Very small bubbles at surfaces—the nanobubble puzzle

Abstract

Atomic Force Microscope images and other experiments show us that very small stable bubbles, known as nanobubbles, can be present on surfaces despite well founded theoretical considerations that predict otherwise. Nanobubbles are thought to play a role in the rupture of thin films during froth flotation, hydrodynamic slip over surfaces, interaction forces between hydrophobic surfaces and influence the electroplating and electrolysis processes. Here we describe what is known of nanobubbles and discuss the challenges in understanding nanobubble morphology and stability.

---

Vince Craig completed his Bachelor of Science (Honours) in Chemistry in 1993 and his PhD in Physical Chemistry in the Department of Chemistry at The Australian National University in 1997. After postdoctoral appointments in the Department of Chemical Engineering and Materials Science at UC Davis in California and The Department of Chemical Engineering at The University of Newcastle in Australia he took up an Australian Postdoctoral Research Fellowship in the Applied Mathematics Department at ANU. This was followed by an Australian Research Fellowship and the recent award of an Australian Research Council Future Fellowship. He is currently the Head of Department. His research interests include bubbles, ion specificity and the adsorption of surfactants and polymers at interfaces.

---

The physics of a bubble

Bubbles and surface tension

We have all had the experience of creating bubbles. One method is to place a tube into a liquid and blow. It is necessary to apply pressure to create a bubble as it requires energy to overcome the effects of surface tension. The surface tension or surface energy describes the energy cost per unit area of producing new interface and is intimately related to the intermolecular forces that hold materials together. Consider a droplet in air. In the bulk of the droplet the molecules interact favourably with surrounding molecules and in doing so adopt a lower energy state. If a molecule is brought to the surface of the material it is no longer surrounded by other molecules and therefore it must lose favourable interactions with some neighbouring molecules. The stronger the intermolecular bonding in the material the greater the energy cost of producing new surface and higher the surface tension. So the energy cost of producing a bubble of a given size in a liquid is proportional to the surface tension. Surface tension is also responsible for the shape of bubbles. Consider a small gas bubble in water. In the absence of gravity, the bubble will form a perfect sphere. A sphere is formed, as for a given volume of gas this shape has the smallest surface area and therefore the smallest energy cost associated with creating interface. A means to reduce the energy cost associated with the interface is to reduce the area of the interface by compressing the gas in the bubble. This effect will be balanced by the increase in pressure within the bubble. The increase in pressure inside a spherical bubble due to the surface tension is described by the Young–Laplace equation, which for a spherical object is given by,

ΔP= 2γ/r (1)

Where, γ is the surface tension of the bubble (i.e. the vapour–liquid interface) and r is the radius of the bubble. Note that the extra pressure inside the bubble is inversely proportional to the radius of the bubble. Therefore, small bubbles have high internal pressure. This effect can be very significant, for a bubble of radius 100 nm in water the extra pressure in the bubble is 1.45 MPa or ∼14.3 atmospheres and for a 10 nm radius bubble, the extra pressure is 14.5 MPa.

Bubble lifetime

The interface of a bubble surface is not impermeable, therefore gas and vapour molecules can move from the bubble to the solution and vice versa. The net flow of gas is determined by the solubility of the gas and the amount of gas dissolved in the liquid in which it is immersed. The solubility of gas varies with both temperature and pressure. The solubility of gas as a function of pressure is described by Henry's law, which states that the solubility of a gas is proportional to the partial pressure of the gas. This has an important consequence for small bubbles. We have shown above that the pressure within a bubble is higher than the surroundings and is dependent on the bubble radius; the smaller the radius, the higher the pressure. This elevation in pressure results in an increase in solubility of the gases that make up the bubble in the surrounding liquid. Consequently, gas leaves the bubble by diffusion and is solvated in the liquid in order to establish equilibrium. The loss of gas from the bubble leads to a decrease in bubble size and an increase in bubble pressure, as described by the Young–Laplace equation (eqn (1)). The increased pressure within the bubble results in a further increase in solubility and more gas leaving the bubble. This positive feedback cycle can rapidly lead to the dissolution and disappearance of small bubbles. For millimetre sized bubbles the effect is small and the bubbles are essentially kinetically stable, but for micron sized bubbles the theory predicts that they should disappear within tens of milliseconds,² and other work has shown that the attachment of the bubble to a surface has only a small effect on the bubble lifetime and is only weakly dependent on the contact angle.³ Further, temperature should have only a weak effect on the lifetime of a dissolving bubble.³ Note that if the liquid is supersaturated with gas, the feedback cycle operates in reverse and this can result in the rapid growth of bubbles from small nuclei, as is commonly observed when the lid on a carbonated beverage is opened and the pressure is released. The predicted lifetime for dissolution of a bubble as a function of the starting radius is shown in Fig. 1. It is worth noting that for small bubbles the bubble lifetime is not a function of surface tension even though surface tension drives dissolution, which is rapid and diffusion limited. Because of the solid basis on which these theories are built, early reports of the existence of stable nanobubbles at surfaces were greeted with scepticism and an accepted description of the stability of nanobubbles is still lacking.

Fig. 1 Theoretically predicted bubble lifetime for a bubble in water, as a function of starting radius and gas type.² The parameters used were diffusion coefficient in water 2 × 10⁻⁹ m² s⁻¹; Henry's Law coefficient for oxygen 7.9 × 10⁴ J mole⁻¹; nitrogen 15.6 × 10⁴ J mole⁻¹ and hydrogen 13 × 10⁴ J mole⁻¹ at temperature 298 K.

