Language：Others   Publishing type：Research paper (scientific journal)  

Classical thermodynamics theory predicts that nanosized bubbles should disappear in a few hundred microseconds. The surprisingly long lifetime and stability of nanobubbles are therefore interesting research subjects. It has been proposed that the stability of nanobubbles arises through pinning of the three-phase contact line, which results from intrinsic nanoscale geometrical and chemical heterogeneities of the substrate. However, a definitive explanation of nanobubble stability is still lacking. In this work, we examined the stability mechanism by introducing a "pinning force." We investigated nanobubbles at a highly ordered pyrolytic graphite/pure water interface by peak force quantitative nano-mechanical mapping and estimated the pinning force and determined its maximum value. We then observed the shape of shrinking nanobubbles. Because the diameter of the shrinking nanobubbles was pinned, the height decreased and the contact angle increased. This phenomenon implies that the stability results from the pinning force, which flattens the bubble through the pinned three-phase contact line and prevents the Laplace pressure from increasing. The pinning force can also explain the metastability of coalesced nanobubbles, which have two semispherical parts that are joined to form a dumbbell-like shape. The pinning force of the semispherical parts was stronger than that of the joint region. This result demonstrates that the contact line of the semispherical parts is pinned strongly to keep the dumbbell-like shape. Furthermore, we proposed a nanobubble generation mechanism for the solvent-exchange method and explained why the pinning force of large nanobubbles was not initially at its maximum value, as it was for small nanobubbles.

DOI： 10.1063/1.4973385

Repository Public URL： http://hdl.handle.net/2324/4793619

Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Langmuir  

Despite the importance of the effect of subnanoscale roughness on contact line behavior, it is difficult to directly observe the local behavior of contact lines at the micro- and nanoscale, leaving significant gaps in our current understanding. In this research, we investigate contact line motions and their relationship with nanoscale surface topography using coherence scanning interferometry. Our experiments were conducted on the substrates with different wettability without changing nanoscale surface topography. Titanium dioxide was used as a substrate, the wettability of which was varied under UV-light irradiation. A ridge-like structure with a height of approximately 1 nm was observed to cause contact line deformation toward the droplet side, regardless of the direction of the contact line motion. This was explained in terms of an imbalance in the local capillary pressure at the nanoscale contact line. We also found that the deformation becomes larger on the more hydrophilic surface, which was rationalized by theoretical prediction based on analysis of the work done by the force acting on the contact line and the change in surface free energy associated with the deformation of the liquid/gas interface. Furthermore, it was revealed by contact angle measurements that the maximum pinning forces on a hydrophilic surface were less than half of those on a hydrophobic surface. We attributed the weak pinning force on the hydrophilic surface to cascading depinning, where the initial depinning event triggers a chain reaction of subsequent depinning events, driven by the conversion of excess surface energy to kinetic energy. Our experimental works provide new insights of the relationship between the subnanoscale surface roughness and macroscopic contact line motion.

DOI： 10.1021/acs.langmuir.4c04227

Scopus

PubMed

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.jcis.2024.09.170

PubMed

Language：English   Publisher：International Journal of Heat and Mass Transfer  

The discovery of ultrafast water flow in graphene channels holds significant implications for various applications like electronics cooling, thermally driven energy harvesting, and solar-driven desalination. However, existing slip flow measurements on graphene are confined to room temperature, limiting our understanding of temperature-dependent behaviors and constraining potential thermal applications. Here, we simultaneously measured water slip lengths and evaporation fluxes in graphene nanochannels with depths ranging from 78 to 290 nm, spanning a temperature range from room temperature up to 85 °C. This was achieved by employing a capillary flow model combined with evaporation, a factor that cannot be disregarded at elevated temperatures. The measured water slip lengths ranged from 20 nm to 80 nm, exhibiting a decrease with increasing temperature and an increase with greater channel depths. The temperature-dependent slip length can be attributed to enhanced momentum transfer at the solid-liquid interface at elevated temperatures, while the observed channel depth dependence may result from stronger electrical-double-layer interactions with decreased channel depths. Additionally, the capillary evaporation fluxes increased with rising temperatures and greater channel depths, effectively explained by the capillary evaporation theory.

