Successful Visualization of “Nano-Water Droplet”
Behavior on Glass Surface
Dr. Yuki Araki, Prof. Toyoko Arai (Graduate School of Natural Science and Technology,Kanazawa University) and Dr. Taketoshi Minato (Institute for Molecular Science, National
Institutes of Natural Sciences) have successfully visualized the behavior of nanometer-sizedwater droplets on glass surfaces. By observing the wetting process of silica glass in a humid air
using frequency modulation atomic force microscopy (FM-AFM) (※1), they discovered thatthese nanoscale droplets, or “nano-water droplets”, form spontaneously with increasing humidity.
Contrary to previous wetting models, these droplets do not grow larger but maintain their droplet
state even in high humidity environment. Further measurements using peak force tapping atomicforce microscopy (PFT-AFM) (※2) revealed that the nano-water droplets form not directly on
the glass surface, but atop a gel-like layer on the glass. The visualization of these nano-water
droplets provided insight into their unique movements across the glass surface, different fromthose of larger droplets, and suggested their potential role in mass transport on solid surfaces andinvolvement in various phenomena such as catalysis. The team plans to further explore theformation mechanisms of these droplets to determine if this phenomenon is universal acrossother materials as well. The findings were published in the online edition of “Scientific Reports”
on May 10, 2024.
Background
The wetting of solid surfaces by water is a common phenomenon in everyday life, yet theintricate mechanisms behind it are not fully understood. Recent advancements in computational
science and spectroscopy have facilitated detailed investigations into the adsorption of watermolecules on glass surfaces in relation to humidity. Previous research suggested that watermolecules uniformly adsorb and form a continuous film as humidity increases. However, more
recent findings from nonlinear spectroscopy have indicated that water adsorption is
non-uniform at around 50% humidity. We have employed Frequency Modulation Atomic ForceMicroscopy (FM-AFM), which enables the nanoscale observation of surface structures, todirectly examine the microscopic dynamics of wetting on glass surfaces as they vary withhumidity.
Summary of Research Findings
We began by increasing the relative humidity from 30% around the sample and observing theglass surface. At around 50% humidity, we detected water droplets approximately several
hundred nanometers in size. Remarkably, these droplets maintained their size even as humidity
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exceeded 80%, remaining distinct in a high-humidity environment (nano-water droplets: Figure1). Contrary to previous studies, which anticipated that these nano-water droplets would growand eventually form a film with rising humidity, we found that it was actually the height of thedroplets that increased, not their lateral size. Further, we measured the adhesive forces on theglass surface utilizing the peak force tapping atomic force microscopy (PFT-AFM). Ourfindings showed an increase in adhesion in areas without nano-water droplets as humidity rose,suggesting the formation of some type of liquid film. Upon comparing adhesion forces, it
became apparent that the liquid film was more viscous than the droplets, hinting that the film
might consist of a substance different from water. Currently, we believed that a silica gel-likelayer, resulting from the reaction between the silica glass surface and the adsorbed water, hasformed. Moving forward, we plan to further verify the presence of this silica gel-like layer. Thevisualization of nano-water droplets also clarified their diffusion across the glass surface,suggesting that these tiny droplets might play a role in the transport of substances on solidsurfaces.
Future Directions
Understanding the behavior and properties of nano-water droplets can lead to more effective
control of wetting, which is crucial for preventing material degradation. Moreover, the observedphenomenon of moving nano droplets in our study raises the possibility that they could also
influence other processes, such as catalytic effects. Moving forward, by examining the behaviorof nano-water droplets in greater detail and elucidating their mechanisms, we aim to gain adeeper understanding of the role of water at the microscopic level on solid material surfaces.
Acknowledgements
This work was supported by the Japan Society for the Promotion of Science (JSPS)
Grants-in-Aid for Scientific Research (Basic Research (C), 23K04581) and the “Initiative forRealizing Diversity in the Research Environment”, MEXT. Additionally, part of this work wascarried out at the Institute for Molecular Science, National Institutes of Natural Sciences, withsupport from the MEXT under the “Nanotechnology Platform” project (S-21-MS-0007) and the“Advanced Research Infrastructure for Materials and Nanotechnology” project(JPMXP1222MS0005).

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Figure 1 Nano-water droplets formed on glass. They were observed above 50% RH, anddisappeared when the humidity was reduced to around 30%. All scale bars represent 500 nm.
🄫🄫 2024 Araki, et al., Scientific Reports, CC BY 4.0(http://creativecommons.org/licenses/by/4.0/)

Journal Name：Scientific Reports
Title：Microscopic behavior of nano-water droplets on a silica glass surface
Authors：Yuki Araki*, Taketoshi Minato, Toyoko Arai
Publication Date：2024. 5. 10
DOI：10.1038/s41598-024-61212-1
URL：https://www.nature.com/articles/s41598-024-61212-1

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Glossary※1：Frequency modulation atomic force microscopy (FM-AFM)
Atomic force microscopy (AFM) is a type of microscope that enables the observation ofsurface structures at atomic resolution. This method involves scanning the sample surface with asharp probe and detecting variations in the cantilever’s vibrations caused by the forces between
the probe and the sample using frequency modulation (FM) techniques. This approach not only
allows for high-resolution observation of solid surfaces but also facilitates the study of thestructure of water adsorbed on these surfaces, known as hydration structure.
※2：Peak force tapping atomic force microscopy (PFT-AFM)
PFT is a type of atomic force microscopy where the probe approaches and retracts from thesample surface while measuring the force exerted on the probe during approach and retraction.
By detecting extremely small forces in the piconewton (pN) range, this technique allows for thecreation of the maps of adhesion force and elastic modulus of the sample surface.

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