Early evidence for nanobubbles

The first report of the existence of nanobubbles in the literature appeared in 1994. Parker et al.⁴ were using a highly sensitive force measurement device to measure the attractive force between two hydrophobic surfaces in water. They found that the attractive force as a function of distance exhibited clear steps when the surfaces were brought together. They interpreted the attraction and the presence of these steps as arising from the presence of nanobubbles on the surfaces. As each nanobubble bridged the gap between the surfaces it would give rise to a stepwise increase in the attractive force. From the experimental data, they could give an estimate of the height of the nanobubbles and found them to be <100 nm. At the time this report was highly controversial as there were theoretical objections to the existence of stable nanobubbles as we have described above. Significantly, this study tied the existence of nanobubbles to the long-range hydrophobic attraction which is measurable between hydrophobic surfaces in aqueous solutions.⁵ It is now widely accepted that the very-long range and variable component of this force is attributable to capillary forces that arise when nanobubbles bridge between hydrophobic surfaces—though other aspects of this force are not fully resolved.

A much earlier study by Blake and Kitchener⁶ into the interaction of macroscopic bubbles with glass surfaces could also be cited as a possible evidence for the existence of nanobubbles. They found that when a macroscopic bubble was pushed towards a native glass (hydrophilic) surface the repulsive electrostatic forces acted to prevent drainage of the liquid film between the bubble and the surface, as predicted by the DLVO theory of colloidal interaction. However, if the silica surface was made hydrophobic by silanation, the liquid film often became unstable at thicknesses of the order of 100 nm. This is notable as the silanation process was found to have almost no effect on the surface charge of the silica surface and therefore one expects that the long-range forces would be unchanged. At the time the authors were concerned that their data may have been affected by the existence of small particles on the surfaces, though it now seems likely that the presence of nanobubbles may have influenced their results.

A significant step in the development of research into nanobubbles came with the arrival of the new millennium when images of nanobubbles obtained using the Atomic Force Microscope (AFM) were published by two independent groups working in China¹ and Japan.⁷ It is noteworthy that both of these reports demonstrated that the presence, or absence, of nanobubbles is strongly dependent upon the history of the sample. This is significant as it explains why some researchers saw evidence of nanobubbles and others did not in experiments using similar systems. They had employed different surface preparation protocols. Protocols that involved cleaning with ethanol or displacement of ethanol by water are now known to reliably produce nanobubbles. This is discussed further below. In the work by Ishida et al.^7,8 they were able to demonstrate that the range of the hydrophobic attractive force was extended when nanobubbles were present on the surface. Nanobubbles were found to have a diameter of ∼650nm and a height of ∼40nm. Importantly they reported that nanobubbles were found when a hydrophobic surface was immersed in water, whereas if the surface was initially hydrophilic and rendered hydrophobic by chemical reaction without exposure to air, no nanobubbles were found. It is expected that nanobubbles will form more easily on hydrophobic surfaces and that they will attach to them more strongly. Further evidence that the features in the images were actually gas pockets was provided by repeating the experiments using degassed water and finding no nanobubbles. The work by the Shanghai group,^1,9,10 focused on the production and imaging of nanobubbles and they presented some high quality detailed images of nanobubbles on both mica surfaces and Highly Ordered Pyrolytic Graphite (HOPG) surfaces (for examples see Fig. 2). Importantly in this work the authors describe the first method of controllably producing nanobubbles. This is achieved by a solvent exchange method which is now widely used and is discussed in more detail below. Both of the groups reported that the nanobubbles observed were stable for hours.

Fig. 2 AFM tapping mode images of nanobubbles on HOPG in water. Image size 10 µm × 10 µm. In the lower image, it can be seen that the imaging process has led to the removal of bubbles in a 4 µm × 4 µm area. Image taken from ref. 1.

Confirmation of the existence of nanobubbles

The most direct evidence for the existence of nanobubbles is provided by Atomic Force Microscope images.^7,9,11–13 These images are obtained in ‘tapping mode’ whereby the cantilever is oscillated at or near the resonant frequency which is generally in excess of 10 kHz in water and the amplitude of the oscillation is used to control the feedback of the instrument. That is, the surface separation is adjusted to keep the oscillation amplitude at a predetermined level and the adjustment in separation required is mapped in two dimensions to produce a height image. This technique is commonly employed to image soft surfaces, such as biological samples, in fluids. Tapping mode imaging of nanobubbles is influenced by the imaging conditions,¹³ nonetheless careful analysis has revealed a great deal about the morphology of nanobubbles. The other common mode of imaging using the AFM is contact mode, which is known to be more suitable for hard surfaces. In general nanobubbles are too soft to be revealed by contact mode imaging. Developments in AFM technology have made imaging of nanobubbles routine, whereas the early studies required a great deal of expertise from the operator of the instrument.