DOI： 10.1016/j.ijheatmasstransfer.2024.125451

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)  

Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： doi.org/10.1016/j.surfin.2024.103923

Repository Public URL： https://hdl.handle.net/2324/7168680

Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： https://doi.org/10.1016/j.ultramic.2023.113747

Repository Public URL： https://hdl.handle.net/2324/6786359

Language：Others   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.ijheatmasstransfer.2021.122001

Repository Public URL： http://hdl.handle.net/2324/4793217

Language：Others   Publishing type：Research paper (scientific journal)  

A dynamics study of surface nanobubbles using liquid phase electron microscopy showcases their unique push–push behavior.

DOI： 10.1039/d1cp03451k

Language：Others   Publishing type：Research paper (scientific journal)  

DOI： 10.1021/acs.langmuir.1c01589

Repository Public URL： http://hdl.handle.net/2324/4793219

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1021/acs.langmuir.1c00145

Language：Others   Publishing type：Research paper (scientific journal)  

We report for the first time a zigzag-shaped gas phase at a highly-ordered pyrolytic graphite/water interface. The novel shape of the gaseous domain is triggered by the holes of the underlying solid-like layers, which are composed of air molecules. Specifically, many holes were created by heating in the thin solid-like layers, which roughened them. The gas domains that formed on these layers deformed from circular to zigzag-shaped as the contact lines expanded while avoiding the holes of the underlying layers. We explained the formation and growth processes of these gas structures in terms of thin film growth, which varies with the mobility of the constituent molecules. This journal is

DOI： 10.1039/d0ra08861g

Repository Public URL： http://hdl.handle.net/2324/4793638

Language：Others   Publishing type：Research paper (scientific journal)  

Hydration structures are ubiquitous at solid/liquid interfaces and play a key role in various physicochemical and biological phenomena. Recently, it has been reported that dissolved gas molecules attracted to hydrophobic surfaces form adsorbed gas layers. Although a hydration structure and adsorbed gas layers coexist on the surface, the relationships between them remain unknown. In this study, we investigated a highly ordered pyrolytic graphite/pure water interface with and without adsorbed gas layers using frequency-modulation atomic force microscopy. We penetrated the adsorbed gas layers with the strong load force of the AFM tip and thereby obtained the frequency shift curves inside them. By comparing the curves with those measured on a bare HOPG surface, we found that the adsorbed gas layers were located at regions where the molecular density of water was low and were sandwiched between hydration layers with high water density. Moreover, the distance between adjacent hydration layers was larger than that predicted by simulations and was the same with and without the adsorbed gas layers. We propose that gas molecules on the hydrophobic surface interact with the hydration structure before forming the adsorbed gas layers, and extend the distance between hydration layers. This journal is

DOI： 10.1039/d0cp01719a

Repository Public URL： http://hdl.handle.net/2324/4793637

Language：Others   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1021/acsomega.0c01207

Repository Public URL： http://hdl.handle.net/2324/4793618

Language：Others   Publishing type：Research paper (scientific journal)  

To accurately characterize the shape of interfacial nanobubbles using atomic force microscopy (AFM), we investigated the effect of wettability of the AFM tip while operating in the peak force tapping (PFT) mode. The AFM tips were made hydrophobic and hydrophilic by Teflon AF coating and oxygen plasma treatment, respectively. It was found that the measured base radius of nanobubbles differed between AFM height images and adhesion images, and that this difference depended on the tip wettability. The force curves obtained during the measurements were also different depending on the wettability, especially in the range of the tip/nanobubble interaction and in the magnitude of the maximum attractive force in the retraction period. The difference suggests that hydrophobic tips penetrate the gas/liquid interface of the nanobubbles, with the three phase contact line being pinned on the tip surface; hydrophilic tips on the other hand do not penetrate the interface. We then quantitatively estimated the pinning position and recalculated the true profiles of the nanobubbles by comparing the height images and adhesion images. As the AFM tip was made more hydrophilic, the penetration depth decreased and eventually approached zero. This result suggests that the PFT measurement using a hydrophilic tip is vital for the acquisition of reliable nanobubble profiles.

