Study on the Formation and properties of Trapped Nanobubbles and

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Study on the Formation and properties of Trapped Nanobubbles and Surface Nanobubbles by Using Spontaneous and Temperature Difference Methods Dayong Li, Litao Qi, Yubo Liu, Bharat Bhushan, Juan Gu, and Jinbo Dong Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b02058 • Publication Date (Web): 19 Aug 2019 Downloaded from pubs.acs.org on August 20, 2019

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Study on the Formation and properties of Trapped Nanobubbles and Surface Nanobubbles by Using Spontaneous and Temperature Difference Methods Dayong Li,1,3* Litao Qi,1 Yubo Liu,1 Bharat Bhushan,3* Juan Gu,2 Jinbo Dong1 1 School of Mechanical Engineering, Heilongjiang University of Science and Technology, Harbin, 150022 China; 2 School of Science, Heilongjiang University of Science and Technology, Harbin, 150022 China; 3Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics (NLB2), The Ohio State University, 201 W. 19th Avenue, Columbus, OH 43210-1142, USA Abstract Trapped nanobubbles are gas domains trapped at nanopits on solid-liquid interface. This is different from surface nanobubbles which usually form at smooth surface. Herein, both trapped nanobubbles and surface nanobubbles formed on nanopitted polystyrene film were studied by using spontaneous formation method and temperature difference method. Trapped nanobubbles show behaving more flexible than surface nanobubbles under different scanning loads. The nanopits under trapped nanobubbles appear at large force scanning, and both trapped nanobubbles and surface nanobubbles can recover after reducing the scan load. The contact angles of the two kinds of nanobubbles were calculated and were found to be approximately constant. Configurations of trapped nanobubbles including under the pit mouth, protruding out but pinning at the pit mouth and protruding out and extending around the pit mouth were experimentally observed. The gas oversaturation in the liquid after replacing the low-temperature water with high-temperature water was evaluated and was found to be a key factor for nanobubble formation and leading to trapped nanobubble protruding out and extending. Our study should be helpful for understanding the formation mechanism and properties of trapped nanobubbles and surface nanobubbles, it will also be useful for further research about the control of nanobubble distribution. Keywords: Trapped nanobubbles; Surface nanobubbles; Gas oversaturation; Nanobubble formation *Corresponding authors: [email protected] and [email protected] 1 ACS Paragon Plus Environment

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Introduction Surface nanobubbles are nanoscopic gas domains forming at the immersed substrates [1-4], which has aroused considerable interest [5-23] since they were imaged by using atomic force microscopic (AFM) in 2000 [1, 2]. When studying surface nanobubbles, two commonly used methods are solvent exchange [1, 16-21] and spontaneous formation at immersion substrates [2, 24-30]. Spontaneous formation of surface nanobubbles is done by immersing the hydrophobic substrate into the saturated water or by dropping water droplets onto a dry hydrophobic surface directly. In the course of doing this, the air in water is absorbed at the solid surface and thus brings about the formation of surface nanobubbles. The spontaneous formation technique is the easiest way to form surface nanobubbles. For the solvent exchange method of studying surface nanobubbles, different organic solvents such as ethanol [1, 17-18], propanol [16, 18], methanol [17], saline [31] and Dimethyl sulfoxide [32]

were used. These solvents have higher air solubility than water, and an oversaturation of gas

will occur and thus leads to nanobubble nucleation. Currently, the solvent exchange technique has become the most often used protocol to produce surface nanobubbles. Another method to generate surface nanobubbles is called the temperature difference method [33], which is by using high-temperature water to exchange low-temperature water; or depositing cold water on room-temperature substrate [15, 34] or heating the substrate directly [32, 35, 36].

Temperature difference method is gradually becoming a commonly used method to study

surface nanobubbles.

