Article pubs.acs.org/Langmuir
Metastable Nanobubbles at the Solid−Liquid Interface Due to Contact Angle Hysteresis Takashi Nishiyama,*,†,‡ Yutaka Yamada,†,‡ Tatsuya Ikuta,† Koji Takahashi,†,‡,§ and Yasuyuki Takata‡,§,∥ †
Department of Aeronautics and Astronautics, ‡CREST, §International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), and ∥Department of Mechanical Engineering, Kyushu University, Fukuoka 819-0395, Japan S Supporting Information *
ABSTRACT: Nanobubbles exist at solid−liquid interfaces between pure water and hydrophobic surfaces with very high stability, lasting in certain cases up to several days. Not only semispherical but also other shapes, such as micropancakes, are known to exist at such interfaces. However, doubt has been raised as to whether or not the nanobubbles are gas-phase entities. In this study, surface nanobubbles at a pure water−highly ordered pyrolytic graphite (HOPG) interface were investigated by peak force quantitative nanomechanics (PF-QNM). Multiple isolated nanobubbles generated by the solvent-exchange method were present on the terraced areas, avoiding the steps of the HOPG surface. Adjacent nanobubbles coalesced and formed metastable nanobubbles. Coalescence was enhanced by the PF-QNM measurement. We determined that nanobubbles can exist for a long time because of nanoscale contact angle hysteresis at the water−HOPG interface. Moreover, the hydrophilic steps of HOPG were avoided during coalescence, providing evidence that the nanobubbles are truly gas phase.
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heterogeneities of the substrates.8,10,21,22 In addition, the relationship between boiling and surface nanobubbles has been investigated theoretically and experimentally.23,24 However, a definitive explanation of nanobubble stability is still lacking. Nanobubbles were first observed by tapping-mode atomic force microscopy (TM-AFM).1,2 Subsequently, a range of other methods have been used to study nanobubbles, such as rapid cryofixation,25 quartz crystal microbalance,26 attenuated total internal reflection Fourier transform infrared (FT-IR) spectroscopy,27,28 and optical methods.29,30 Although TM-AFM remains the most commonly used technique in nanobubble research,9−15 it suffers from the drawback that it detects changes in the amplitude of the resonance frequency oscillation of a cantilever. This means that there is a high likelihood of perturbing the shape of surface nanobubbles at the solid−liquid interface during TM-AFM measurements. Therefore, it is difficult to obtain a true image of nanobubbles using TM-AFM. In particular, when measuring soft samples, the height profiles obtained by TM-AFM may be unrealistic. Thus, to better
INTRODUCTION For over a decade, numerous experimental studies have confirmed the existence of a highly stable nanoscale gas phase, referred to as nanobubbles, at solid−liquid interfaces.1−5 However, classical thermodynamics predicts that small bubbles, with diameters of micro- or nanometers, should disappear in less than a few hundred microseconds.6,7 The surprisingly long lifetime and stability of nanobubbles are the subject of considerable research interest.8−16 In addition to sphericalcapped nanobubbles, micropancakes have also been observed, although the conditions under which they form are still poorly understood. In one study, micropancakes were found to be irregularly shaped, with a typical height of 1.1−1.3 nm and width of over 1 μm on a hydrophobic monolayer film.17 In another study, micropancakes with a height of 2 nm and width of several micrometers formed on highly ordered pyrolytic graphite (HOPG). Nanobubble−micropancake composites, i.e., spherical-cap nanobubbles on top of flat gas layers, have also been observed.18,19 It has been proposed that nanobubbles are stabilized by the gas outflux driven by Laplace pressure being balanced by a gas influx at the contact line.12,20 Other studies have suggested that the stability arises through contact line pinning, which results from intrinsic nanoscale physical roughness or chemical © 2014 American Chemical Society
Received: September 10, 2014 Revised: December 25, 2014 Published: December 25, 2014 982
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Langmuir understand nanobubbles, more accurate measurement of their shape is required. Additionally, doubt has been raised as to whether or not the nanobubbles are gas-phase entities because some reports31,32 revealed the absence of nanobubbles at solid−liquid interfaces. In general, AFM is a tool for measuring surface structure and cannot be used to obtain direct evidence of the gas phase. The gaseous nature of nanobubbles has been confirmed using FT-IR spectroscopy,27,28 although such experiments are restricted to infrared-active gases, such as carbon dioxide. Therefore, evidence of the gas phase and the underlying mechanism of formation of nanobubbles derived from air should be examined using a reliable experimental technique. Recently, a novel AFM measurement mode, peak force quantitative nanomechanics (PF-QNM), has been applied to nanobubble measurements.33−35 PF-QNM enables high-resolution topographical images and quantitative mapping images of mechanical properties to be obtained simultaneously. In PFQNM, the probe and sample are intermittently brought into contact for a short period, eliminating lateral forces. Unlike in TM-AFM, in which the feedback loop keeps the cantilever vibration amplitude constant, PF-QNM controls the maximum force applied by the tip. PF-QNM makes it possible to control the interactions between the AFM tip and nanobubble and is therefore a suitable technique for nanobubble measurement.36 In this study, we use PF-QNM to investigate the stability of nanobubbles and the reasons why they form unstable shapes.
