Stability of Nanobubbles Formed at the Interface between Cold Water

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Stability of Nanobubbles Formed at the Interface between Cold Water and Hot Highly Oriented Pyrolytic Graphite Hongjie An, Beng Hau Tan, Qingyun Zeng, and Claus-Dieter Ohl* Cavitation Lab, Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore ABSTRACT: For the wider application of nanobubbles, a simple and reproducible nucleation process is not readily available. Here we describe a method for nucleating nanobubbles using only the most basic of conditions: depositing cold water at 4 °C on heated highly oriented pyrolytic graphite substrates. This method thus avoids the need, as in previous studies, to use secondary liquids, salts, or electrolysis to nucleate the nanobubbles and provides a pure system in which the properties of nanobubbles can be studied. The nanobubbles generated with this method are observed to survive for at least 5 days, barely changing their contact angles or heights after the first few hours. The stability of the nanobubbles in our system is discussed within the framework of some recently published theories.

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INTRODUCTION Interfacial nanobubbles are gaseous domains that are attached to immersed hydrophobic surfaces. Their existence at ambient temperature and pressure, atypically large contact angles, and unusual stability have attracted intensive interest. Perhaps the most intriguing property of nanobubbles is their stability against dissolution. The well-established diffusion theory of Epstein and Plesset1 (and later Ljunggren and Eriksson2) suggests that a freely suspended spherical nanobubble should dissolve within a second. However, nanobubbles are known to survive for many days, several orders of magnitude longer than the lifetimes predicted by the classical theory. Over the past decade, various groups have developed theories to account for the long lifetimes of nanobubbles, such as the assumption of a dynamic equilibrium between the nanobubble and its surrounding liquid3 or the possibility that a barrier of contamination physically prevents dissolution.4 Some of these explanations were later ruled out by experiment. For instance, the dynamic equilibrium model predicts a strong directed outflow around nanobubbles, but two independent groups5,6 later found that no strong, directed flows can be detected around nanobubbles. The current understanding is that contact line pinning,7,8 supersaturation, or both9,10 facilitate the stability of nanobubbles. Naturally, the predictions of these new theories should be compared against experiments. The unusual stability of surface-attached nanobubbles has motivated significant interest in using them for a number of industrial applications, such as surface cleaning, flotation,11 lubrication in microfluidic environments, and nanofabrication.12,13 Unfortunately, protocols for nucleating nanobubbles are notoriously susceptible to contamination, which is likely to be the cause of a number of mutually conflicting results in the literature.14 This issue has raised uncertainty about which papers in the literature are correctly characterizing nanobubbles and which ones are actually imaging drops of contamination.15 © 2016 American Chemical Society

The standard approach for producing nanobubbles is to exchange water and an organic solvent over a substrate. The working mechanism of solvent exchange is that the difference in gas solubility between water and ethanol generates a substantial oversaturation that promotes nucleation.16−19 The surfaceattached nanobubbles are then characterized, usually with atomic force microscopy (AFM), while the substrate is immersed in water. The substrate chosen is typically an atomically flat hydrophobic surface such as highly oriented pyrolytic graphite (HOPG). However, in many laboratories, liquids are delivered into the atomic force microscope imaging cell using medical-use plastic syringes and/or cannulas. Berkelaar et al. found that when plastic syringes or plastic cannulas for medical purposes are used to transport liquids, polymeric contamination can inadvertently be introduced into the nucleation experiments.15 The contamination originates from the use of polydimethylsiloxane or other medically inert silicone oils to lubricate the barrels and needles of syringes.20,21 In medical practice, such lubrication is necessary to reduce pain while delivering topical injections to patients. The use of plastic syringes to nucleate nanobubbles may be the origin of a number of controversial and mutally conflicting results in the literature. Several research groups have reported what they believe to be gaseous micropancakes, irregular micrometer-sized gaseous domains of nanometric thickness, using the solvent exchange method.11,13,22,23 Unfortunately, liquid polymers also form micropancakes with a virtually identical morphology.24,25 There is therefore some suspicion that these pancakes may actually be polymeric layers rather Special Issue: Nanobubbles Received: April 21, 2016 Revised: June 6, 2016 Published: June 12, 2016 11212

