Unexpected Homogeneous Bubble Nucleation Near a Solid-Liquid

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Unexpected Homogeneous Bubble Nucleation Near a Solid-Liquid Interface Yoko Tomo, Qin-Yi Li, Tatsuya Ikuta, Yasuyuki Takata, and Koji Takahashi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09200 • Publication Date (Web): 03 Dec 2018 Downloaded from http://pubs.acs.org on December 8, 2018

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The Journal of Physical Chemistry

Unexpected Homogeneous Bubble Nucleation near a Solid-Liquid Interface Yoko Tomo,1,2 Qin-Yi Li,1,2 Tatsuya Ikuta,1,2 Yasuyuki Takata,2,3 and Koji Takahashi* 1,2 1

Department of Aeronautics and Astronautics, Graduate School of Engineering, Kyushu

University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan 2

International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University,

744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan 3

Department of Mechanical Engineering, Graduate School of Engineering, Kyushu University,

744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan *E-mail: [email protected]

Submitted to Journal of Physical Chemistry C Manuscript type: Article

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ABSTRACT

We report a quasi-three-dimensional observation of electron-beam-induced nanobubbles inside a 1000-nm-thick layer of water using the liquid cell electron microscopy. In the early stage of observation, heterogeneous bubble nucleation occurred and small bubbles coalesced with the adjacent bubbles when they come in contact with each other. However, for the first time, we found that, after prolonged electron beam irradiation, heterogeneous nucleation did not occur more, and then homogeneous nucleation started even though a solid surface was available for heterogeneous nucleation. We conclude that Ostwald ripening effect disturbs heterogeneous nucleation to occur and that the lower surface tension due to the generation of ions and radicals boosts the homogeneous nucleation. It was also discovered that the generation sites of homogeneous nucleation are beneath the three-phase contact lines of existing interfacial bubbles.


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INTRODUCTION Micro and nanobubbles have attracted intense research interest owing to not only their unique properties1–5 but also their technological applications such as aquaculture,6 washing machines,7

nano-material

engineering,8

and

microelectronics.9,10

Moreover,

interfacial

nanobubbles affect chemical and physical phenomena near the solid-liquid interface, such as electrolysis and drag reduction.11,12 Bubbles disturb electrolysis by covering the surface of electrodes but enhance the liquid slip especially in microchannels. Therefore, an understanding of the generation and stabilization mechanisms of nanobubbles in the solid-liquid boundary is desired to improve many kinds of liquid-based systems. However, the initial stage of bubble generation is a complicated phenomenon influenced by mechanical,13 electrical,14–16 and chemical17,18 elements. Reported theories, such as classical nucleation theory and the Laplace equation, are based on idealized conditions, and cannot accurately describe the generation and growth of nanobubbles near the solid-liquid interface. Thus, experimental data are key to the understanding of nanobubble dynamics and for the design and optimization of wet processes in which gas phases in a liquid act as important mediators. Dynamic visualization is a useful method for the investigation of nanobubble behaviour. However, in-situ observation of nanobubbles in water at the nanoscale is experimentally challenging. Recently, liquid cell electron microscopy19–27 has enabled us to monitor bubble generation and growth with a spatial resolution of sub-10 nm using transmission electron microscopy (TEM). Coalescence and the Ostwald ripening have been observed between two small interfacial nanobubbles with diameters of less than 10 nm in ultrathin water layers with a thickness of several tens of nanometers using a graphene liquid cell.24,25 Although the graphene liquid cell achieves the highest spatial resolution, the bubble location and motion are limited to the horizontal


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direction. On the other hand, a silicon nitride liquid cell, whose water layer was confined between a couple of silicon nitride membrane windows, can control the thickness of the water layer and could allow a bubble to move three-dimensionally. The three-dimensional sequential process of nanobubbles from nucleation to growth has yet to be explored because TEM images are two dimensional. However, we previously developed a Fresnel fringe method,26 which realizes threedimensional nanobubble observation by shifting focus position slightly using two-dimensional TEM imaging. Then, it was a well-known fact that heterogeneous nucleation was induced by the electron beam irradiation and interfacial nanobubbles located on the surfaces of the top and bottom membrane windows at the early stage of TEM observation. In this paper, we demonstrate longterm observations of the dynamic behaviour of nanobubbles in a confined layer of water whose thickness is ten times as high as that of the nanobubbles. Our observation, for the first time, confirmed that homogeneous nucleation occurred following heterogeneous nucleation, even though all bubble generation near solid surfaces has been expected to arise from heterogeneous nucleation.21,23,26,27 We discuss the detailed mechanisms of growth and nucleation of nanobubbles after the hemispherical gas phases are initially created on the solid-liquid interface.


