Ultrahigh density inside a nanobubble
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gust 2020 Vol. 63 No. 8: 287031 https://doi.org/10.1007/s11433-020-1577-2
Ultrahigh density inside a nanobubble HaiPing Fang
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Department of Physics, East China University of Science and Technology, Shanghai 200237, China; Zhangjiang Laboratory, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210, China Received April 10, 2020; accepted May 9, 2020; published online June 3, 2020
Citation:
H. P. Fang, Ultrahigh density inside a nanobubble, Sci. China-Phys. Mech. Astron. 63, 287031 (2020), https://doi.org/10.1007/s11433-020-1577-2
Gas bubbles are very common in our daily life and are often seen in water and beer. For a large gas bubble in a liquid, the pressure difference ΔP within and outside the bubble is governed by the Laplace equation ΔP = γ/R, where γ is the surface tension at the interface between the bubble and liquid, and R is the bubble radius. Clearly, for tiny bubbles, inner gas pressure will be very high; thus, the bubbles will be unstable and rapidly dissolve in water. For example, a bubble with a radius of 10 nm will have the inner pressure of 7 1.459×10 Pa [1]. Surprisingly, in 2000, Lou et al. [2] have reported their direct experimental observation of bubbles at the nanometer level (named as nanobubbles) on hydrophobic graphite or hydrophilic mica surfaces in water by atomic force microscopy. Those nanobubbles were stable overnight and for several days. Later, nanobubbles have been widely observed on various surfaces and even in bulk water and have attracted great attention [3]. Currently, a series of international conferences focused on nanobubbles and their applications have been founded. There are wide applications based on nanobubbles, such as wastewater recovering, aquaculture, agricultural planting, surface cleaning, mineral floating, and healthcare. Recently, nanobubbles have been used to explain a mystery puzzled us for over a century, i.e., anesthetics caused by inert gases [4]. However, the phenomenon of stability of nanobubbles is still puzzling although many efforts have been made to understand it. A key aspect for this problem is the structure of gas molecules inside nanobubbles. There is an empirical *Corresponding author (email: [email protected])
equation that shows the relationship between surface tension 4 and densities of a liquid and a vapor, γ = [A(ρl − ρg)/M] , where M, A, ρl, and ρg are the molecular weight (kg/mol), 3 parachor, density of liquid (kg/m ), and density of gas 3 (kg/m ), respectively. Based on this relationship, in 2008, Zhang et al. [5] have shown that if density of gas molecules inside nanobubbles is sufficiently large, the life of the nanobubbles in bulk water will considerably increase and even approach the timescale of the experimental observations. At the same time, Wang et al. [6], on the basis of a molecular dynamics simulation, showed that densities of N2 and H2 gas bubbles accumulated at water/graphite interfaces greatly increased and reached 41% of the density of liquid N2 and 52% of that of liquid H2,
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