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Similar to surface nanobubbles, trapped bubbles can be regarded as another gas form and have been extensively investigated on micro/nanopitted surfaces. For the formation of trapped bubbles, several techniques including fast reducing the liquid pressure [37, 38], acoustics [39], solvent exchange [40-42] and direct immersion of the hydrophobic substrate into water [43, 44] were applied in experiments. Theoretically, these nucleation methods can be explained that gas molecules are entrapped in crevices, scratches or pits on surfaces and then grow by gas diffusion or by gas expansion. This seems to be similar and suited to surface nanobubbles according to the crevice model proposed by Atchley and Prosperetti [45]. However, trapped bubbles in experiments were usually trapped at artificial micro holes, while surface nanobubbles were investigated on smooth surfaces [1, 2, 12, 15-22], this might lead to differences between the two kinds of bubbles. At the same time, there are very few studies paying attention to nano-sized trapped bubbles, which should be helpful to understand the nucleation mechanism and the characteristics of surface nanobubbles. Therefore, it is significant to investigate and compare the properties and the formation of the two types of nanobubbles. In this study, the spontaneous formation method and temperature difference method were used to study the trapped nanobubbles and surface nanobubbles formed on nanopitted polystyrene (PS) film. The change in morphology of trapped bubbles and surface nanobubbles were investigated under different scanning force loads, and the typical configuration of trapped nanobubbles was experimentally studied. In addition, the effects of gas oversaturation on the formation of trapped nanobubbles and surface nanobubbles were investigated by changing the temperature of hot water to replace the low-temperature water. 3 ACS Paragon Plus Environment

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Experiments Section Materials Preparation Pitted PS (polystyrene) films were prepared on 1 cm  1 cm silicon wafer (100) by spin-coating the emulsion of PS solution (0.2%, weight, molecular weight 350,000, Sigma-Aldrich) and water with a ratio of 500:1 (volume) at a span speed of 2000 rpm. Before spin-coating, 2 silicon (100) wafers were put in piranha solution boiling for about 30 min, then were rinsed in an acetone ultrasonic bath for about 5 min, and then were cleaned in ethanol and Milli-Q water for 2 min (3 times). After coating, the PS substrates were dried with nitrogen and annealed at 52°C in an oven for more than 5 hours. The thickness of the pitted PS ﬁlm was about 30 ± 2 nm estimated by using the method of AFM nanoshaving. Water used in our experiments was Milli-Q water (18.2MΩ.cm, Milli-Q system, Millipore, Corp.). Before experiments were carried out, 50 mL Milli-Q water was put in a glass beaker and equilibrated in air overnight. Measurement Methods AFM (NTEGRA Prima, NT-MDT Company, Moscow) was used in our experiments to measure the morphology of substrates both in air (contact mode, CM) and in water (tapping mode, TM). The cantilever (CSG30, NT-MDT) used was rectangular shaped with a tip radius of Rt ≤ 10 nm and a nominal spring constant k = 0.13 ~ 2 Nm-1 (the calibrated spring constant is k = 0.65 ± 0.02 Nm-1). When measuring surface nanobubbles and trapped nanobubbles in water, the pitted PS substrate was fixed in an open fluid cell by two clip springs. The resonance frequency of the cantilever used in water was about 68 kHz. The scanning frequency was 1 Hz and the value of free amplitude A0 was calculated about 4.5 ± 0.2 nm. All the experiments were carried out at a room temperature 25 ± 1°C and humidity 45-55 % RH.


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Results and Discussions Imaging of Trapped Nanobubbles and Surface Nanobubbles Fig. 1a shows the 3D image of pitted PS film captured from CM-AFM in air. Nanopits dotted on 2 μm  2 μm scan area are in nanosize with a diameter and depth on the order of ~120 nm and ~15 nm, respectively. Fig. 1b shows the cross-section on two typical nanopits in Fig. 1a, from which the depth of the two nanopits is about 12 nm and 10 nm, respectively. Based on the section analysis for each nanopits in Fig. 1a, a statistical analysis about the diameter and depth of nanopits was obtained, and the average diameter and depth are about 90.5±4.5 nm and about 8.5±0.4 nm, respectively. Obviously, these nanopits do not puncture the PS film by comparing the pit depth with the thickness of PS film (about 30 ± 2 nm). After immersing the nanopitted PS substrate into air-equilibrated water, Fig. 2a was captured at a set-point of 95% and a scanning frequency of 1 Hz, from which a number of nanobubbles together with several nanopits dotted among them can be seen clearly. The distribution density of nanopits shows obvious decrease as compared with that in Fig. 1a, this indicates that some of nanopits should have trapped nanobubbles. Fig. 2b shows the section analysis of nanopits labeled as pit 1, pit 2 and pit 3 in Fig. 2a. The depth of the three nanopits are about 1.2 nm, 2.5 nm and 3.3 nm from pit 1 to pit 3, respectively (the average depth value of nanopits in water obtained from a statistic analysis is about 2.1±0.1 nm), showing a significant decrease as compared with the depth of nanopits measured in air (8.5±0.4 nm). Note that the AFM cantilever will have a negative charge in water, the PS film also has a lower negative charge in water, thus there could be an electrostatic repulsion between the AFM tip and the surface of pits. Whether the surface charge will cause the decrease in depth of nanopits in water than that in air? A contrast experiment was carried out in degassed water