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EXPERIMENTAL METHODS We used a silicon nitride liquid cell whose gap-thickness for storing water were more than 1000 nm (K-kit, Materials Analysis Technology Inc., Hsinchu, Taiwan). Water was sandwiched between two silicon nitride membrane windows with the thickness of 50 nm and sealed using an epoxy resin. Water-gas interfaces were imaged using a TEM (JEM-3200FSK, JEOL Ltd., Tokyo, Japan) operated at 300 kV. When water is irradiated by the electron beam, water molecules are decomposed into hydrogen gas and oxygen gas.21,28 The temperature rise was not considered because the electron beam power was expected to be very low under these experimental conditions.21 We conducted another TEM observation under 200 kV and the result is provided in the Supporting Information.


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RESULTS AND DISCUSSION At the early stage of TEM observation, all bubbles were generated on the surfaces of the top or bottom windows owing to an electron beam irradiation.26 These bubbles mainly consist of hydrogen and oxygen gases which were produced due to the decomposition of water. We determined the vertical locations of the bubbles based on the brightness of the fringe around the

Figure 1

Time evolution of bubble nucleation in the liquid cell. Blue, red, and white dashed

lines indicate homogeneous bubbles, heterogeneous bubbles on the surface of the top window, and those on the bottom window, respectively. The white arrows indicate diminishing bubbles. The yellow square indicates a spacious area where homogeneous nucleation did not occur. The yellow arrows indicate the homogeneous bubbles generated beneath a couple of close threephase contact lines of interfacial nanobubbles. Time refers to the electron beam irradiation time. All scale bars correspond to 20 nm.


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bubble using the Fresnel fringe method,26 since the brightness of the bubble fringe depends on the vertical distance from the focus position of the electron beam. Figure 1 shows three kinds of nanobubbles with different fringes. The bubbles with red and blue dashed lines have dark fringes while the bubbles with the white dashed lines have bright fringes, which indicated that the former bubbles were located upstream of the electron beam and the latter bubbles were downstream, according to the Fresnel fringe method. The original movie, Video S1, is provided in the supporting information. The order of bubble nucleation was as follows. First, bubbles with red lines, second, those with white lines and then those with blue lines were generated. We found that the bubbles newly generated after approximately 100 s were located below the existing bubbles that had already been generated on the surface of the top window, and we observed growth, coalescence and Ostwald ripening processes of the top-window bubbles. As shown in Figure 1 at 72 s and 104 s, the small heterogeneous bubbles on the top-window surface grew by themselves, came into contact with the adjacent bubbles of about the same size, and eventually coalesced with each other. We did not find any new heterogeneous nucleation in the tiny space between the three-phase contact lines of the adjacent large bubbles. Some new heterogeneous bubbles were generated close to the existing large bubbles, as indicated by the white arrows in Figure 1 at 142 s. After some while, these small bubbles near the large ones gradually shrank and disappeared, which indicates the typical Ostwald ripening process.29,30 This Ostwald ripening phenomenon was also observed on the bubbles that grew more slowly than the neighbouring ones generated at the same time. As shown in Figure 1 at 104 s, 142 s and 228 s, it should be noted that the blue lines overlap the red lines even for bubbles with a small diameter and they did not coalesce with each other, which suggests that the blue lines corresponded to bubbles of homogeneous nucleation. Moreover, homogeneous nucleation occurred after the small heterogeneous bubbles started to be diminishing


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as mentioned above. Figure 2 shows the number of new heterogeneous and homogeneous nucleation in the upstream side of the electron beam in each period of time. After the onset of homogeneous nucleation, we could not find any new heterogeneous nucleation. As shown in Figure 1 at 142 s, 197 s and 228 s, it is worth noting that there was a solid surface area that was not occupied by any gas phase even after many homogeneous bubbles were generated. Our observation was inconsistent with the well-known nucleation theory, which states that the heterogeneous nucleation is predominant over the homogeneous one. Furthermore, we found that, in the beginning, the sites of the homogeneous nucleation were located beneath the three-phase contact lines of the interfacial nanobubbles growing on the surface of the top window, as shown in Figure 1 at 197 s and no homogeneous nucleation occurred below the spacious surface area between the interfacial nanobubbles on the top window, as indicated by the yellow square in Figure 1 at 197 s. After about 200 s, homogeneous nucleation occurred below the interfacial nanobubbles as well as near the triple lines, when many nanobubbles had grown enough. In addition, it should be noted that the acceleration voltage of electron beam influences the bubble nucleation and we observed that the number of nucleation under 200 kV is much smaller than under 300 kV, as shown in Figure S1.


