Abstract
The existence of surface nanobubbles has already been confirmed by variable detection methods, but the mechanism of their extraordinary stability remains unclear and has aroused widespread research interest in the past 2 decades. Experiments and theoretical analyses have tried to account for these stabilities such as the very long lifetime, very high pressure and very small contact angle. Attractive hydrophobic potential was applied to complement the pinning-oversaturation theory and successfully explain the survival of surface nanobubbles in undersaturation environment by some researchers. However, the survival of nanobubbles on hydrophilic surface still requires sizeable oversaturation. In this paper, we introduce the variable surface tensions, namely Tolman-dependence and state-dependence, and show that they effectively promote the stability of nanobubbles. The decrease in surface tension can lead to larger contact angle and even make the nanobubbles survivable on the highly hydrophilic surface. In Tolman-dependence, the changing rate in the contact angle evolution slows down, which is more obvious when the bubble size is close to the Tolman length. The contact angle is also getting larger in the state-dependence, and the increase of the gas saturation degree is beneficial to the stability of surface nanobubbles. With the gas saturation ratio of 3, the bubbles on the quite hydrophilic surface can also be stable, while grow up on the hydrophobic surface. The variable surface tensions weaken the need of saturation degree for the surface nanobubbles’ stability.
Highlights
Small bubble can exist everywhere in the nature and industrial liquid systems, which is a spherical void under or in the liquid
With the smaller surface tension in the gas-liquid interface, the results show that the surface nanobubbles (SNBs) can exist with larger contact angle in the same environment and even survive on a highly hydrophilic surface
Many researches about SNBs focus on their long life and small contact angles
Summary
Small bubble can exist everywhere in the nature and industrial liquid systems, which is a spherical void under or in the liquid. In 2000, Lou [12] and Ishida [13] got the first images of SNBs on different substrates using the atomic force microscope (AFM) These earliest experimental results verified the assumption of SNBs proposed by Parker [14] and laid a solid foundation for the later study of SNBs. Researchers are interested in the extraordinary properties especially for the small gas-side contact angle θ [15,16,17] and the long-term stability of SNB [18,19,20]. Petsev argued that the adsorption of gas molecules at the substrate modifies the energy of solid-gas interface, and reduces the gas-side contact angles and the pressure inside [38] None of those models can account for all the outstanding properties of SNBs, especially their stability on the hydrophilic surface, such as mica or glass. With the smaller surface tension in the gas-liquid interface, the results show that the SNBs can exist with larger contact angle in the same environment and even survive on a highly hydrophilic surface
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