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which can be seen in Fig. S1 in supporting information. There is no nanobubbles observed on nanopitted PS film, the average depth value of nanopits in degassing water obtained from a statistic analysis is about 6.2 ± 0.3 nm, and the depth of typical nanopits (with similar diameter as that of nanopits in Fig. 1b) is about 8 nm in the degassing water. These indicate that the decrease in depth of nanopits in water should be caused mainly by air adsorption, i.e., the nanopits in degassing water were not trapped air while the nanopits in Fig. 2a had trapped air in them but did not protrude out of the pit mouth. In addition, we found that the nanopits without nanobubbles protruding out of the pit mouth are usually with a lateral size larger than 80 nm. This is similar to the results of Wang and his coauthors [44], they found in their experiments that nanopits with size of about 60 nm are fit for trapped nanobubbles protruding the pit mouth.

Figure 1. (a) Height image of pitted PS film shows the PS film dotted with nanopits, (b) shows the section line along two nanopits obtained from image (a) (not to scale).


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Figure 2. Nanobubbles generated on pitted PS film by using spontaneous formation method. (a) Height image of nanobubbles, which was captured after the pitted PS film immersed in air-equilibrated water, (b) the section analysis of nanopits labeled pit 1, pit 2 and pit 3 in image (a). Nanopits measured in water shows a decrease in depth as compared with that measured in air (as can be seen from Fig. 1b).

To further investigate the formation of trapped nanobubbles, we use high-temperature water (60°C) to replace the low-temperature water (room temperature) in the liquid cell, and then Figure 3 was captured. In Fig. 3a, there are both surface nanobubbles and trapped nanobubbles scattered with a small blister in the middle of the image. Three typical surface nanobubbles are labeled as S1, S2 and S3, two typical trapped nanobubbles are labeled as T1 and T2, and the blister is labeled as B1. Different from that in Fig. 2a, most of the nanopits including those on the blister B1 had trapped nanobubbles. In detail, our statistical results show that the number of nanopits without trapping nanobubbles for the two experiments is about 6.3 / μm2 (spontaneous formation method) and about 2.0 / μm2 (temperature difference method, 60 °C water replacement) respectively. This can be explained by the increase of gas saturation in the liquid after the high/low-temperature water exchange. Fig. 3b(i) was obtained after scanning at a set-point of 85%, most of the nanopits under the trapped nanobubbles emerged and then recovered by 7 ACS Paragon Plus Environment

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trapped nanobubbles in Fig. 3b(ii) which was captured at set-point of 95%. Fig. 3c(i) was scanned with set-point of 80% at the central area (1μm × 1μm) of Fig. 3a, all the nanopits can be seen to be exposed. The depth of trapped nanobubble T1 at this moment is about 11 nm based on the section analysis (in Fig. 4a). This indicates that the AFM tip might have reached to the bottom of the nanopit as compared with the value of pit depth measured in air (Fig. 1). Fig. 3c(ii) was obtained at the original scan area (2μm × 2μm) at set-point of 95%. The squashed trapped nanobubbles and surface nanobubbles recovered again in Fig. 3c(ii). These results provide a direct and visual evidence for the gas nature and formation of trapped nanobubbles. In addition, the phenomenon of nanobubble coalescence has also been observed in this experiment. About three nanobubbles in Fig 3b(ii) circled in a blue ellipse under nanobubble S3 merged together and formed a large one in Fig. 3c(ii) after subjecting large load scan (set-point 80%). This is in line with the results of surface nanobubble coalescence in our previous study [24].


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Figure 3. Height images of trapped nanobubbles (two typical trapped nanobubbles are labeled as T1 and T2), surface nanobubbles (three typical surface nanobubbles are labeled as S1, S2 and S3), and a blister (labeled as B1) formed on pitted PS film (after replacing the room-temperature water with high-temperatuer water, 60°C).