DOI: 10.1021/acs.langmuir.6b01531 Langmuir 2016, 32, 11212−11220

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Figure 1. Height images of nanobubbles formed by placing a drop of 4 °C Milli-Q water on the HOPG surfaces at room temperature (25 °C) and 40, 60, and 80 °C. Scan size of 5 μm × 5 μm. Height scale of 50 nm.

than gaseous ones.4,26 An et al. used the usual solvent exchange protocol to nucleate nanobubbles using both plastic and glass syringes, finding that the glass syringes did not produce micropancakes while the plastic ones did.27 They also reported that the pancake structures formed by the use of plastic syringes were stable to dissolution when the surrounding liquid was degassed. Other nucleation methods in the literature tend to require the addition of secondary liquids or ions for electrolysis or require that the bubbles be coated in fluorescent dyes for optical visualization. Surface-attached nanobubbles can also be generated by water−salt solution exchange.28,29 As with solvent exchange, it is thought that the exchange of water and salt solutions generates sufficiently high oversaturation to promote nucleation of nanobubbles, but other researchers doubt this because these nanobubbles do not dissolve when the salt water is replaced with deionized water.14 Finally, nanobubbles can also be nucleated by performing electrolysis of an ionic solution over a conductive substrate.30−32 With electrolysis, factors such as the surface area of electrodes33 and the distance between the working electrode and counter electrode play a role in the morphology, distribution, and chemistry of the nanobubbles produced. Perhaps the greatest downside of these alternative

methods is the fact that the need to introduce additional liquids, ions, and dyes changes the experimental setup fundamentally from the pure system that researchers hope to understand. In this Article, we introduce a straightforward and reproducible nucleation technique for nanobubbles using a bare minimum of conditions for nucleation; only cold water and a heated graphite surface are required. The simplicity of our technique avoids the need to use secondary liquids to generate the oversaturation necessary for nucleation. We also examine the stability of the nanobubbles against dissolution over several days and compare our findings against theory.

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MATERIALS AND METHODS

Formation of Surface-Attached Nanobubbles. Twenty milliliters of Milli-Q water (Millipore Corp., Boston, MA) in a 100 mL glass bottle (Duran Screw Cap GL 45) was refrigerated at 4 °C overnight before use. The glass bottle was washed with acetone, isopropanol, and ethanol thrice before being used. Highly oriented pyrolytic graphite (HOPG) (ZYB grade, SPI) was cleaved with Scotch tape (3M) and kept in an oven at different temperatures (40, 50, 60, 70, and 80 °C) for at least 2 h before the experiments. Once the samples were removed from the oven, an aliquot of 100 μL of Milli-Q water (4 °C) was dispensed immediately on the HOPG substrate. The 11213