DOI： 10.1063/1.5010131

Repository Public URL： http://hdl.handle.net/2324/4793620

Event date： 2025.1

Language：English   Presentation type：Poster presentation  

Event date： 2024.12

Language：English   Presentation type：Oral presentation (general)  

Event date： 2024.12

Language：English   Presentation type：Oral presentation (general)  

Event date： 2024.11

Language：English   Presentation type：Oral presentation (general)  

Event date： 2024.10

Language：English   Presentation type：Oral presentation (invited, special)  

Event date： 2024.10

Language：English   Presentation type：Oral presentation (general)  

Event date： 2024.10

Language：English   Presentation type：Poster presentation  

Event date： 2024.6

Language：English   Presentation type：Oral presentation (general)  

Event date： 2024.6

Language：English   Presentation type：Oral presentation (general)  

Event date： 2024.2

Language：English  

Venue：Fukuoka   Country：Japan  

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Event date： 2024.1

Language：Japanese  

Venue：東京   Country：Japan  

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Event date： 2023.11

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：アスティとくしま（徳島県徳島市）   Country：Japan  

Event date： 2023.11

Language：Japanese  

Venue：Washington, DC   Country：United States  

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Event date： 2023.11

Language：Japanese  

Venue：熊本   Country：Japan  

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Event date： 2023.11

Language：English  

Venue：Washington, DC   Country：United States  

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Event date： 2023.10

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：神戸   Country：Japan  

Event date： 2023.10

Language：Japanese   Presentation type：Public lecture, seminar, tutorial, course, or other speech  

Venue：オンライン   Country：Japan  

Event date： 2023.10

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東京大学 本郷キャンパス   Country：Japan  

Event date： 2023.10

Language：Japanese  

Venue：神戸   Country：Japan  

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Event date： 2023.10

Language：Japanese  

Venue：神戸   Country：Japan  

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Event date： 2023.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：Magdeburg, Germany   Country：Japan  

Event date： 2023.9

Language：English   Presentation type：Oral presentation (general)  

Venue：Padova   Country：Italy  

Event date： 2023.9

Language：English   Presentation type：Oral presentation (general)  

Venue：Padova   Country：Italy  

Event date： 2023.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：Busan   Country：Korea, Republic of  

Event date： 2023.8

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：北海道大学   Country：Japan  

Event date： 2023.7

Language：English   Presentation type：Oral presentation (general)  

Venue：San Diego   Country：United States  

As the system size decreases to the nanoscale, fluids show unique behavior because the forces derived from interfaces and three-phase contact line become dominant. Fortunately, recent advancements in measurement techniques have allowed for direct observation of fluids at the nanoscale. In this talk, we report our recent progress on experimental investigation of solid-liquid interfacial phenomena by using optical microscopy, atomic force microscopy (AFM) and transmission electron microscopy (TEM). Condensation of ethanol microdroplets was observed using reflection interference contrast microscopy. Our results showed that only when using the hydrophilic substrate, the microdroplets spontaneously moved towards the relatively larger droplet with a contact angle hysteresis as low as ~0.5°. This is due to the existence of precursor film less than 20 nm thickness near the three-phase contact lines, which reduces the resistive force against the thermocapillary motion [1]. TEM is a promising technique for observing the dynamics of interfacial phenomena with sub-nanometer spatial resolution, and we utilized liquid cell electron microscopy. For example, we found that the contact lines of nanobubbles trapped in the two graphene layers are pinned by contamination in two different ways: direct contact and non-direct contact. This mechanism was explained by the Derjaguin, Landau, Verwey, and Overbeek theory and nanoconfinement effect [2]. On the other hand, AFM is the sole technique that enables simultaneous three-dimensional observation of the liquid-gas interface and solid surface topography at the nanoscale. Glycerol nanodroplets on an ideally homogeneous graphene surface exhibit the same contact angles as those in macroscale, whereas size-dependent contact angles were observed on a SiO2 surface with angstrom-scale heterogeneity, suggesting a breakage of Young’s equation [3]. In water, high-sensitivity frequency modulation AFM revealed that dissolved gases strongly adsorb onto graphene-water interfaces, forming molecular-thick layers that act as pinning sites [4].