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Image (a) was obtained at 95%, follow by (b) obtained at 85% set-point then 95%, and then (c) obtained at 80% and 95% set-point. The nanopits under trapped nanobubbles can be seen clearly after being subjected to large force scanning.

In Fig. 4, the changes in morphology of trapped nanobubble T1, surface nanobubble S1 and blister B1 with the increase of scanning force are analyzed. After the scanning at set-point of 85%, the trapped nanobubble T1 was pressed to be under the pit mouth (about -3 nm) from about 7 nm

(Fig. 4a); as a comparison, the height of surface nanobubble S1 at this moment is about

2.5 nm, which shows about 50% decrease in height (Fig. 4b). When increasing the scan load to set-point 80%, the height of trapped nanobubble T1 decreases to about -11 nm, and the height of surface nanobubble S1 changes to be about 1 nm. The morphology of both trapped nanobubble and surface nanobubble recovered when set-point was increased to be 95%. The change in morphology with the increase of scan load shows that trapped nanobubbles behaves with more flexibility than surface nanobubbles. As for the blister B1 (Fig.4c) there is no obvious change in morphology at set-point of 85% and shows a slight decrease in both lateral size and height at setpoint of 80%, and then recovers to the original morphology at set-point of 95%. This is in line with the results in our recent study [46].


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Figure 4. Section analysis of (a) trapped nanobubble T1, (b) surface nanobubble S1 and (c) blister B1 in Figure 3 (not to scale).

Figure 5a shows that the contact angle θ of both trapped nanobubbles and surface nanobubbles keeps almost constant (about 11°) with the increase of bubble lateral size (left Yaxis), this indicates that the pits under trapped nanobubbles might have little or no effect on their contact angle. From the right Y-axis in Fig.5a, there is a linear function between the lateral size 11 ACS Paragon Plus Environment

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and the height of both the trapped nanobubble and surface nanobubble. Similarly, Fig. 5b shows that the contact angle of trapped nanobubbles keeps constant (about 11°) with the increase in the lateral size of pit mouth (left Y-axis), and the lateral size of trapped nanobubbles increases with the size of pit mouth linearly (right Y-axis). Based on the statistical data in Fig. 5b, the trapped nanobubbles are with a lateral size in the scale of about 60-150 nm, which are usually larger than that of pit mouth (about 50-100 nm), an example can be seen from the section of trapped nanobubble T1 in Fig.4a. In this case, the trapped nanobubble do not pin but extend its three-phase contact line around the pit mouth. Note that trapped nanobubbles extending around the pit mouth must have experienced the process of protruding out of the pit mouth, thus the configuration of trapped nanobubbles can be categorized into three groups (as can be seen in Fig. 6), that is under the pit mouth, protruding out but pinning at the pit mouth and protruding out and extending around the pit mouth. This is in line with the simulation study

[47]

and theoretical prediction

[48]

and the

experimental results about micro sized trapped bubbles in the Ref [49], where micro trapped bubbles were observed protruding and extending around the mouth of micro-pits with the increase of negative pressure. By comparing the experimental results in Fig.2 (obtained by spontaneous formation method) with that in Fig.3 (obtained by temperature difference method), more nanopits were trapping nanobubbles in the process of high/low-temperature water exchange. Considering that high/low-temperature water exchange will lead to an increase in gas oversaturation; thus, we can conclude that the gas oversaturation should be a key factor for trapped nanobubbles protruding out of the pits and extending around the pit mouth.


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Figure 5. Statistical analysis of morphology parameters of nanobubbles. (a) Contact angle θ (gas side) of both trapped nanobubbles and surface nanobubbles keeps constant with their lateral size (left Y-axis) and the height of trapped nanobubbles behaves as a function of their lateral size (right Y-axis); (b) Contact angle (gas side) of trapped nanobubbles keeps constant with the pit lateral size (left Y-axis) and the lateral size of trapped nanobubble behaves as a function of the pit lateral size (right Y-axis). The bubble size shows much larger than the corresponding pit width. The inset in Fig. 4b is a sectional view of trapped nanobubble, where the


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geometric parameters of trapped nanobubbles are defined.

Figure 6. Diagram showing the different stages of trapped nanobubble nucleation.