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Langmuir samples were mounted onto the atomic force microscope stage and left to equilibriate under the ambient laboratory conditions [room temperature (25 °C) and atmospheric pressure] for 30 min before AFM imaging. AFM Measurements. AFM measurements were conducted with a Bioscope Catalyst atomic force microscope (Bruker Corp.). Height images were captured in ScanAsyst mode under manual control so that the bubbles were scanned with the same force. In this mode, the maximal force on the cantilever is directly used as a feedback mechanism to control the tip’s position, allowing the user to select the imaging force,6 also known as the peak force. v-Shaped silicon nitride cantilevers (OMCL-TR400PSA-1, Olympus, Tokyo, Japan) with nominal spring constants of 0.08 N/m were used for imaging. The cantilevers were pretreated with oxygen plasma (Harrick Plasma) for 15 s to make them hydrophilic. The deflection sensitivity was measured on the bare HOPG substrate, and the spring constant was calibrated in air by the thermal tune program built into the atomic force microscope software. To minimize system drift, we waited 2 h from the time that the atomic force microscope tip was immersed in liquid before making the first AFM scans. Force spectroscopy data were captured using a point and shoot scheme at marked areas, so that nanomechanical properties at various areas of interest could be measured quickly. Both images and force curves were captured with the closed loop activated. Force Spectroscopy. In peak force mode (also known as ScanAsyst mode), a module known as “point and shoot” allows users to capture force curves at various points of interest in a given scan. A height image incorporating several nanobubbles is first captured, and some points of interest in this map are identified by the user (“point”), before a vertical ramp (“shoot”) is performed at the chosen points. Force curves are then acquired within a few minutes. The advantage of this program is that it saves considerable amounts of time in performing force volume imaging in contact mode, which usually takes several hours. The force−separation curves were generated from the deflection versus z-piezo displacement curves by the in-built software. To generate accurate curves, the deflection sensitivity and spring constant of the cantilever were first calibrated using built-in software. Calculating the Contact Angles of the Nanobubbles. Contact angles in this work are measured from the denser phase. The crosssectional profile of each nanobubble (see the inset of Figure 3 for a representative example) was fitted to a spherical cap by a custom algorithm in Python. The left and right boundaries of the bubble are characterized by a rise in gradient relative to the flat substrate. The raw data points are smoothed with a two-point rolling mean, and the gradient of the smoothed signal is calculated. The two dominant peaks of the smoothed signal correspond to the positions of the left and right boundaries. All the raw data points between the two detected boundaries were then fitted using a standard least-squares algorithm to a circle (spherical cap in axis symmetry). Finally, to determine the contact angle, a chord was constructed on the fitted circle by interpolating points on the substrate outside the bubble. An elementary geometrical relation then yields the contact angle.

Table 1. Sizes of Nanobubbles in Water at Room Temperature at 2, 24, 48, 72, and 96 ha H

L

CA

R

ζ

time (h)

1

2

3

4

5

2 24 48 72 96 2 24 48 72 96 2 24 48 72 96 2 24 48 72 96 2 24 48 72 96

15 21 21 22 22 218 219 222 206 192 165 161 160 157 155 404 297 311 249 224 3.4 4.2 4.4 5.4 6.3

13 16 18 16 16 144 146 148 129 118 163 156 153 157 156 209 173 161 135 114 5.8 7.9 8.7 8.6 9.8

15 20 21 21 22 200 198 197 196 186 165 159 156 158 153 341 250 247 236 208 3.7 5.2 5.9 5.4 7.0

15 19 21 22 20 187 178 176 175 157 164 160 157 156 152 295 217 199 189 167 4.2 5.5 6.3 6.6 8.5

12 15 16 17 16 139 138 149 131 122 164 159 156 153 151 214 168 184 136 124 5.6 7.4 7.8 9.9 11.3

a

The bubbles were selected from Figure 5. H (nm) is the apparent height. L (nm) is the diameter of the footprint. CA (nm) is the nanoscopic contact angle. R (nm) is the radius of curvature of the selected nanobubble. ζ (dimensionless) is oversaturation, where ζ = [(4γ)/(PatmL)] sin(π − θ) (see ref 9).

because of local conditions at the substrate. For instance, it is known that the positions of cleave steps on HOPG surfaces affect their distribution.29 Unfortunately, AFM imaging covers only a relatively small area of the substrate. Using a top-view CCD camera, we are able to obtain a much wider view of some much larger microbubbles with base diameters of ∼100 μm, as shown in Figure 2. (Note that optical visualization is diffraction-limited, so the surface nanobubbles imaged via AFM in Figure 1 cannot be resolved optically.) These large bubbles were formed in all of the experiments on HOPG surfaces at different temperatures and coalesced into even larger millimeter-sized bubbles that could be discerned with the naked eye after incubation for ∼30 min under laboratory conditions. While we did not observe any casual trend relating the size of microbubbles to the temperature, we were not able to collect detailed statistics of the distribution of the microbubbles because of a lack of lateral range in the microscope stage and sparse coverage of such large bubbles. Nanoscopic contact angles of nanobubbles were calculated from AFM height profiles and were measured through the water phase for this work. For each nanobubble, a crosssectional profile was extracted and fitted to a spherical cap using the algorithm described in Materials and Methods. The inset panel in Figure 3 shows a representative outcome of the fitting method. The average nanoscopic contact angle calculated is in the range of 155−172° and is independent of surface temperature. This range of contact angles is broadly consistent