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EXPERIMENTAL SECTION Figure 1. Topographical PF-QNM images (5 × 5 μm) of a HOPG− water interface (a) 45 min, (b) 90 min, (c) 150 min, (d) 150 min (mapping image using the DMT model), (e) 180 min, and (f) 230 min after solvent exchange with a setpoint of 462 pN, 770 pN, 2.3 nN, 2.3 nN, 462 pN, and 462 pN, respectively. An example of nanobubble coalescence is indicated by the white arrows in (a), (b), and (e). The white lines extending from the upper left corner to the lower right corner of the image in (a) indicate the positions of the steps on the HOPG surface.
Nanobubbles have been measured at the interface between a hydrophobic surface and pure water in many previous studies.1−5 In the present study, the interface between pure water and HOPG (SPI-1 grade, 10 × 10 mm, Alliance Biosystems Inc., Japan) was observed by PF-QNM. A HOPG sample with a thickness of ∼0.5 mm was fixed on a stainless-steel Petri dish. Pure water was prepared by a water purifier (RFP742HA, Advantec, Japan) without degassing. A solvent-exchange method, which is a standard method for generating nanobubbles,2,18,28,37−39 was then performed. The HOPG sample was first immersed in ethanol, which has higher air solubility than water, for several minutes before the ethanol was displaced by pure water. This process creates supersaturated conditions at the HOPG−liquid interface and thereby enhances the formation of nanobubbles. Supersaturation is believed to be an important factor in nanobubble formation.40,41 When the solvent-exchange method was not performed, it was difficult to observe nanobubbles. AFM measurements were performed with a Dimension Icon (Bruker AXS) instrument. A ScanAsyst Fluid+ cantilever (tip radius: 2 nm; spring constant: 0.7 N m−1) was used, which was suitable for PFQNM measurements in liquid.
Semispherical nanobubbles (diameter: 100−500 nm; height: 3−22 nm) can be seen in Figure 1a. Although the macroscopic contact angle for pure water on HOPG is between 88° and 92°, the measured contact angle of the nanobubbles is 170°−174° with no size dependence. The contact angles of the nanobubbles are similar to those reported in the literature and can be explained by line tension.42 Moreover, the contact angles are close to those of nanobubbles in pure water without solvent exchange; this demonstrates that there is no residual ethanol. A topographical PF-QNM image taken 90 min after solvent exchange is depicted in Figure 1b. Coalescence of some adjacent nanobubbles is observed. Figure 1c is a topographical image taken 150 min after solvent exchange. The peak force setpoint was gradually increased from 462 pN (Figure 1a) to 2.3 nN (Figure 1c) at the time of measurement. Although the nanobubbles disappeared from the topographical image after 150 min, they were still detectable using other characteristics of the PF-QNM, for instance, in the image obtained using the Derjaguin−Muller− Toropov (DMT) model, as illustrated in Figure 1d. Figure 1e is a topographical image measured after 180 min after solvent exchange with a 462 pN peak force setpoint. The coalescence of many nanobubbles can be observed in this image. Nanobubble coalescence has been observed in some
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RESULTS AND DISCUSSION Images of the nanobubbles at the HOPG−water interface obtained by PF-QNM after solvent exchange are shown in Figure 1. The image in Figure 1a was obtained 45 min after solvent exchange. The peak force setpoint determines the strength of the AFM tip approach and was set at 462 pN, which is a sufficiently weak loading force to measure soft nanobubbles. The white lines extending from the upper left corner to the lower right corner of this image indicate the positions of the steps on the HOPG surface (the steps can be seen clearly in Supporting Information Figure S1). The height of each step is about 1 nm. Thus, at the HOPG−water interface, nanobubbles form on the terraced areas and not on the steps. 983
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Langmuir TM-AFM studies after increasing the tapping force.43−46 In these studies, the nanobubbles increased in size upon coalescence while retaining an approximately semispherical shape. The metastability of the nanobubbles in the present study depended on the AFM measurement mode. For TMAFM, the resonance frequency of the cantilever is used as the operating frequency; this is determined by the type of cantilever and is generally above 100 kHz in liquids. In contrast, PF-QNM makes it possible to operate at frequencies as low as 2 kHz. It is considered that a high tapping frequency promotes nanobubble coalescence. Therefore, the semispherical shape is already formed when the tapping force is returned to the lower value in TM-AFM measurements. In the case of PF-QNM, for which the rate of coalescence is low because of the low frequency of tapping, a metastable gas phase can be observed before the semispherical shape forms. Figure 1f is a topographical image measured 230 min after solvent exchange. The PF-QNM measurement was conducted with a low peak force setpoint of 462 pN, as was used for the image in Figure 1e. However, in this case, no nanobubble coalescence was observed. Thus, the coalescence of nanobubbles is caused by a beating and flattening effect during the higher setpoint PF-QNM measurement. Once the position of the contact line of a nanobubble has been moved by deformation because of the large setpoint, the nanobubble does not return to its original shape even if the setpoint is decreased to a low load. In addition, when the measurement is made with a sufficiently small setpoint, nanobubble coalescence and deformation are not promoted. PF-QNM measurements were performed at three different times at 462 pN (Figure 1e,f). However, the topographic images obtained revealed almost no change in the nanobubbles. Although coalesced metastable nanobubbles seem to be unstable, they exist stably for least several tens of minutes. There are shape irregularities with a thickness of approximately 1 nm apparent in the micropancakes; however, this is the first time that metastable nanobubbles with heights of about 10 nm have been confirmed. We attribute their stability to contact angle hysteresis. The cross sections of an individual nanobubble at the HOPG−water interface indicated by circles in Figure 1a,b,e are shown in Figure 2. The shape and contact angle of the nanobubble changed during the PF-QNM measurements; specifically, its diameter increased and height decreased, leading