Further research about the influence of gas oversaturation on the formation of trapped nanobubbles and surface nanobubbles Considering that gas oversaturation will occur in the liquid when using high temperature water to replace low temperature, thus we can evaluate the effect of gas saturation on the formation of trapped nanobubbles and surface nanobubbles. Here we used high-temperature water (80 °C) to replace the water in the liquid cell, and then Figure 7 was obtained immediately. In Fig. 7 (i), the number of nanobubbles shows an obvious increase in unit area as compared with that in Fig. 3a, which was obtained after using 60 °C high-temperature to replace the room-temperature water. Almost all the nanopits were trapped nanobubbles, except for some in the two bleak areas circled with white dotted line, where should be caused by micro-bubbles formed in the process of water replacement. More specifically, a statistical analysis (without considering bleak areas mentioned above) was made and the result shows that the number of nanopits without trapping nanobubbles decreases from 2.0 / μm2 (60 °C water replacement) to about 0.5 / μm2 (80 °C water replacement). 14 ACS Paragon Plus Environment

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Fig. 7(iii) was captured at the rectangular area circled with blue dotted line. Based on Fig. 3a and Fig. 7(iii), the statistical analysis of nanobubbles in unit area was performed (here both trapped nanobubbles and surface nanobubbles were called nanobubbles). The number of nanobubbles in unit area (1 μm2) increases from 32 to 51 (see in Table 1). In detail, the nanobubbles with lateral size from 50 nm to 110 nm have an obvious increase after the water exchange (80 °C), especially for nanobubbles with the size of about 100 nm which increases about 70%, although the number of nanobubbles with lateral size larger than 150 nm shows no obvious increase. The mean area and volume of single nanobubble were also calculated, which can be seen in Table 1 increasing from about 11.7 to 16.2 nm × μm and from about 39.3 to 75.8 nm2× μm, respectively. Note that the pit volume under trapped nanobubbles was not considered when calculating the gas volume, due to some trapped nanobubbles might be newly formed in Fig. 7(iii), so the actual increase in gas volume should be larger than the calculated one.

Figure 7. Height image (i) and phase image (ii) of trapped nanobubbles and surface nanobubbles after replacing the water in liquid cell with hot water (80 °C), (iii) was captured at the blue rectangular area in image (i). Almost all the nanopits were trapped at nanobubbles and the number of nanobubbles shows an obvious increase as compared with that in Figure 3.


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Table 1. Quantitative analysis of nanobubbles with the change in gas oversaturation

Item

In figure 3(a) In figure 7(iii)

Count (mean, number / μm2) 32 51

Area (mean, nm × μm)

Volume (mean, nm2× μm) 39.3 75.8

11.7 16.2

Figure 8. (a) Histogram of nanobubble nucleation after the exchange of water in the liquid cell with hightemperature water (60 °C and 80 °C); (b) The relationship between sinθ and nanobubble lateral size Lb. The slope in image (b) corresponds to sinθ/L, the values of local gas oversaturation ζ after twice water replacement can be calculated according to Eq. (1), which are 2.1 nm-1 (60 °C, Fig. 3a) and 3.7 nm-1 (80 °C, Fig. 7(iii)), respectively.

The gas oversaturation ζ for nanobubbles at the two stage (after 60 °C and 80 °C high /lowtemperature water exchange) was estimated. Based on the model of Lohse and Zhang [50], the gas oversaturation ζ can be depicted as a function of bubble shape as follows



4 sin  e P0 Lb

(1)

where 𝜃𝑒 is equilibrium contact angle, 𝛾 is the air-liquid interface surface tension (0.072 N/m),