Additional evidence for nanobubbles has been provided by rapid cryofixation and freeze fracture,¹⁴ which has been used to study the interface between water and a silicon substrate in both the native state and following treatment with hexamethyldisilazane vapor, which renders the surface hydrophobic. By freezing the sample at a very rapid rate, the ice is trapped in an amorphous state and any structures present are preserved without modification. It is then necessary to fracture the sample and image the interface. This is done using techniques that are well established for the imaging of biological samples. This study found that no structures were visible when a hydrophilic surface was employed or if the hydrophobic surface was used in conjunction with degassed water. In contrast, voids were found on hydrophobic substrates in contact with gassed water that are commensurate in size with the images of nanobubbles obtained by AFM studies. These findings parallel the imaging studies of Ishida et al.⁷ and others.^15,16 Whilst this study is important in that evidence for nanobubbles was found using an alternative technique, it was unable to provide additional morphological information.

Both AFM images and the cryofixation and freeze fracture techniques provide evidence for the existence of structures at the interface but they do not provide chemical information on the make-up of these structures. It is possible, for example, that these structures are formed by an organic contaminant rather than being gas-filled bubbles. Experiments where nanobubbles are removed upon degassing of solutions were performed to address this^7,15,16 and in an elegant experiment Zhang et al.¹⁷ demonstrated that nanobubbles do indeed consist of gas. They used CO₂ saturated water to produce nanobubbles by the solvent exchange method.¹CO₂ has an infrared spectrum that varies considerably between the gaseous and aqueous states due to the rotational fine structure that is visible in the gaseous state and therefore it can be used to directly reveal the phase state of the nanobubbles. Furthermore, from the rotational fine structure the pressure of the CO₂ gas within the nanobubbles could be approximately determined and was found to be consistent with that expected from the Young–Laplace equation for the curvature of the nanobubble interface.

Nanobubble characteristics

The size, shape, contact angle, surface tension and internal pressure within a nanobubble are of interest not only in describing nanobubbles but also in addressing the issue of nanobubble stability. When looking at AFM images it should be remembered that most are presented with a considerable vertical exaggeration. The scale in the z dimension can be 100 times smaller than the scale in the x and y dimensions. This can easily give rise to an incorrect perception of the shape of nanobubbles. A sectional profile through a nanobubble can be very useful in gaining an understanding of their true shape. However, due to imaging artefacts caused by the deformability of nanobubbles a section taken through a standard tapping mode image can be misleading. The interaction of the cantilever tip with the nanobubble is determined by both colloidal and capillary forces and depends on the solution conditions and any material adsorbed at the interface. In water and electrolyte solutions, the tip of the cantilever may penetrate the nanobubble and this leads to an underestimate of the height. If surfactant is present it stabilises the liquid film between the nanobubble and the cantilever tip and prevents the tip from penetrating the nanobubble. In this case the nanobubble is deformed by the pressure of the tip and the degree of deformation is dependent upon the imaging conditions, in particular the amount of energy that the tip loses when contacting the surface (amplitude setpoint). Fig. 3 illustrates the influence of setpoint on the image of a nanobubble in the presence of surfactant. As the setpoint amplitude is reduced (A to G) the nanobubbles appear smaller due to the tip deforming the interface. Images H to J, show that the nanobubbles recover when the setpoint amplitude is increased (i.e. when the imaging force is reduced). This shows that the nanobubbles are resilient to the action of the cantilever tip.

Fig. 3 Images of the same two nanobubbles in a non-ionic surfactant solution under different imaging conditions. The force with which the tip hits the bubble is increased on moving from image A to G and then decreased again from H to J. The nanobubbles appear smaller as the setpoint amplitude is reduced as the nanobubble-tip force is increased and the tip induced deformation of the nanobubble increases. That is, the image is formed from the position of the loaded interface and when it is under a larger load it deforms to a greater extent and further underestimates the apparent size of the nanobubble. Image taken from Zhang et al.¹³

Zhang et al.¹³ have shown that a truer measure of the shape of a nanobubble is gained by using the information in deflection versus piezo displacement data. This method which was first developed for measuring nano-droplets,¹⁸ allows the height of the nanobubble at a particular point to be determined accurately and can be obtained at each point of an image using a technique widely known as force-volume imaging.

Researchers have consistently reported that the nanobubble shape is accurately described as a spherical cap (except for reports on “nanopancakes” which are discussed below). This infers that the curvature of the interface is everywhere the same and that the pressure drop across the interface is constant. If surface forces had a significant impact on the stability of nanobubbles one would expect that this would result in a change in curvature of the interface with separation from the surface.¹⁹ The constant interfacial curvature is evidence that long-range surface forces are not responsible for the stability of nanobubbles as the magnitude of such forces reduces rapidly with distance and this would be revealed as a variable contribution to the pressure with distance from the surface. That is, the curvature of the interface would change. However, it is typically difficult to determine the shape of the interface within the last 20 nm of the surface, due to the finite size of the cantilever tip, so the influence of surface forces within the last 20 nm cannot be gauged at this stage.