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RESULTS AND DISCUSSION Nucleation of Surface Nanobubbles with Hot Water and Heated Surfaces. We dispensed cold water on HOPG heated to temperatures ranging from room temperature to 80 °C. AFM scans of the bubbles are presented in Figure 1. The nucleation of nanobubbles with this method is highly reproducible; for substrates heated from 40 to 80 °C, we found nanobubbles in 100% of experiments, while the unheated substrate at room temperature still produced nanobubbles in ∼90% of our experiments. The size of selected bubbles at room temperature can be found in Table 1 at 2 h. In general, the nanobubbles appear to be larger in base diameter and are more densely packed with a higher substrate temperature, as seen in Figure 1, but there is some variation to this trend, perhaps 11214

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Figure 2. Optical images showing microbubbles formed after deposition of 4 °C water on HOPG. (a) HOPG substrate at room temperature. (b) HOPG at 60 °C. Images were captured using a top view CCD equipped with a 10× magnification objective.

Figure 4. Solubility of air as a function of temperature. The empirical fitting curve uses fitting parameters given in the literature.35,36

Figure 3. Nanoscopic contact angles of nanobubbles formed at the interface between 4 °C water and HOPG at different temperatures (25−80 °C). The angles reported here are averaged over 30 bubbles for each temperature and are measured through the denser water phase. In the inset, a cross-sectional profile of a typical bubble is shown together with a least-squares fit of a spherical cap, demonstrating the calculation of the contact angle.

such as solvent exchange to generate sufficient gas oversaturation for nucleation. Although there is not enough information to determine whether the early experiments were contaminated, our results show that it is at least plausible that the early experiments were able to nucleate nanobubbles without solvent exchange. The key ingredient seems to be the use of cold water. Even when deposited on an unheated substrate, cold water provides sufficient oversaturation to nucleate surface-attached nanobubbles. This is expected because oversaturation is a necessary condition for nucleation,13 and a smaller temperature difference generates a lower oversaturation in the liquid. Another factor that improves the likelihood of bubble nucleation is the fact that the substrates used in early experiments are considerably rougher than the atomically flat HOPG substrates that are used in more recent experiments. A rougher surface reduces the free energy barrier for nucleation and therefore favors the creation of surface nanobubbles.5 AFM Scans at Large Imaging Forces. The literature gives conflicting accounts about what happens when nanobubbles are imaged with large forces. While Walczyk et al. report that their nanobubbles maintain visible at peak forces as high as 27 nN,36 other studies in peak force mode using cantilevers of similar stiffness show that the objects disappear altogether when bubbles are imaged with much smaller forces on the order of 1−2 nN.13,31,34,40 It is worth noting that in the Walczyk paper, plastic syringes were used to dispense liquids and nanobubbles were nucleated without the aid of solvent exchange, temper-

with previous studies of nanobubbles produced by solvent exchange methods.34 Nucleation Mechanism. We next investigate the mechanism by which nucleation occurs when cold water is incident on a substrate. It can be established from the literature35,36 that the cold water has a solubility substantially larger than that of the hot interface between the HOPG substrate and the water. The maximal difference in solubility between water at 4 and 80 °C is approximately 2-fold (see Figure 4), which means that at the instantaneous point of nucleation, the gas content is ∼200% of that of the ambient 4 °C water. The gas is thus highly supersaturated in the liquid close to the substrate, promoting the nucleation of nanobubbles. However, this is an idealized estimate; heat transfer is assumed to be perfect, so that the water adjacent to the HOPG surface is 80 °C at the point of contact while the rest of the water remains at 4 °C. The substantial supersaturation generated by temperature differences may explain the nucleation mechanism of some early works on nanobubbles. In these papers, nanobubbles were discovered to form on hydrophobic surfaces immersed in water at room temperature.37−39 Although early works give the impression that nanobubbles are abundant on immersed hydrophobic surfaces, most research groups have found it difficult to nucleate nanobubbles using only water and a hydrophobic substrate and also find it necessary to use methods 11215

