1 nm in separation but ∼1 μm laterally. When measuring surface forces between neutral (uncharged) hydrophobic surfaces, Christenson and Claesson observed that upon separation the hydrophobic surfaces were bridged by a cavity.76 In this experiment, it was not possible to determine if the cavity was gas-filled (i.e., a bubble) or vapor-filled. The resolution was not sufficient to determine if nanosized bubbles were present on the surfaces before they were brought into contact, but the cavity was observed to rupture when the surfaces were separated by a sufficient distance. Discovery of Surface Nanobubbles. Over the next decade, researchers reported unexpected and unexplained attractive forces between hydrophobic surfaces that extended to extreme separations in excess of 100 nm. Moreover, it was noted that the strength of the interaction varied with different surfaces and over time during an experiment. How the hydrophobic surfaces interacted over such an extended range in water was extremely difficult to explain. In 1994, Parker, Claesson, and Attard2 proposed that the attraction was due to the presence of nanosized bubbles on hydrophobic surfaces. They posited that when two surfaces were brought into proximity a bridging bubble led to attraction between the surfaces. Using a new highly sensitive force-measuring technique, they observed distinct steps in the attractive force between hydrophobic surfaces. Each step in the force curve was attributed to the effect of an additional nanobubble bridging the surfaces (Figure 10). It is notable that this explanation for the extremely long range attractions measured between hydrophobic surfaces is now accepted, but it was not widely accepted at the time. The concern was due to the Laplace pressure bubble catastrophe that predicted that the nanobubbles responsible for the attraction should not be stable. This argument with regard to nanobubbles influencing surface force

Figure 10. Attractive steps between two hydrophobic surfaces attributed to the bridging of nanobubbles between hydrophobic surfaces. The image was taken from Parker et al.2

measurements was presented by Ljunggren and Eriksson,3 who using a treatment parallel to Epstein and Plesset1 showed that the nanobubbles required to explain the observed attraction were expected to exist for less than a second, whereas experiments showed that the attraction persisted for many hours after the surfaces were immersed in water. This led them to conclude that “air bubbles of colloidal size in water have a short lifetime, ranging from about 1 to 100 micro seconds···it is··· difficult to speculate that the proposed bridging cavity mechanism yields a valid explanation of the longrangeness of the hydrophobic attraction”.3 However, in the same year it was demonstrated that the range of the hydrophobic attraction between hydrophobic surfaces was reduced when the solution was degassed.77 Resolving the contradiction between observations of surface nanobubble lifetimes and the expected lifetime from the Epstein and Plesset theory remains today as the central challenge in the field of surface nanobubbles.78 Early Research on Surface Nanobubbles. The first images of surface nanobubbles were published at the turn of the millennium by groups working independently in China and Japan. Ishida et al. imaged surface nanobubbles by tapping mode atomic force microscopy and showed a clear correlation between the existence of surface nanobubbles and the range of the long-range hydrophobic attractive force.4 They also showed that the nanobubbles were likely produced when the hydrophobic surface was initially immersed in water because surfaces that were made hydrophobic after immersion were shown to have an absence of nanobubbles and the very long-range attractive force was absent. Cross sections of nanobubbles taken from AFM images showed that the nanoscopic contact angle was substantially different from the microscopic contact angle for a bubble on a hydrophobic surface. This observation of the anomalous contact angle has been reproduced by many other researchers and is yet to be adequately explained. For a recent discussion of this issue, see An et al.79 Around the same time Lou et al. produced high-quality AFM images of swarms of nanobubbles on a highly ordered pyrolytic graphite (HOPG) substrate.5 The nanobubbles were produced by what has become known as the solvent exchange technique. Here the hydrophobic surface is immersed in ethanol, which is subsequently displaced with aqueous solution. This solvent 11093