When the sectional profile of a nanobubble is accurately determined the contact angle can also be determined with reasonable accuracy. Surprisingly, the nanoscopic contact angle differs considerably from the macroscopic contact angle. In many cases the contact angle has been reported to be in the range of 10° to 40°,^7,11 where the angle has been measured through the gas phase. However, the correct definition of the contact angle at the three phase line is to measure the angle through the more dense phase, which in this case is the liquid, thus the true value is given by 180°–x°, where x° is the angle measured through the gas phase and the contact angles are more correctly reported as being in the range of 140° to 170°.^7,13 These exceptionally high contact angles are much larger than the corresponding macroscopic contact angles which are in the range of 80° to 120°. This has several significant implications. The first is on the shape of a nanobubble. This is illustrated in Fig. 4 where two bubbles of the same volume are depicted one with the macroscopic contact angle and the other with the nanoscopic contact angle. The shape is influenced dramatically. When the contact angle is 170° the bubble has a maximum height of only 15 nm and the diameter of the base is 347 nm, whereas for a contact angle of 110° the bubble has a maximum height of 58 nm and the diameter of the base is only 165 nm. The curvature of the interface is greatly reduced by an increase in nanobubble contact angle, in the example in Fig. 4, the radius of curvature is 1000 nm when the contact angle is 170° (see the bubble on the right), whereas the same volume with a contact angle of 110° has a radius of curvature of 88 nm (left bubble). Thus the Laplace pressure is also greatly reduced—for the high contact angle case it is 1.4 atmospheres compared to 16.4 atmospheres for the lower contact angle. In this case the greater contact angle reduces the Laplace pressure by a factor of 11.7. Recent work has found contact angles for nanobubbles on HOPG surfaces that are somewhat lower (but still much higher than the bulk value) of approximately 120°. This work was based on the assumption that the AFM images obtained gave a true undeformed image of the nanobubble and suggested that much higher contact angles obtained on HOPG surfaces have their origin in surface contamination.²⁰ However, images from other researchers reporting high contact angles do not appear to show such contamination, which is reported to cause a higher degree of roughness of the HOPG surface. Evaluation of the contact angle requires further work before this issue is fully resolved.

Fig. 4 Schematic diagram indicating the influence of the contact angle on shape for two spherical cap bubbles of equal volume (7.25 × 10⁵ nm³) with the normal macroscopic contact angle (on the left, θ₁ = 110°) and the observed nanoscopic contact angle (on the right, θ₂ = 170°). The radius of curvature is much smaller for the macroscopic contact angle, (R₁ = 88 nm < R₂ = 1000 nm) as is the bubble diameter (D₁ = 83 nm < D₂ = 174 nm). Whilst the maximum height is larger (H₁ = 58 nm > H₂ = 15.2 nm) for the bubble with the macroscopic contact angle. The increase in the radius of curvature associated with the nanoscopic contact angle results in a much reduced internal pressure and contributes to the stability of nanobubbles.

A reduction in interfacial curvature and a consequent significant reduction in the internal pressure within a bubble aid greatly in our understanding of the stability of nanobubbles, as this means that the driving force for dissolution is greatly reduced and with a small degree of gas supersaturation we can expect that the bubbles are kinetically stable for hours and even days. A small degree of gas supersaturation can arise from solvent exchange or temperature changes. Whilst a high contact angle aids in our understanding of the stability of nanobubbles it is not clear as to why the contact angle should be so high. It has been suggested that the line tension at the three-phase line is significant and for such small volumes can have a significant effect on the contact angle.¹¹ Though line tension should be revealed as a consistent change in contact angle as a function of nanobubble size and this is generally not seen.²⁰ An alternative explanation is that very small scale roughness can lead to pinning of the interface and results in a contact angle that is far from the equilibrium value.¹⁶ It is worth noting that nano-scale liquid droplets do not have contact angles that differ from the macroscopic value. This suggests that the nanobubble contact angle anomaly is associated with the low density of molecules in the gaseous phase and possibly their confinement in the z direction. Interestingly, the height of a nanobubble is of the order of the mean free path, so in the z direction a molecule can travel across a nanobubble without colliding with any others.²¹

The internal pressure in a nanobubble is not only controlled by the curvature of the interface but also the surface tension of the interface. One can imagine that the high energy air–water interface could readily adsorb contaminants from solution, particularly as it ages. This will result in a lower surface tension and a reduced Laplace pressure. Zhang et al.¹³ have been able to estimate the surface tension of a nanobubble in a non-ionic surfactant solution by evaluating the deformation of the nanobubble interface under the pressure of the AFM tip. A value of 43 mN m⁻¹ was obtained which is commensurate with the literature values for macroscopic interfaces with the same concentration of this surfactant. Consequently the Laplace pressure would be reduced to ∼60% of the Laplace pressure present for a clean water interface and the same bubble geometry. Unfortunately, this method cannot be applied to nanobubbles in the absence of surfactant as the tip of the cantilever penetrates the nanobubble. The difference in the interaction between the tip and nanobubbles in water and surfactant solutions suggests that nanobubbles in water have an interface that is not highly contaminated, though the presence of small amounts of contamination cannot be ruled out.¹³ Currently we have no information on the nature of possible contaminants, but one can hypothesise that the material is surface active and of low solubility and may therefore consist of hydrocarbons with chain lengths greater than 16 which are partially oxidised and therefore have carboxylic acid or alcohol functionality as such material is ubiquitous and will be preferentially adsorbed at the air–water interface. Alternatively, minute particles (or clusters of atoms) could be present at the interface. The development of a general method to measure the surface tension of nanobubbles would be of great utility in studying nanobubbles as would a method of determining the presence of and identifying the nature of surfactants, particles and organic materials at the interface.