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Figure 5. AFM height images of nanobubbles imaged with different peak forces (a−d) from 0.2 to 3.0 nN. After this set of scans, a final scan (e) was performed in which the peak force was reduced to 0.2 nN. Nanobubbles were nucleated by placing a drop of 4 °C Milli-Q water on the HOPG substrate at 80 °C. Scan size of 2 μm × 1 μm. Height scale of 50 nm.

ature differences, or other techniques that are usually typically required to generate supersaturation of gas in the liquid. Because our nucleation method is very simple and does not utilize plastic syringes or other secondary liquids, the nanobubbles in our system are generated in a pure environment. How do the bubbles in this pure system then respond to an increase in imaging force? We find that the nanobubbles disappear cleanly under imaging forces larger than ∼3 nN (Figure 5), in line with studies that have shown that the bubbles disappeared under relatively small imaging forces.13,31,34,40 When the imaging peak force was restored back to the original value of 0.2 nN, the bubbles reappeared at their original positions in the scan image. Note that the bubbles are not displaced from their positions during this invasive scanning; nanobubbles are known to pin strongly onto their substrates, while contamination drops are weakly pinned and easily displaced.14 Force Spectroscopy. We also observed the mechanical response of the nanobubbles as an atomic force microscope tip approaches and leaves them during force ramping. When the cantilever is descending in liquid and approaching the bubble, the cantilever deflection is zero, but as the tip comes into contact with the bubble, the cantilever deflection changes by an amount proportional to the force acting on the cantilever. The force measured as a function of the vertical separation is therefore a measure of the bubble’s stiffness. Figure 6 shows the force−separation curves on HOPG surfaces and on nanobubbles in water. The approach force− separation curve of a nanobubble is quite different from that of HOPG, but consistent with previous studies of nanobubbles formed by solvent exchange methods. There is a jump when the atomic force microscope tip first comes into contact with the surface of the bubble (the red curves in Figure 5a,b), after which a linear compliance regime follows. We can calculate the slope of the linear regime with the equation slope = force/ separation = 1.0 nN/14 nm ≈ 71 mN/m, which agrees closely with the surface tension of the water−air interface. The adhesion force when the atomic force microscope tip jumps off the nanobubble can be directly read from Figure 5b and is very close to when the tip jumps off the HOPG substrate. It has been reported in the literature, both theoretically and experimentally, that nanobubbles respond linearly upon approach by an atomic force microscope tip,18,34,41 as opposed to the nonlinear response reported for oil drops.42 We therefore believe that the linear slopes we report here provide

Figure 6. Force vs separation curves. (a) Approach curves show the cantilever response when the atomic force microscope tip is approaching the substrate. (b) Retract curves show the interactions between the tip and surfaces when the tip is withdrawn.

a reliable indicator that the object is a nanobubble as compared to a contaminant drop. Life Span of Nanobubbles. It is well-known that nanobubbles nucleated using the standard solvent exchange techniques are known to survive for many days.8 Theoretically, the lifetime of a nanobubble depends on the gas content in the liquid generated by the nucleation technique. Note that the standard solvent exchange technique generates a substantial oversaturation of gas throughout the liquid, but our temperature difference method produces only a localized oversaturation at the substrate. It would therefore be useful to understand whether the nanobubble lifetime would be significantly different in our method versus that in solvent exchange. To study the stability of the nanobubbles nucleated with our temperature difference method, AFM images (Figure 7) were captured 2, 24, 48, 72, and 96 h after nucleation. All images were captured in peak force mode with a peak force of 0.2 nN. We found the nanobubbles can survive for at least 5 days, with 11216

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Figure 7. Life span of nanobubbles in water at the same scan area at room temperature. Nanobubbles can survive for 5 days with little change in size over time. Scan size of 5 μm × 5 μm. Height scale of 50 nm.