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Figure 2

Histogram of heterogeneous and homogeneous nucleation in the upstream side of the

electron beam. Inserted illustrations are schematic side views of heterogeneous and homogeneous nucleation.

Figure 3

Schematic illustration of the sequential process from heterogeneous to homogeneous

nucleation in the liquid cell. Blue dashed arrows indicate the transfer of gas molecules from a smaller bubble to a larger one owing to the Ostwald ripening effect. Black arrows indicate the absorption of supersaturated gas molecules into a grown-up bubble. Red arrows indicate the movement of the liquid-gas interfaces. The deeper pink region indicates higher densities of ions and radicals.


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The mechanism of bubble nucleation near a solid–liquid boundary is explained as follows. Figure 3 shows the schematic illustrations of the sequential process from the heterogeneous nucleation to homogeneous one in the upstream side of the liquid cell. When electron beam irradiation started, gas molecules were generated owing to the radiolysis of water.28 First, the density of gas molecules was higher at the upstream side of the electron beam than the downstream side, which induced heterogeneous nucleation on the top window. Second, heterogeneous nucleation started on the bottom window, because heterogeneous nucleation requires less energy than homogeneous one. As shown in Figure 1 at 72 s and 104 s, small bubbles coalesced with the adjacent bubble when they contacted with each other, while new bubble generation continues to occur on the surface of the top window. As shown in Figure 1 at 142 s, 197 s and at 228 s, although some of the small interfacial nanobubbles did not grow anymore and diminished, the larger interfacial bubbles were still growing, which suggests that the supersaturated gas molecules and very small nanobubbles near the solid surface were absorbed into the grown-up bubbles due to the Ostwald ripening as illustrated in Figure 3. This is the reason why no more heterogeneous nucleation occurred despite the available surface area, but homogeneous nucleation started below the grown-up bubbles. This discussion is corroborated by the results that no heterogeneous nucleation occurred between the large bubbles and that homogeneous nucleation was launched after the Ostwald ripening effect began to work. For a better understanding on this new phenomenon, we calculated the mean bubble radius of heterogeneous bubbles and compared it with the Lifshitz-Slyozov-Wagner (LSW) model for Ostwald ripening. The projected area of each interfacial nanobubble was measured in each image and the radius was approximated as 𝑟𝑟𝑖𝑖 ≈ (𝐴𝐴⁄𝜋𝜋)1⁄2 .31 The mean bubble radius < 𝑟𝑟 >= (1⁄𝑁𝑁) ∑𝑁𝑁 𝑖𝑖 𝑟𝑟𝑖𝑖 was calculated in each irradiation time ACS Paragon Plus Environment

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and the logarithmic relationship between and t is plotted in Figure 4. Before t = 143 s, gradually increased with time, and after homogeneous nucleation started, increased rapidly. According to the LSW theory,29–32 is expected to fit a power law function of the form of
= 𝛼𝛼𝑡𝑡𝛽𝛽 , where 𝛽𝛽 equals 1/3 for the diffusion-limited case and 𝛽𝛽 equals 1/2 for the reactionlimited case. As shown in Figure 4, between t = 91 s and 143 s, linear regression between lg()

and lg(t) yields the power exponent 𝛽𝛽1 = 0.511 ± 0.068, which is consistent with the reaction-

limited process in the LSW theory, indicating that the growth rates of the interfacial bubbles were

limited by the radiolysis of water rather than molecule diffusion. Between t = 153 s and 203 s, the evolution of can also be well fit to a power law with the exponent 𝛽𝛽2 = 1.665 ± 0.166 , which is much larger than that before t = 143 s. This rapid growth of heterogeneous bubbles is

caused by the coalescence process, which occurs more easily during this period as enlarging bubbles were approaching each other. This rapid increase of owing to coalescence is consistent with the previous report of in-situ observation of single colloidal platinum nanocrystal growth.32


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Figure 4

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Logarithmic relationship of the mean bubble radius (unit: nm) versus irradiation time

(unit: s). The black lines are the piecewise linear fits of the logarithmic values, which yield piecewise power law relations between and t in the form of < 𝑟𝑟 >= 𝛼𝛼𝑡𝑡𝛽𝛽 , where 𝛽𝛽1 =

0.511 ± 0.068 between t = 91 s and 143 s, and 𝛽𝛽2 = 1.665 ± 0.166 between t = 153 s and 203 s.