Recently a remarkably different novel morphology for a nano-scale gaseous state at a hydrophobic surface has been reported.²² This differs markedly from the spherical caps that are generally reported and consists of a gaseous layer <3 nm thick that extends in the lateral direction for microns with constant height. This shape has led to these gaseous states being dubbed ‘nanopancakes’. Imaged on HOPG, the shape of these nanopancakes is clearly determined by the atomic layer steps in the underlying substrate. The Van der Waals force between the HOPG and the liquid across such nanopancakes should be attractive and contribute to the instability of such structures. It remains a mystery as to how this morphology may persist, but it is known that it can be present along with nanobubbles and it has only been observed on HOPG. Even more mysteriously, composite structures are seen where a nanobubble is present and free to move around on top of a nanopancake and even stepped pancakes have been seen.²³ It has been proposed that nanopancakes are stabilised at a specific thickness due to a minimum in the disjoining force that consists of an attractive Van der Waals interaction between the liquid and the solid across the nanobubble that is prevented from collapsing the gas phase altogether by a repulsive component.²⁴ However, an acceptable candidate for this repulsive component has not been identified. These exotic structures can be transformed into nanobubbles under the action of the AFM tip.²³ This strongly suggests that a thin film remains between the two structures. Perhaps very small graphitic particles or organic material that arises from the substrate are operating to stabilize such films. These would confer stability by the same mechanism by which Pickering emulsions are stabilized by partially hydrophobic particles. These discoveries pose several significant challenges to our understanding of stable gas states at surfaces and clearly warrant further investigation.

How to produce nanobubbles

One of the earliest publications reporting images of nanobubbles also included a clear description of a method by which nanobubbles could be produced on a surface¹ and several subsequent papers have adopted this methodology. Despite this, many investigations of nanobubbles have been conducted without any effort to control the conditions that may influence nanobubble production, though degassing has commonly been employed to prevent the formation of nanobubbles or remove nanobubbles already present. It is likely that differences in surface preparation are responsible for much of the confusion in the literature, indeed many studies reporting null findings for nanobubbles did not employ conditions under which one might expect to find them. In experiments that may be influenced by nanobubbles, particularly when non-wetting surfaces are employed it would be wise to do experiments under conditions where nanobubbles are purposely excluded (for example by degassing) or where they are purposely produced by employing electrolysis or solvent exchange techniques, as described below, in order to clarify the role of nanobubbles.

The solvent exchange technique¹ for producing nanobubbles operates by inducing a supersaturation of gas at the interface. Common atmospheric gases such as nitrogen and oxygen are present in water in millimolar quantities at atmospheric pressure and room temperature. In many other solvents even greater quantities of gas are dissolved. A solvent miscible with water and with a greater solubility of atmospheric gases is chosen, ethanol is most commonly used as it is available in very high purity. The substrate is immersed in ethanol and then the ethanol is displaced by water. Ethanol preferentially wets hydrophobic substrates and therefore the ethanol at the surface is not easily displaced, but as it is miscible with water it will mix with the aqueous phase. In doing so, the gases that were dissolved in the ethanol will be ‘precipitated’ at the interface, producing nanobubbles as the solubility of gas in the aqueous phase is exceeded. It is unclear at this stage if the solvent exchange method also leads to supersaturation of the bulk aqueous phase or even if it produces (if only momentarily) nanobubbles in the bulk aqueous phase. Though we note here that there are reports of long lived nanobubbles in the bulk of a liquid in the presence of organic material,²⁵ there is some doubt about the interpretation of the light scattering data used to arrive at this conclusion. Recent work has shown that nanobubbles are not stable in the bulk, rather small particles of contaminant produced by solvent exchange can give rise to increased light scattering.²⁶ The generality of the solvent exchange technique has been demonstrated by using the same approach to precipitate liquid organic materials at hydrophobic interfaces.²⁷ Similarly nanobubbles can be produced by gaseous supersaturation induced by temperature change.²⁸

Electrochemical methods have also been used for the generation of nanobubbles.²⁹ Using HOPG as a conducting hydrophobic substrate, a battery is used to complete an electrochemical cell such that hydrogen gas is formed at the surface of the HOPG by electrolysis of water. After only a small amount of current is allowed to flow the local concentration of gas is such that the solubility is exceeded and nanobubbles are produced. This method has not been employed as extensively as the solvent exchange method due to the necessity of having a conducting hydrophobic substrate. Gold substrates have also been employed to produce nanobubbles by electrochemical means.³⁰

Whilst these methods are well established and both are dependent upon the nucleation of a gas phase nothing is currently known about the nucleation process. For example it is not known if there is a minimum size that nanobubbles have to reach in order to be stable. Surprisingly it appears that nanobubbles do not act as nucleation sites for the production of macroscopic bubbles.³¹

Why nanobubbles were thought not to exist

The existence of nanobubbles until recently was a controversial topic despite mounting evidence for their existence. The controversy was clearly demonstrated when in 2007 the prestigious journal Nature ran an article in the research highlights section which headlined “No nanobubbles”,³² this was some seven years after high quality AFM images were first published by two separate groups. Here we will address the theoretical and experimental work that has been cited as evidence that nanobubbles do not exist. We address these arguments and demonstrate that none of this work is able to rule out the existence of nanobubbles.