While a full calculation that takes the truncated sphere geometry of the nanobubble into account is rather involved, an understanding of its essential physics does not require a detailed calculation. It can be shown8 that the rate of loss of gas molecules from the bubble is a function of the gas concentration between the bubble surface and the far field:

no sign of imminent dissolution by the end of the experiment. Base diameter L, nanoscopic contact angle θ, and radius of curvature R of the selected nanobubbles labeled in Figure 6a are provided in Table 1. The apparent height of nanobubbles increased from 2 to 24 h and fluctuated slightly after that. The base diameters of nanobubbles were not found to change significantly but were found to be slightly smaller by the last day. Both the radii of curvature and the nanoscopic contact angles of the bubbles tended to decrease over a 5 day observation period, corresponding to a slight increase in bubble height. Despite the difference in gas oversaturation generated in this technique compared to that generated via solvent exchange, both techniques produce nanobubbles that are long-lived for at least several days. Comparison with Stability Theories. A recently published theory on the stability of nanobubbles suggests that the combination of the nanobubble’s pinned contact line and the existence of gas oversaturation in the liquid can stabilize the bubble against dissolution. Although the theory was independently established by two separate groups,9,10 we will refer to it in this Article as the Lohse−Zhang framework as an acknowledgment to the group who published first.

⎤ ⎡P 4γ sin(π − θ ) dN ∼ −⎢ atm + − c∞⎥f (θ , D , L , t ) dt kHL ⎦ ⎣ kH (1)

where kH is Henry’s law constant, γ is the surface tension of the water−air interface, L is the footprint diameter of the bubble, Patm is atmospheric pressure, and f is a term dependent on the diffusion geometry that is derived from a full solution of the diffusion equation. The gas concentration at the bubble is obtained by coupling the gas pressure of the nanobubble PG = Patm + 4γ sin(π − θ)/L to Henry’s law. The working principle of the Lohse−Zhang model9 is that for some far-field gas concentration c∞, the term in brackets in eq 1 can be equated to zero, thus canceling the molecular flux to zero and stabilizing the nanobubble against dissolution. By defining the oversaturation ζ = c∞/csat − 1 and csat = Patm/kH, we can show that 11217

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estimate how long the liquid should be saturated, we may utilize a simple dimensional estimate7 τ ∼ S 2/D, where S is the length scale of the water layer and the order of magnitude of the diffusion constant for most gases in water is D ∼ 10−9 m2 s−1. With the generous assumption that S ∼ 1 cm, we find that the diffusion time scale is on the order of 12 h. After this time, it is theoretically expected (provided the water has not evaporated) from thermodynamic equilibrium and Henry’s law that the gas concentration is approximately at saturation c∞ = Patm/kH; this means that the oversaturation ζ should relax to zero, instead of remaining constant or even increasing, as Figure 9 suggests. In other words, if it is accepted that nanobubbles are stabilized by contact line pinning and gas oversaturation, it is still not understood why the oversaturation persists for a time much longer than the diffusion time scale of the system, let alone increase significantly over time. The Lohse−Zhang theory makes the strict requirement that the liquid must be oversaturated; a detailed calculation shows that if the oversaturation falls below even several percent, the bubble would still dissolve over a time scale of microseconds.10 It therefore cannot account for the findings of a previous experimental paper in which nanobubbles survived despite being incubated overnight in a marginally undersaturated environment at ζ ≈ −0.08.8 Despite the clear disparity between the theory and experiments, we want to highlight the fact that a separate set of experiments by our group does appear to support the existence of an equilibrium contact angle suggested by Lohse and Zhang. In total internal reflection fluorescence microscopy experiments, Chan et al.10 showed that merging nanobubbles nucleated by the solvent exchange method grew toward an equilibrium contact angle with a fitted oversaturation of ζ ≈ +0.16, albeit with the caveat that a near-exact fit between theory and experiment was only possible when the surface area of the bubble used in the calculation was 10% of the actual value (their interpretation was that the coating of fluorescent dye inhibited the total surface area over which gas diffusion occurred). Our findings therefore indicate that the theoretical description of the stability of nanobubbles is still not complete. One possible reason why the nanobubbles survive far longer than the time expected for the liquid to lose its excess gas is that because the imaging is performed in an open system, a physical barrier of dirt from the ambient environment may accumulate on the bubble within 24 h, to the point that it physically prevents gas from escaping the nanobubble. The bubbles are thus kept intact independent of the oversaturation in the ambient liquid. This interpretation is supported by the observation that the bubble heights sharply increase (and, conversely, the contact angles decrease) from 2 to 24 h but then remain approximately constant over the next several days.