to an increase in contact angle from 169° to 174°. It has been reported that an increase in the setpoint decreases both the diameter and height of nanobubbles.33,34 Thus, our results are not consistent with the findings of these previous studies. A possible reason for this is differences in the strength of the pinning effect resulting from different conditions of the solid surface. Our results show that nanobubble deformation would result not only from temporal changes but also from the pressing and tapping of the AFM cantilever. During nanobubble coalescence, the steps of HOPG are avoided, as shown in Figure 1; that is, coalescence does not occur if a step is present between the nanobubbles. The contact angle of the HOPG surface is approximately 90°, whereas that of the side wall of bulk HOPG is about 75°, so the steps can be expected to exhibit hydrophilic characteristics. Yang et al.15 reported that the formation of nanobubbles is greatly enhanced on the upper steps of HOPG, whereas no nanobubbles are formed on the lower steps. In contrast, our results did not indicate a preference for the upper or lower steps. The step height was similar in both studies. However, the sizes of the measured nanobubbles differed considerably: Yang et al. reported nanobubbles with diameters of about 100 nm, whereas in the present study, the diameters were about 100− 500 nm. We consider that the smaller nanobubbles are strongly affected by the nanoscopic hydrophilic and hydrophobic regions of the HOPG surface. Results of condensation experiments with an environmental scanning electron microscope (ESEM) have also indicated that the steps of HOPG are hydrophilic.47 Condensation of nanobubbles occurs preferentially at the steps. The fact that nanobubbles did not adhere at the HOPG steps is evidence for their gaseous nature. A schematic diagram of the metastable nanobubbles is depicted in Figure 3. On a macroscopic scale, the coalesced
Figure 3. Schematic diagram of semispherical and metastable bubbles (top view) with a very large advancing contact angle and very small receding contact angle.
bubble shape immediately becomes semispherical because of surface tension. To maintain metastability, a very large advancing contact angle (i.e., the surface is difficult to wet during recession of the contact line) and very small receding contact angle (i.e., once the substrate surface at the waist of the metastable gas phase is wet, it does not dry easily) are required. That is, hysteresis between the advancing and receding contact angles is necessary to form metastable nanobubbles.
Figure 2. Cross sections of an individual nanobubble (indicated by circles in Figure 1a,b,e) at the HOPG−water interface after solvent exchange. 984
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Langmuir
vibration of the cantilever. (iii) Metastable nanobubbles generated by coalescence exist stably for at least several tens of minutes because of nanoscale contact angle hysteresis. (iv) Coalescence proceeds while avoiding the hydrophilic steps of HOPG, providing evidence that the nanobubbles are truly gaseous in nature. These results suggest that it is possible for a metastable gas phase to exist stably at the nanoscale, unlike at the macroscale. The stability of micropancakes can also be explained by considering distinctive characteristics on the nanoscale.
At the macroscopic scale, the advancing and receding contact angles for pure water on an HOPG surface are 90° and 70°, respectively,48 which is a relatively small hysteresis. We conducted an experiment to measure the contact angle hysteresis at the nanoscale at the interface between pure water and graphite. A multiwalled carbon nanotube (MWCNT), which has a graphite-like surface, was penetrated into the droplet by a manipulator, and the advancing and receding contact angles at the MWCNT−water interface were observed. ESEM images of the penetration and pull-out experiment using the manipulated MWCNT probe are presented in Figure 4. A very high advancing contact angle is
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ASSOCIATED CONTENT
* Supporting Information S
A peak force error image of the HOPG−water interface after solvent exchange and a movie of MWCNT penetration and pull-out. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (T.N.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partially supported by JSPS KAKENHI Grants 23360101 and 25420164, JST-CREST. PF-QNM measurements were performed at the Center of Advanced Instrumental Analysis, Kyushu University.
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CONCLUSION Nanobubbles at the solid−liquid interface of HOPG and pure water were measured using PF-QNM. The following observations were made: (i) Many nanobubbles are generated by the solvent exchange method, but not on the steps of the HOPG surface. (ii) During the PF-QNM measurement, coalescence of adjacent nanobubbles is promoted by the 985
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