Bubble lifetime

As described in the Introduction, the existence of nanobubbles is thought to be precluded by their rapid dissolution under a high internal Laplace pressure. However, we have seen that the morphology of nanobubbles is such that the interfacial curvature only gives rise to a moderate internal pressure and nanobubbles can be maintained under these conditions for long periods with a moderate supersaturation of gas. Further, if the interface is contaminated with a film that is impermeable to gas, the bubble is essentially a closed system and is stabilised—though there is as yet no evidence of such a film. Brenner and Lohse³³ have proposed a theory of nanobubble stability that involves flux of gas from solution to a hydrophobic surface. This gas then passes into the nanobubble and in doing so compensates for loss of gas from the bubble. It is not clear that this non-equilibrium flux can be maintained long enough to account for the stability of nanobubbles—which can last for days—therefore this explanation is not currently widely accepted. Further, using electrochemical means, stable nanobubbles are also produced on hydrophilic surfaces.^30,34 Recently, Ducker has argued that a small amount of insoluble material at the interface is sufficient to provide a surface pressure that counters the influence of surface tension and thereby prolongs the lifetime of the nanobubble.³⁵ The presence of small amounts of insoluble contaminant at the air–water interface, even when very clean procedures are followed, is probable as such this simple mechanism is plausible. Evidence for this is that addition of surfactants above the cmc destabilise nanobubbles presumably by solubilising this material within micelles.³⁵ Earlier studies using sub cmc concentrations of surfactant showed no influence of surfactant on the nanobubble stability.¹³

Surface investigations

A number of surface sensitive experiments have been conducted on hydrophobic surfaces in water that should be capable of revealing the presence of nanobubbles. These can be categorized as ellipsometric studies and scattering studies.

Ellipsometry

Nanobubbles present at a hydrophobic interface should provide excellent contrast in ellipsometric studies, as the refractive index of the gas phase will be near unity and as such it will be very different to both the aqueous phase (n = 1.333) and solid substrate (n ≈ 1.35). However, at least two investigations that have sought to find vapour or air layers at the interface between hydrophobic surfaces and water have reported that no such layers are found.^36,37 The sensitivity limits of these studies demand that a reasonable surface coverage of nanobubbles exist if they are to be revealed. It has been estimated that in each 10 µm × 10 µm area more than 1000 nanobubbles would need to be present, in order for them to be detected ellipsometrically.³⁶ In neither case were procedures such as solvent exchange purposely adopted to produce nanobubbles, so it is unsurprising that nanobubbles were not found in these studies, particularly as this was not their aim. A repetition of these investigations using the solvent exchange technique or electrochemical methods to produce nanobubbles is likely and should show the presence of nanobubbles.

Scattering at the interface

There is much interest in the structure of water adjacent to hydrophobic interfaces and as such numerous scattering studies aimed at the interface have been conducted. Given the large mass density and electron density difference between a gas phase and either the hydrophobic substrate or water one might expect that nanobubbles would be readily revealed by such investigations, yet several scattering studies report no evidence for nanobubbles. Neutron reflectivity studies have reported results that have been interpreted as consistent with the precursor of nanobubbles^38,39 but later investigations found no evidence for the existence of nanobubbles.⁴⁰ These latter experiments are supported by X-ray scattering studies^41,42 that reveal evidence for a lower density of water adjacent to a hydrophobic surface but no evidence of nanobubbles. None of these investigations employed conditions that would be expected to produce nanobubbles, so it is unsurprising that evidence of nanobubbles was not found. Again experiments that combine the established techniques for the production of nanobubbles and scattering experiments should be revealing.

Some of the presented evidence for nanobubbles have been questioned. Tyrrell and Attard,^43,44 reported AFM images depicting very high density coverage of features at a hydrophobic surface. These were interpreted to be nanobubbles. However, the features presented in the images are consistent with a polymeric layer formed by the reaction of the dichlorodimethylsilane (used to make the surface hydrophobic) with water vapor.⁴⁵ It is worth noting that the images of nanobubbles presented in this work have a morphology that differs from the prior investigations by both Chinese¹ and Japanese⁷groups and subsequent studies, in that the three phase contact line is highly convoluted.

During the imaging of nanobubbles using AFM it is necessary that the tip come into contact with the nanobubble. This brings the tip in close proximity to the substrate and it has been suggested that this process actually nucleates nanobubbles.^36,40 That is, it is proposed that nanobubbles are not present prior to the commencement of the imaging process but rather arise as an artefact of the imaging process. This idea has arisen from the force measurement community where the existence of nanobubbles has been associated with the measurement of the hydrophobic attraction between two hydrophobic surfaces. It is known that upon separating two hydrophobic surfaces in water a vapour phase can be produced.⁴⁶ However, AFM tips are hydrophilic and as such there is no reason to believe that proximity to another surface can nucleate bubbles. Further, one would expect that if the tip were nucleating bubbles that the distribution and size of bubbles would increase with continued imaging, whereas in most cases the image remains very stable for many hours^1,7,13 and in other cases the tip is seen to sweep the surface free of nanobubbles (see Fig. 2, lower frame).¹ More evidence that the tip is not responsible for the nucleation of nanobubbles comes from investigations using the solvent exchange technique to produce nanobubbles. This technique causes supersaturation of gas in the aqueous phase and on occasion this causes the nucleation of micron sized bubbles, in areas adjacent to the presence of nanobubbles, which can be seen with standard optical microscopy.¹³ Also, the production of nanobubbles both by electrochemical³⁰ and solvent exchange⁴⁷ means has been followed using QCM (where no AFM tip is present).