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P0 is the atmospheric pressure and Lb is the lateral size of nanobubbles. Fig.8b shows the relation between sin θ and nanobubble lateral size Lb. By fitting these data, the values of sin θ/L (slope values in Fig.8b) can be obtained. According to Eq. (1), the gas oversaturation ζ can be calculated. The calculated ζ for the case 60 °C water replacement and 80 °C water replacement is 2.1 nm-1 and 3.7 nm-1, respectively. This is in line with the experimental results in Figure 3 and Figure 7, i.e., the number and the size for both surface nanobubbles and trapped nanobubbles show an obvious increase, which indicates that gas oversaturation should be a key factor for the formation of surface nanobubbles and trapped nanobubbles protruding out of the nanopits. Conclusions In this paper, the properties and difference of trapped nanobubbles and surface nanobubbles were first studied on nanopitted PS film by using spontaneous formation method and temperature difference method. Trapped nanobubbles and surface nanobubbles show having the same morphology. The change in morphology of trapped nanobubbles and surface nanobubbles under different scanning force loads were studied, and trapped nanobubbles behaves with more flexibility than surface nanobubbles. The nanopits under the trapped nanobubbles will appear as being subjected to large force scanning, both trapped bubbles and surface nanobubbles can recover with the reduction of scan loads. The typical configuration of trapped nanobubbles was experimentally investigated. There are three configurations of trapped nanobubbles including under the pit mouth, protruding out but pinning at the pit mouth and protruding out and extending around the pit mouth. This is consistent with the simulation study [47] and theoretical prediction [48] and the experimental results about trapped microbubbles [49]. When using high17 ACS Paragon Plus Environment

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temperature water to replace the liquid in the liquid cell, both surface nanobubbles and trapped nanobubbles show obvious increase in number and size. The gas oversaturation ζ of high/lowtemperature water exchange experiments (60 °C and 80 °C) was evaluated based on the model of Lohse and Zhang [50], which is in line with the simulation [51,52] and theoretical [50] results and shows that the gas oversaturation is a key factor for the formation of the two kinds of nanobubbles. Our results should be of great interest to the nanobubble community, it should also be helpful for further research about the control of nanobubble distribution and their application in drag reduction [53].

Acknowledgement We thank Dong Song for discussions and the support from the Key laboratory of MicroSystems and Micro-Structures Manufacturing in HIT and Feiran Li for preparing PS film. We gratefully acknowledge the financial support from National Natural Science Foundation of China (No. 11972150 ), the Heilongjiang Province Natural Science Foundation of China (E2016059, E2017060) and Heilongjiang Provincial University Basic Scientific Research Service Project (Hkdqg201803). Dayong Li gratefully acknowledge the financial support from China Scholarship Council to spend one year at The Ohio State University (B. Bhushan).

Supporting Information Supporting Information is included with one figure.

References [1] Lou, S. T.; Ouyang, Z. Q.; Zhang, Y.; Li, X. J.; Hu, J.; Li, M. Q.; Yang, F. J. Nanobubbles on Solid Surface Imaged by Atomic Force Microscopy. J. Vac. Sci. Technol. B: 2000, 18, 25732575. 18 ACS Paragon Plus Environment

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Graphical TOC Entry


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Figure 1. (a) Height image of pitted PS film shows the PS film dotted with nanopits, (b) shows the section line along two nanopits obtained from image (a) (not to scale). 135x55mm (300 x 300 DPI)

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Figure 2. Nanobubbles generated on pitted PS film by using spontaneous formation method. 113x60mm (300 x 300 DPI)


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Figure 3. Height images of trapped nanobubbles (two typical trapped nanobubbles are labeled as T1 and T2), surface nanobubbles (three typical surface nanobubbles are labeled as S1, S2 and S3), and a blister (labeled as B1) formed on pitted PS film (after replacing the room-temperature water with high-temperatuer water, 60°C). 98x186mm (300 x 300 DPI)


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Figure 4. Section analysis of (a) trapped nanobubble T1, (b) surface nanobubble S1 and (c) blister B1 in Figure 3 (not to scale). 165x167mm (300 x 300 DPI)


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Figure 5. Statistical analysis of morphology parameters of nanobubbles. 94x154mm (300 x 300 DPI)


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Figure 6. Diagram showing the different stages of trapped nanobubble nucleation. 77x53mm (300 x 300 DPI)


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Figure 7. Height image (i) and phase image (ii) of trapped nanobubbles and surface nanobubbles after replacing the water in liquid cell with hot water (80 °C), (iii) was captured at the blue rectangular area in image (i). 97x53mm (300 x 300 DPI)


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Figure 8. (a) Histogram of nanobubble nucleation after the exchange of water in the liquid cell with hightemperature water (60 °C and 80 °C); (b) The relationship between sinθ and nanobubble lateral size Lb. 167x78mm (300 x 300 DPI)


Study on the Formation and properties of Trapped Nanobubbles and

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