Event date： 2023.7

Language：Japanese  

Venue：San Diego   Country：United States  

Event date： 2023.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：島根県松江市   Country：Japan  

Event date： 2023.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：島根県松江市   Country：Japan  

Event date： 2023.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：島根県松江市   Country：Japan  

Event date： 2023.5

Language：Japanese  

Country：Japan  

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Event date： 2023.5

Language：Japanese  

Country：Japan  

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Event date： 2023.5

Language：Japanese  

Venue：福岡   Country：Japan  

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Event date： 2023.3

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：オンライン   Country：Japan  

Event date： 2023.3

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：鹿児島大学工学部   Country：Japan  

Event date： 2023.2

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：伊都キャンパス・I²CNER１棟・I²CNERホール   Country：Japan  

Event date： 2022.11

Language：Japanese  

Venue：アスティとくしま（徳島県徳島市）   Country：Japan  

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Event date： 2022.10

Language：Japanese  

Venue：東京大学 本郷キャンパス   Country：Japan  

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Event date： 2022.10

Language：Japanese  

Country：Japan  

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Event date： 2022.9

Language：English  

Country：Japan  

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Event date： 2022.8

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：オンライン   Country：Japan  

Event date： 2022.6

Language：English   Presentation type：Symposium, workshop panel (public)  

Venue：大阪大学   Country：Japan  

Event date： 2022.5

Language：Japanese   Presentation type：Oral presentation (general)  

Country：Japan  

Event date： 2022.5

Language：Japanese  

Country：Other  

Event date： 2022.5

Language：Japanese   Presentation type：Oral presentation (general)  

Country：Japan  

Event date： 2021.12

Language：Japanese  

Country：Other  

Event date： 2021.10

Language：Japanese  

Country：Other  

Event date： 2021.10

Language：English  

Country：Other  

Event date： 2021.8

Language：Japanese  

Country：Other  

Event date： 2021.8

Language：English   Presentation type：Public lecture, seminar, tutorial, course, or other speech  

Venue：I2CNER(オンライン)   Country：Japan  

Event date： 2021.5

Language：Japanese  

Country：Other  

Event date： 2021.5

Language：Japanese  

Country：Other  

Event date： 2020.12

Language：Others  

Country：Other  

Event date： 2020.10

Language：Others  

Country：Other  

Event date： 2020.8

Language：Others  

Country：Other  

Event date： 2020.1

Language：Others  

Country：Other  

Event date： 2019.12

Language：Others  

Country：Other  

Event date： 2019.11

Language：Others  

Country：Other  

Event date： 2019.6

Language：Others  

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Event date： 2018.10

Language：Others  

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Event date： 2018.10

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Event date： 2018.8

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Event date： 2018.8

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Event date： 2018.5

Language：Others  

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Event date： 2018.5

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Country：Other  

Event date： 2017.11

Language：Others  

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Event date： 2017.5

Language：Others  

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Event date： 2017.3

Language：Others  

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Event date： 2016.10

Language：Others  

Country：Other  

Event date： 2016.9

Language：Others  

Country：Other  

Type：Competition, symposium, etc. 

Type：Competition, symposium, etc. 

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Type：Peer review 

Number of peer-reviewed articles in foreign language journals：5

Type：Peer review 

Number of peer-reviewed articles in foreign language journals：1

Number of peer-reviewed articles in Japanese journals：1

Type：Competition, symposium, etc.