Applications of nanobubbles

Nanobubbles are inherently interesting as they pose a number of challenges to our understanding of bubble behaviour. They may also be useful. It is likely that they are already playing an important role in froth flotation.⁴⁸ Nanobubbles are also at the heart of a form of “nanomotor” in which the catalytic generation of oxygen nanobubbles is used to propel a nanorod.⁴⁹ We can already control the conditions under which they are produced and a means by which the size and location of nanobubbles can be controlled has been described,⁵⁰ so it appears that we will be able to manipulate nanobubbles to our advantage. It has been shown that nanobubbles can be used to prevent surface fouling by proteins^51,52 and to clean surfaces that are already fouled.^30,34 By combining these technologies it should be possible to controllably pattern proteins on a surface, where the nanobubbles are used as masks. Alternatively, nanobubbles may also be used as templates for the controlled deposition of materials at the interface in order to produce nanoparticles of controlled size and shape. The presence of a gas phase at the interface is crucial in superhydrophobicity and can lead to hydrodynamic boundary slip.⁵³ Thus the manipulation of nanobubbles could be used to actively change the contact angle of a surface or the flow properties of a fluid. Such manipulations are likely to be employed in microfluidic devices either as switching or mixing technologies. Nanobubbles for these applications could be produced on demand using electrolysis or temperature pulses.

Future research

The investigation of nanobubbles is still in its infancy and many challenges remain. An understanding of the very high contact angle produced by nanobubbles is important as it may be related to their stability. The newly reported nanopancakes pose several challenges. Why are they stable? What is the internal pressure? How can we reconcile the rather flat interface with the high interfacial curvature at the edge of the pancake? How can a nanopancake and a nanobubble interact but remain as separate entities? It is likely that contaminant materials are contributing to the stability of nanobubbles and these more exotic gaseous nanophases but elucidating precisely how they operate is necessary. Recalling that the macroscopic and nanoscopic contact angles differ there are implications for super-hydrophobic surfaces where small gas pockets are trapped at surfaces. If these gas pockets are made very small what contact angle will they adopt and how will this influence the superhydrophobicity? Finally, enormous external pressures are used to ‘crush’ Harvey nuclei which are required for the heterogenous bubble formation, how would nanobubbles survive such a high external pressure? Perhaps they are immune to it as they seem to be resilient to large pressure fluctuations due to acoustic waves?³¹ As our understanding of nanobubbles grows, it is likely not only that we will develop applications that utilise them but that we will become aware of more situations where it is desirable or necessary to remove them. For example it is likely that the formation of nanobubbles during electroplating leads to film defects and the presence of nanobubbles can confound some scientific studies such as the measurement of forces between hydrophobic surfaces.