Langmuir


more, the contact angle did not show a significant trend as a function of the radius of curvature of the nanobubble, indicating that line tension is not responsible for the contact angle anomaly.88 To determine if surface nanobubbles were stabilized by contaminants, nanobubbles were studied in cationic, anionic, and nonionic surfactants.8 These surfactants should dissolve or displace contaminants at the surface of the nanobubbles. The nanobubbles were stable in all cases. Surfactants changed the contact angle slightly and relaxed the pinning of the three-phase line on OTS silicon, leading to circular three-phase lines. The addition of surfactant changes the imaging process. Surfactant adsorbs to the AFM tip and the bubble surface, preventing the AFM tip from penetrating the nanobubble. This enabled the level of deformation of the nanobubble to be controlled by the imaging conditions and the surface tension of the surfactantcoated nanobubble to be estimated as 43 mN m −1 . Furthermore, it was shown that under load the nanobubbles deformed such that they appeared to be smaller, and when the load was reduced, the bubbles returned to their original size. This is consistent with the objects being gas-filled bubbles. Surface nanobubbles were also shown to be stable in a wide range of electrolytes at concentrations of up to 1 M and at both very low and very high pH. This discounted the possibility that the stability was due to electrostatic interactions. Additionally, such extremes of pH should strongly influence any contaminants adsorbed at the surface, but no changes were observed. The conditions that generate nanobubbles usually involve supersaturation, and if this supersaturation is maintained, then this can lead to large increases in the expected lifetime of nanobubbles within the Epstein and Plesset theory.1 Therefore, it is worth noting that these experiments involved exchanging the solution for a surfactant or salt solution that was not supersaturated.8 Despite this, the nanobubbles were observed to be stable for long periods of time (Figure 12.). Other Methods and Techniques for Studying Surface Nanobubbles. Methods other than solvent exchange have been used to produce surface nanobubbles. The electrochemical generation of surface nanobubbles by the electrolysis of water has been demonstrated at the electrode surface by atomic force microscopy.89−91 White et al. have examined the

exchange results in the supersaturation of dissolved gas at the interface and the nucleation of nanobubbles. This technique has become a standard method for the production of surface nanobubbles, though the effectiveness of the solvent exchange technique is variable, perhaps reflecting the stochastic nature of nucleation. Saturation levels of up to 3 times the equilibrium concentration can be obtained by ethanol−water solvent exchange.80 Thus, these early papers established three pillars of surface nanobubble research: the use of the solvent exchange technique for producing surface nanobubbles, the use of tapping mode atomic force microscopy for imaging surface nanobubbles, and degassing solutions in order to remove or avoid the generation of surface nanobubbles. They also showed that nanobubbles were long-lived and highlighted the anomalous contact angle of nanobubbles. The scene was set for a host of research over the next 15 years that has led to the existence of the field of surface nanobubble research. Characterization of Surface Nanobubbles. Following from these initial reports of surface nanobubbles, researchers sought to characterize them more fully. Yang et al.81 produced nanobubbles on surfaces made hydrophobic by reaction with trimethylchlorosilane by the introduction of solution supersaturated with CO2. The silanation process produces a heterogeneous surface, and this is reflected in the three-phase line of nanobubbles on silanated surfaces deviating from a circle as a result of strong pinning effects. They examined the cross section of the nanobubbles and confirmed that the bubble shape was that of a truncated sphere and the contact angles (as measured through the aqueous phase) were anomalously high. Zhang et al. sought to measure the physical properties of surface nanobubbles, such as the surface tension, and examined the stability in the presence of electrolytes and surfactants.8 The imaging process causes deformation and penetration of the nanobubble and a significant underestimate of the height of the nanobubble; therefore, they compared the section profile of a nanobubble from a tapping mode image to the reconstruction of a nanobubble profile using a series of force curves, as shown in Figure 11. The force data can be used to determine the point

Figure 11. Profile of a surface nanobubble obtained from a series of force curves (upper curve) and an AFM image. The height in the AFM image is underestimated because of the penetration of the AFM tip into the nanobubble. The image was taken from ref 8.

at which the cantilever tip first contacts the nanobubble and the point at which it contacts the underlying substrate and therefore provides a means to accurately determine the height at a given point. A comparison of the data shows that the underestimation of the height from a tapping mode image leads to an overestimation of the contact angle. However, this overestimate is typically

A History of Nanobubbles - Langmuir (ACS Publications)

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