Water, which is most abundant in Earth's surface and is very closely related to all forms of living organisms, has a simple molecular structure but exhibits very unique physical and chemical properties. Even though tremendous effort has been paid to understand this core substance of nature, there amazingly still lefts much room for scientist to explore its novel behaviors. Especially, as the scale goes down to nano-regime, water shows extraordinary properties that are not observable in bulk state. One of such interesting features is the formation of nanoscale bubbles showing unusual long-term stability.
Nanobubbles can be spontaneously formed in water on hydrophobic surface or by decompression of gas-saturated liquid. In addition, the nanobubbles can be generated during electrochemical reaction at normal hydrogen electrode (NHE), which possibly distorts the standard reduction potential at NHE as the surface nanobubble screens the reaction with electrolyte solution. However, the real-time evolution of these nanobubbles has been hardly studied owing to the lack of proper imaging tools in liquid phase at nanoscale. Here we demonstrate, for the first time, that the behaviors of nanobubbles can be visualized by in-situ transmission electron microscope (TEM), utilizing graphene as liquid cell membrane.
The results indicate that there is a critical radius that determines the long-term stability of nanobubbles. In addition, we find two different pathways of nanobubble growth. The distinctively small and large nanobubbles show Ostwald ripening like process that a small bubble disappears near the surface of a growing larger bubble. In this case, its boundary is not destroyed until the merging is completed. It seems that the gas diffusion from one bubble to another occurs at invisible time scale.
On the other hand, two similar-sized nanobubbles show a coalescing process, followed by reshaping into dumbbell-like and spherical morphology. It is interesting that such Ostwald ripening-like behaviour can be observed in gaseous particles. We also observed that a small nanoparticle nucleates at the inter-bubble boundary area and rapidly grow into larger nanoparticles, which is possibly due to higher diffusion rate induced by dynamic equilibrium and high flux rate on nanobubble interface.
Furthermore, we observed that the nanoparticles can be included inside the nanobubble cavity to minimize hydration energy and grow into larger nanoparticles. The nanoparticles are often included inside drifting nanobubbles. We suppose that it is one of the catalysing or cleaning pathways common in nature. Our finding is expected to provide a deeper insight to understand unusual chemical, biological and environmental phenomena where nanoscale gas-state is involved.