Abstract
Water containing suspended nanobubbles is utilized in various applications. The observed lifetime of suspended nanobubbles is several weeks, whereas, according to the classical theory of bubble stability, a nanosized bubble should dissolve within microseconds. Explanations for the longevity of nanosized bubbles have been proposed but none of them has gained general acceptance. In this study, we derive an explanation for the existence of metastable nanobubbles solely from the thermodynamic principles. According to our analysis, the dissolution of nanosized aqueous bulk bubbles is nonspontaneous below 180 nm diameter due to the energy requirement of gas dissolution. Hydrophobic surfaces have a further stabilizing effect, and the dissolution becomes nonspontaneous in surface nanobubbles having a diameter below 600 nm.
Highlights
The lifetime of nanosized bubbles has been in focus of active debate since 1990s
The existence of nanobubbles is not restricted to hydrophobic surfaces, as several studies have reported nanosized bubbles in bulk solution
Gibbs energy is a useful tool to evaluate if a process happens spontaneously
Summary
The lifetime of nanosized bubbles has been in focus of active debate since 1990s. In 1994, Parker et al published observations of nanobubbles on hydrophobic surfaces.[1]. Matsumoto and Tanaka[20] studied stability conditions of bulk nanobubbles and concluded that, under vacuum or high tensile stress, bulk nanobubbles are stable.[20] Ohgaki explained nanobubble stability with hard hydrogen bonds in the bubble surface.[9] Zhang stated that nanobubble longevity originated for high internal density.[21] In 2020, Tan et al proposed ζpotential as an explanation of bulk nanobubble longevity.[22] Michailidi et al attributed nanobubble stability to hydrogenbonding interactions on the bubble surface.[23] None of these theories has gained general acceptance. The generally accepted view is that this high pressure causes a rapid dissolution of nanobubbles This theoretical paradigm is often stated as: “the air inside the nanobubble cannot be in equilibrium with the surroundings since larger pressure means larger chemical potential, and the nanobubble should dissolve”.27.
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