Kyushu University Academic Staff Educational and Research Activities Database
List of Papers
Yoko Tomo Last modified date:2022.03.16

Assistant Professor / Department of Mechanical Engineering / Faculty of Engineering

1. Sarthak Nag, Yoko Tomo, Hideaki Teshima, Koji Takahashi and Masamichi Kohno , Dynamic interplay between interfacial nanobubbles: Oversaturation promotes anisotropic depinning and bubble coalescence, Physical Chemistry Chemical Physics, 10.1039/D1CP03451K, 2021.10.
2. Yoko Tomo, Hibiki Koga, Takanobu Fukunaga, Kosaku Kurata, Hisao Matsuno, Keiji Tanaka, Hiroshi Takamatsu, Thermal conductivity measurement of solid materials using an “ITX” method–A pilot study using DNA solid films, International Journal of Heat and Mass Transfer, 10.1016/j.ijheatmasstransfer.2021.121501, 176, 121501, 2021.09, [URL], A new method is proposed to measure the longitudinal thermal conductivity of fibers and films. The thermal conductivity of novel one- and two-dimensional materials has previously been measured using a T-type probe. Although this method is applicable to polymer fibers and films, the measurements are influenced by the thermal contact resistance between the probe and the sample. We therefore propose a so-called ITX method, which determines the inherent thermal conductivity of a sample from measurements made in three geometric configurations (“I”, “T”, and “X” shape) of the sample, heat sinks, and measurement probe. Application of the method to DNA solid films demonstrated that the thermal conductivity can be accurately determined irrespective of the relative contribution of the thermal contact resistance to the overall thermal resistance..
3. Sarthak Nag, Yoko Tomo, Koji Takahashi, Masamichi Kohno, Mechanistic Insights into Nanobubble Merging Studied Using In Situ Liquid-Phase Electron Microscopy, Langmuir, 10.1021/acs.langmuir.0c03208, 37, 2, 874-881, 2021.01, Nanobubbles have attracted great interest in recent times because of their application in water treatment, surface cleaning, and targeted drug delivery, yet the challenge remains to gain thorough understanding of their unique behavior and dynamics for their utilization in numerous potential applications. In this work, we have used a liquid-phase electron microscopy technique to gain insights into the quasistatic merging of surface nanobubbles. The electron beam environment was controlled in order to suppress any new nucleation and slow down the merging process. The transmission electron microscopy study reveals that merging of closely positioned surface nanobubbles is initiated by gradual localized changes in the physical properties of the region between the adjoining nanobubble boundary. The observed phenomenon is then analyzed and discussed based on the different perceptions: localized liquid density gradient and bridge formation for gas exchange. In this study, it is estimated that the merging of the stable nanobubbles is initiated by the formation of a thin gas layer. This work not only enhances our understanding of the merging process of stable surface nanobubbles but will also lead to exploration of new domains for nanobubble applications..
4. Sota Hirokawa, Hideaki Teshima, Pablo Solís-Fernández, Hiroki Ago, Yoko Tomo, Qin-Yi Li, Koji Takahashi, Nanoscale Bubble Dynamics Induced by Damage of Graphene Liquid Cells, ACS Omega, 10.1021/acsomega.0c01207, 5, 19, 11180-11185, 2020.05, Graphene liquid cells provide the highest possible spatial resolution for liquid-phase transmission electron microscopy. Here, in graphene liquid cells (GLCs), we studied the nanoscale dynamics of bubbles induced by controllable damage in graphene. The extent of damage depended on the electron dose rate and the presence of bubbles in the cell. After graphene was damaged, air leaked from the bubbles into the water. We also observed the unexpected directional nucleation of new bubbles, which is beyond the explanation of conventional diffusion theory. We attributed this to the effect of nanoscale confinement. These findings provide new insights into complex fluid phenomena under nanoscale confinement..
5. Yoko Tomo, Yosuke Okuno, Takanobu Fukunaga, Kosaku Kurata, Hiroshi Takamatsu, Examination of the rarefied gas effect on detecting thermal conductivity using a micro-beam MEMS sensor, Proceedings of the 30th international symposium on transport phenomena (ISTP30), 2019.11.
6. Yoko Tomo, Koji Takahashi, “In Situ Nanoscale Observation of Water Using TEM”, Journal of the Heat Transfer Society of Japan, vol. 58, No. 245, pp. 2-7, 2019.10.