at this equilibrium oversaturation is a function of the bubble’s shape 4γ ζ= sin(π − θ ) PatmL (2) The consequence of this relation is that the oversaturation of the liquid alone determines the contact angle of the bubble, because contact line pinning fixes the nanobubble’s footprint diameter L. Figure 8 shows the oversaturation as a function of

Figure 8. Oversaturation against contact angle relation, based on the Lohse−Zhang theory (eq 2).

the contact angle for typical base radii in our experiments. From Table 1, it is clear that if the Lohse−Zhang theory is correct, nanobubbles with footprint diameters of 100−200 nm must be sustained by a oversaturation of at least ζ ≈ 4−7. In fact, the contact angles of the nanobubbles in our experiments decrease over time, which would imply even larger oversaturations ζ > 8 (Table 1). The difference in gas solubility in water at 4 and 80 °C suggests that the maximal estimated oversaturation present in our experimental system is ζ ≈ 1. This value is clearly much lower than the oversaturation required for stability predicted by the Lohse−Zhang theory. Another puzzling feature is the apparent coarsening of the nanobubbles in our stability experiments; the increased heights (smaller contact angles) we observe would be explained in the Lohse−Zhang model as beibg due to a further increase in oversaturation. In Figure 9, we show how the purported oversaturation would increase during the 5 day waiting period. The Lohse−Zhang model requires that the nanobubbles always be sustained by gas-oversaturated water for the entirety of their lifetime. In other words, a nanobubble will survive as long as its ambient liquid remains oversaturated with gas. To

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CONCLUSIONS In this Article, we introduce a remarkably simple method of nucleating nanobubbles, introducing cold water onto HOPG surfaces at different temperatures. We find reproducible nucleation of nanobubbles on surfaces at room temperature and heated surfaces (40−80 °C). The nucleation mechanism lies in the substantial difference in solubility in the heated water closest to the hot substrate and the much colder ambient water, which establishes a localized region of supersaturation at the substrate that favors the nucleation of surface-attached

Figure 9. Oversaturation vs time profile for nanobubbles in Figure 6, calculated on the basis of the Lohse−Zhang theory (eq 2). 11218

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Langmuir

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nanobubbles. Our results thus explain one long-standing mystery about nanobubbles. The earliest papers on nanobubbles purport that they are formed simply by immersing a hydrophobic surface in water, but the mechanism by which the bubbles in these papers were formed has never been explained. We also performed a series of stability experiments by imaging nanobubbles over 5 days. Despite the clear difference in gas oversaturation in our method as compared to that in traditional nucleation methods like solvent exchange, the nanobubbles in our temperature difference method also survive for at least 5 days. Comparing the contact angles to the Lohse− Zhang theory suggests that the oversaturation theoretically required to sustain the nanobubbles’ stability is well in excess of the oversaturation generated by the nucleation method. Moreover, the oversaturation appears to survive for much longer than the time required for the oversaturated liquid to lose its excess gas to the atmosphere. Our nucleation method is of significance beyond being a simple method for creating nanobubbles. Because existing methods of nucleating nanobubbles require secondary liquids, ions, or additional procedures such as electrolysis or exchange of liquids, they are prone to contamination from a range of unknown sources. By keeping the number of ingredients and steps for bubble nucleation to a bare minimum, our method avoids obvious sources of contamination while providing a pure environment for studying the properties of nanobubbles. It is therefore our hope that our work forms a foundation for establishing a new body of reproducible results on nanobubbles that, going forward, may help to resolve remaining questions in this exciting field.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge funding from a competitive research programme under the auspices of the Singapore government’s National Research Foundation (Program NRF-CRP9-201104). B.H.T. acknowledges financial support from the Agency of Science, Technology and Research in Singapore.

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DOI: 10.1021/acs.langmuir.6b01531 Langmuir 2016, 32, 11212−11220

Article

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DOI: 10.1021/acs.langmuir.6b01531 Langmuir 2016, 32, 11212−11220