In addition to the Ostwald ripening effect, there is sufficient space between the top and bottom interfacial bubbles, where the transmitted electron beam causes the generation of gas molecules as the same way near the solid-liquid interface. Then, the thermodynamic fluctuation of the gas molecules causes homogeneous nucleation. Moreover, owing to the generation of ions and radicals, the surface tension of water irradiated by the electron beam should be lower than that of pure water over time, which lowers the energy barrier for nucleation. Next, we discuss the site of homogeneous nucleation as below. Radiolysis of water continues to occur near the liquid-gas interfaces of the grown-up interfacial nanobubbles, which suggests that the densities of ions and radicals are higher there than any other areas, as shown in Figure 3. If two grown-up bubbles are


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closely located, the region near their adjacent triple lines is of the lowest surface tension. Thus, homogeneous nucleation was promoted beneath a couple of close three-phase contact lines of the interfacial nanobubbles but did not occur below the surface area without nanobubbles on the top window. Gas molecules are absorbed into the grown-up bubbles, which delays the homogeneous nucleation below the interfacial nanobubbles. The densities of ions and radicals are increasing over time, which causes homogeneous nucleation occurs even below the interfacial nanobubbles as well as near the three-phase contact line.

CONCLUSIONS In summary, we report for the first time the experimental observation of the long-term growth process of interfacial nanobubbles and the homogeneous nucleation in a water layer with a thickness of 1000 nm using TEM. Electron beam irradiation induced the gas generation from water and heterogeneous nucleation occurred on the surfaces of the top and bottom windows. Small interfacial bubbles grew by themselves and coalesced with the adjacent interfacial bubbles. Some small bubbles near grown-up larger bubbles disappeared owing to the Ostwald ripening effect. After that, homogeneous nucleation occurs though the solid surface is not fully occupied by the gas phase, probably because the ripening effect prevents further heterogeneous nucleation. The preferential region of homogeneous nucleation at their early stage is also identified. These results should advance our understanding of the dynamics of liquid-gas interfaces and nucleation near a solid-liquid boundary at the nanoscale and provide useful information to many applications, such as microfluidic devices, and chemical reactors with bubble generation.


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SUPPORTING INFORMATION The following files are available free of charge. Details of Video S1 and TEM observation under 200 kV (PDF) The original movie of TEM observation of bubble nucleation in the liquid cell (AVI)

ACKNOWLEDGMENT This work was partially supported by JSPS KAKENHI Grant Numbers JP16H04280, JP16H02315, JP17H03186, JP18K13704, and Grant-in-Aid for JSPS Research Fellow Number JP18J11761 TEM observations were performed at the Ultramicroscopy Research Center, Kyushu University.

AUTHOR INFORMATION *Corresponding Author Koji Takahashi E-mail: [email protected] TEL: +81-92-802-3015 Notes The authors declare no competing financial interest.

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TOC Graphic


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Figure 1　 Time evolution of bubble nucleation in the liquid cell. Blue, red, and white dashed lines indicate homogeneous bubbles, heterogeneous bubbles on the surface of the top window, and those on the bottom window, respectively. The white arrows indicate diminishing bubbles. The yellow square indicates a spacious area where homogeneous nucleation did not occur. The yellow arrows indicate the homogeneous bubbles generated beneath a couple of close three-phase contact lines of interfacial nanobubbles. Time refers to the electron beam irradiation time. All scale bars correspond to 20 nm. 99x66mm (600 x 600 DPI)



Figure 2　 Histogram of heterogeneous and homogeneous nucleation in the upstream side of the electron beam. Inserted illustrations are schematic side views of heterogeneous and homogeneous nucleation. 66x53mm (600 x 600 DPI)


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Figure 3　Schematic illustration of the sequential process from heterogeneous to homogeneous nucleation in the liquid cell. Blue dashed arrows indicate the transfer of gas molecules from a smaller bubble to a larger one owing to the Ostwald ripening effect. Black arrows indicate the absorption of supersaturated gas molecules into a grown-up bubble. Red arrows indicate the movement of the liquid-gas interfaces. The deeper pink region indicates higher densities of ions and radicals. 49x13mm (600 x 600 DPI)



Figure 4　Logarithmic relationship of the mean bubble radius (unit: nm) versus irradiation time (unit: s). The black lines are the piecewise linear fits of the logarithmic values, which yield piecewise power law relations between and t in the form of =αt^β, where β_1=0.511±0.068 between t = 91 s and 143 s, and β_2=1.665±0.166 between t = 153 s and 203 s. 81x69mm (600 x 600 DPI)


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60x43mm (300 x 300 DPI)


Unexpected Homogeneous Bubble Nucleation Near a Solid-Liquid

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