7. Qin Yi Li, Ryo Matsushita, Yoko Tomo, Tatsuya Ikuta, Koji Takahashi, Water Confined in Hydrophobic Cup-Stacked Carbon Nanotubes beyond Surface-Tension Dominance, Journal of Physical Chemistry Letters, 10.1021/acs.jpclett.9b00718, 10, 13, 3744-3749, 2019.01, Water confined in carbon nanotubes (CNTs) can exhibit distinctly different behaviors from the bulk. We report transmission electron microscopy (TEM) observation of water phases inside hydrophobic cup-stacked CNTs exposed to high vacuum. Unexpectedly, we observed stable water morphologies beyond surface-tension dominance, including nanometer thin free water films, complex water-bubble structures, and zigzag-shaped liquid-gas interface. The menisci of the water phases are complex and inflected, where we measured the contact angles on the CNT inner wall to be 68-104°. The superstability of the suspended ultrathin water films is attributed to the strong hydrogen-bonded network among water molecules and adsorption of water molecules on the cup-structured inner wall. The complex water-bubble structure is a result of the stability of free water films and interfacial nanobubbles, and the zigzag edge of the liquid-gas interface is explained by the pinning effect. These experimental findings provide valuable knowledge for the research on fluids under nanoscale confinement..
8. Yoko Tomo, Qin-Yi Li, Tatsuya Ikuta, Yasuyuki Takata, Koji Takahashi, Unexpected homogeneous bubble nucleation near a solid-liquid interface, The Journal of Physical Chemistry C, 10.1021/acs.jpcc.8b09200, 122, 50, 28712-28716, 2018.12, 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 the Ostwald ripening effect prevents heterogeneous nucleation from occurring 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..
9. Yoko Tomo, Alexandros Askounis, Khellil Sefiane, Yasuyuki Takata, Koji Takahashi, Study on liquid-gas interface at nanoscale using transmission electron microscopy, 16th International Heat Transfer Conference, IHTC 2018 International Heat Transfer Conference, 10.1615/IHTC16.bae.023136, 1169-1174, 2018.08, Control for the bubble nucleation at the onset of nucleate boiling (ONB) ensures the stable start of boiling heat transfer. However, the bubble nucleation mechanism at the ONB remains unclear, because of the difficulty of in-situ observation, which is due to the small size of nucleation. Thus, in order to break through the current technological barrier of boiling heat transfer, a new experimental technique enabling the investigation of the dynamics of bubbles near the solid-liquid interface is highly desirable. Liquid cell electron microscopy is the most useful method for the in-situ observation of liquid samples at the nanoscale. We prepared a closed liquid cell fabricated using MEMS technology and observed the generation and growth of bubbles at the nanoscale and in real time using transmission electron microscopy (TEM). In the growing process, the water meniscus between smaller bubbles becomes thinner and thinner and eventually ruptures. However, when the bubbles grow, the meniscus between larger bubbles do not rupture and the bubble overlaps with others, suggesting that thin meniscus can be stable only in the case of larger bubbles because of the difference of the curvature of their liquid-gas interfaces between smaller bubbles and larger bubbles. Our experimental results lead to the insight of the mechanism of the stability and the phase change phenomena at the liquid-gas interface at the nanoscale..
10. Yoko Tomo, Alexandros Askounis, Tatsuya Ikuta, Yasuyuki Takata, Khellil Sefiane, Koji Takahashi, Superstable ultrathin water film confined in a hydrophilized carbon nanotube, Nano Letters, 10.1021/acs.nanolett.7b05169, 18, 3, 1869-1874, 2018.02, Fluids confined in a nanoscale space behave differently than in the bulk due to strong interactions between fluid molecules and solid atoms. Here, we observed water confined inside “open” hydrophilized carbon nanotubes (CNT), with diameter of tens of nanometers, using transmission electron microscopy (TEM). A 1–7 nm water film adhering to most of the inner wall surface was observed and remained stable in the high vacuum (order of 10–5 Pa) of the TEM. The superstability of this film was attributed to a combination of curvature, nanoroughness, and confinement resulting in a lower vapor pressure for water and hence inhibiting its vaporization. Occasional, suspended ultrathin water film with thickness of 3–20 nm were found and remained stable inside the CNT. This film thickness is 1 order of magnitude smaller than the critical film thickness (about 40 nm) reported by the Derjaguin–Landau–Verwey–Overbeek theory and previous experimental investigations. The stability of the suspended ultrathin water film is attributed to the additional molecular interactions due to the extended water meniscus, which balances the rest of the disjoining pressures..
11. Yoko Tomo, Koji Takahashi, Takashi Nishiyama, Tatsuya Ikuta, Yasuyuki Takata, Three dimensional investigation of bubble formation in liquid cell electron microscopy, Proceedings of the 6th International Symposium on Micro and Nano Technology, 2017.03.
12. Yoko Tomo, Koji Takahashi, Takashi Nishiyama, Tatsuya Ikuta, Yasuyuki Takata, Nanobubble nucleation studied using Fresnel fringes in liquid cell electron microscopy, International Journal of Heat and Mass Transfer, 10.1016/j.ijheatmasstransfer.2017.01.013, 108, 1460-1465, 2017.01.