ABSTRACT
Nanobubbles have attracted significant attention due to their unexpectedly long lifetimes and stabilities in liquid solutions. However, explanations for the unique properties of nanobubbles at the molecular scale are somewhat controversial. Of special interest is the validity of the Young-Laplace equation in predicting the inner pressure of such bubbles. In this work, large-scale molecular dynamics simulations were performed to study the stability and diffusion of nanobubbles of methane in water. Two types of force field, atomistic and coarse-grained, were used to compare the calculated results. In accordance with predictions from the Young-Laplace equation, it was found that the inner pressure of the nanobubbles increased with decreasing nanobubble size. Consequently, a large pressure difference between the nanobubble and its surroundings resulted in the high solubility of methane molecules in water. The solubility was considered to enable nanobubble stability at exceptionally high pressures. Smaller bubbles were observed to be more mobile via Brownian motion. The calculated diffusion coefficient also showed a strong dependence on the nanobubble size. However, this active mobility of small nanobubbles also triggered a mutable nanobubble shape over time. Nanobubbles were also found to coalesce when they were sufficiently close. A critical distance between two nanobubbles was thus identified to avoid coalescence. These results provide insight into the behavior of nanobubbles in solution and the mechanism of their unique stability while withstanding high inner pressures.
ABSTRACT
Gas hydrate is usually regarded as a huge potential energy resource that has promising industrial applications in gas separation, storage, and transportation. Previous research studies have shown that a gas hydrate phase transition is mainly controlled by heat and mass transfer, while there are limited works on the mass transfer effects of gas micro-bubbles on hydrate crystallization. In this study, variations in the microscopic morphology of the hydrate crystal growth in a liquid-gas interface were observed using a microscope imaging system. The results indicated that the nucleation of the hydrate first tends to occur at the bubble surface. The cooling rates increased exponentially with the crystal growth rates and played an important role in the morphology of the hydrate crystal growth. In addition, the hydrate crystals tended to grow in the direction of the bubbles affected by the Ostwald ripening effects, which suggested that bubbling was an efficient measure to promote the application of hydrate-based technologies. In turn, reducing the concentration of the bubbles on the surface of the hydrate, inhibiting their generation, and enhancing the process of gas mass transfer in water around the hydrate surface were also conducive to further accelerate the decomposition of the hydrate, which may provide some guidance for the resource exploitation of gas hydrate.