References

1. S. T. Lou, Z. Q. Ouyang, Y. Zhang, X. J. Li, J. Hu, M. Q. Li and F. J. Yang, J. Vac. Sci. Technol., B, 2000, 18, 2573–2575.
2. S. Ljunggren and J. C. Eriksson, Colloids Surf., A, 1997, 130, 151–155.
3. T. A. Shedd, J. Microlithogr., Microfabr., Microsyst., 2005, 4, 033004.
4. J. L. Parker, P. M. Claesson and P. Attard, J. Phys. Chem., 1994, 98, 8468–8480.
5. J. N. Israelachvili and R. M. Pashley, J. Colloid Interface Sci., 1984, 98, 500–514.
6. T. D. Blake and J. A. Kitchener, J. Chem. Soc., Faraday Trans. 1, 1972, 68, 1435–1442.
7. N. Ishida, T. Inoue, M. Miyahara and K. Higashitani, Langmuir, 2000, 16, 6377–6380.
8. N. Ishida and K. Higashitani, Miner. Eng., 2006, 19, 719–725.
9. S. T. Lou, J. X. Gao, X. D. Xiao, X. J. Li, G. L. Li, Y. Zhang, M. Q. Li, J. L. Sun and J. Hu, Chin. Phys., 2001, 10, S108–S110.
10. S. T. Lou, J. X. Gao, X. D. Xiao, X. J. Li, G. L. Li, Y. Zhang, M. Q. Li, J. L. Sun, X. H. Li and J. Hu, Mater. Charact., 2002, 48, 211–214.
11. J. W. Yang, J. M. Duan, D. Fornasiero and J. Ralston, J. Phys. Chem. B, 2003, 107, 6139–6147.
12. X. H. Zhang, G. Li, Z. H. Wu, X. D. Zhang and J. Hu, Chin. Phys., 2005, 14, 1774–1778.
13. X. H. Zhang, N. Maeda and V. S. J. Craig, Langmuir, 2006, 22, 5025–5035.
14. M. Switkes and J. W. Ruberti, Appl. Phys. Lett., 2004, 84, 4759–4761.
15. J. Zhang, R.-H. Yoon, M. Mao and W. A. Ducker, Langmuir, 2005, 21, 5831–5841.
16. X. H. Zhang, G. Li, N. Maeda and J. Hu, Langmuir, 2006, 22, 9238–9243.
17. X. H. Zhang, A. Khan and W. A. Ducker, Phys. Rev. Lett., 2007, 98, 136101.
18. S. D. A. Connell, S. Allen, C. J. Roberts, J. Davies, M. C. Davies, S. J. B. Tendler and P. M. Williams, Langmuir, 2002, 18, 1719–1728.
19. P. C. Wayner, J. Colloid Interface Sci., 1982, 88, 294–295.
20. B. M. Borkent, S. de Beer, F. Mugele and D. Lohse, Langmuir, 2010, 26, 260–268.
21. W. Ducker, personal communication.
22. X. H. Zhang, X. D. Zhang, J. L. Sun, Z. X. Zhang, G. Li, H. P. Fang, X. D. Xiao, X. C. Zeng and J. Hu, Langmuir, 2007, 23, 1778–1783.
23. L. J. Zhang, X. H. Zhang, C. H. Fan, Y. Zhang and J. Hu, Langmuir, 2009, 25, 8860–8864.
24. X. H. Zhang, N. Maeda and J. Hu, J. Phys. Chem. B, 2008, 112, 13671–13675.
25. F. Jin, J. Ye, L. Z. Hong, H. F. Lam and C. Wu, J. Phys. Chem. B, 2007, 111, 2255–2261.
26. A. Habich, W. Ducker, D. E. Dunstan and X. H. Zhang, J. Phys. Chem. B, 2010, 114, 6962–6967.
27. X. H. Zhang and W. Ducker, Langmuir, 2007, 23, 12478–12480.
28. X. H. Zhang, X. D. Zhang, S. T. Lou, Z. X. Zhang, J. L. Sun and J. Hu, Langmuir, 2004, 20, 3813–3815.
29. L. J. Zhang, Y. Zhang, X. H. Zhang, Z. X. Li, G. X. Shen, M. Ye, C. H. Fan, H. P. Fang and J. Hu, Langmuir, 2006, 22, 8109–8113.
30. G. M. Liu, Z. H. Wu and V. S. J. Craig, J. Phys. Chem. C, 2008, 112, 16748–16753.
31. B. M. Borkent, S. M. Dammer, H. Schonherr, G. J. Vancso and D. Lohse, Phys. Rev. Lett., 2007, 98, 204502.
32. Nature, 2007, 445, 129.
33. M. P. Brenner and D. Lohse, Phys. Rev. Lett., 2008, 101, 214505.
34. G. M. Liu and V. S. J. Craig, ACS Appl. Mater. Interfaces, 2009, 1, 481–487.
35. W. A. Ducker, Langmuir, 2009, 25, 8907–8910.
36. M. Mao, J. H. Zhang, R. H. Yoon and W. A. Ducker, Langmuir, 2004, 20, 1843–1849.
37. Y. Takata, J. H. J. Cho, B. M. Law and M. Aratono, Langmuir, 2006, 22, 1715–1721.
38. D. Schwendel, T. Hayashi, R. Dahint, A. Pertsin, M. Grunze, R. Steitz and F. Schreiber, Langmuir, 2003, 19, 2284–2293.
39. R. Steitz, T. Gutberlet, T. Hauss, B. Klosgen, R. Krastev, S. Schemmel, A. C. Simonsen and G. H. Findenegg, Langmuir, 2003, 19, 2409–2418.
40. D. A. Doshi, E. B. Watkins, J. N. Israelachvili and J. Majewski, Proc. Natl. Acad. Sci. U. S. A., 2005, 102, 9458–9462.
41. M. Mezger, H. Reichert, S. Schoder, J. Okasinski, H. Schroder, H. Dosch, D. Palms, J. Ralston and V. Honkimaki, Proc. Natl. Acad. Sci. U. S. A., 2006, 103, 18401–18404.
42. A. Poynor, L. Hong, I. K. Robinson, S. Granick, Z. Zhang and P. A. Fenter, Phys. Rev. Lett., 2006, 97, 266101.
43. E. Tyrode, M. W. Rutland and C. D. Bain, J. Am. Chem. Soc., 2008, 130, 17434–17445.
44. J. W. G. Tyrrell and P. Attard, Phys. Rev. Lett., 2001, 87, 176104.
45. D. R. Evans, V. S. J. Craig and T. J. Senden, Physica A, 2004, 339, 101–105.
46. H. K. Christenson and P. M. Claesson, Science, 1988, 239, 390–392.
47. X. H. Zhang, Phys. Chem. Chem. Phys., 2008, 10, 6842–6848.
48. N. Mishchuk, J. Ralston and D. Fornasiero, J. Colloid Interface Sci., 2006, 301, 168–175.
49. W. F. Paxton, K. C. Kistler, C. C. Olmeda, A. Sen, S. K. St Angelo, Y. Y. Cao, T. E. Mallouk, P. E. Lammert and V. H. Crespi, J. Am. Chem. Soc., 2004, 126, 13424–13431.
50. A. Agrawal, J. Park, D. Y. Ryu, P. T. Hammond, T. P. Russell and G. H. McKinley, Nano Lett., 2005, 5, 1751–1756.
51. Z. H. Wu, H. B. Chen, Y. M. Dong, H. L. Mao, J. L. Sun, S. F. Chen, V. S. J. Craig and J. Hu, J. Colloid Interface Sci., 2008, 328, 10–14.
52. Z. H. Wu, X. H. Zhang, X. D. Zhang, L. Gang, J. L. Sun, Y. Zhang, M. Q. Li and J. Hu, Surf. Interface Anal., 2005, 37, 797–801.
53. O. I. Vinogradova, Langmuir, 1995, 11, 2213–2220.

---