Ultrahigh Density of Gas Molecules Confined in Surface Nanobubbles in Ambient Water

Abstract

To understand the unexpected and puzzling long-term stability of nanoscale gas bubbles, it is crucial to probe their nature and intrinsic properties. We report herein synchrotron-based scanning transmission X-ray microscopy (STXM) evidence of highly condensed oxygen gas molecules trapped as surface nanobubbles. Remarkably, the analysis of absorption spectra of a single nanobubble revealed that the oxygen density inside was 1–2 orders of magnitude higher than that in atmospheric pressure, and these bubbles were found in a highly saturated liquid environment with the estimated oxygen concentration to be hundreds of times higher than the known oxygen solubility in equilibrium. Molecular dynamics simulations were performed to investigate the stability of surface nanobubbles on a heterogeneous substrate in gas-oversaturated water. These results indicated that gas molecules within confinement such as the nanobubbles could maintain a dense state instead of the ideal gas state, as long as their surrounding liquid is supersaturated. Our findings should help explain the surprisingly long lifetime of the nanobubbles and shed light on nanoscale gas aggregation behaviors.

The interplay among gas, liquid and solid interactions determines the stability of surface nanobubbles

“…This large localized oversaturation is consistent with recent experimental results. 18 More difficult is the comparison with computational results available in the literature, which also predicts a large oversaturation at the solidliquid interfaces. 17,41 In fact, in these works, the authors use a different computational approach consisting of preparing a sample with a very high gas oversaturation and determining the gas local oversaturation profile as a function of the distance from the surface.…”

“…[19][20][21] Among the others, Zhou et al have performed near-edge X-ray absorption fine structure experiments showing a large gas oversaturation in the liquid surrounding nanobubbles. 18 Overall, these results call for a new comprehensive microscopic theory addressing the open questions concerning the stability of nanobubbles in a broader range of experimental conditions and their relationship with the possible existence of a permanent local oversaturation.…”

“…Thus, a local oversaturation in this region can effectively counteract the gas outflux. A number of recent experiments have shown that, indeed, one can produce a large local gas oversaturation at the solid/water interface 17,18 and when the oversaturation reaches a critical value, of the order of ζ ∼150-250, nanobubbles are formed. [19][20][21] Among the others, Zhou et al have performed near-edge X-ray absorption fine structure experiments showing a large gas oversaturation in the liquid surrounding nanobubbles.…”

The effects of nanoscale nuclei on cavitation

“…Mørch (2018) discussed the possibility of nanoscale gas bubbles and/or droplets serving as cavitation nuclei, which can be stabilized by the surface tension forces of the bounding water molecules, and can be in gas diffusion balance with the surrounding liquid. The stabilization of a nanobubble on a heterogeneous substrate is studied by molecular dynamics simulations (Zhou 2020). Li, Gu & Chen (2018) simulated cavitation from particles of 0.5 to 2 nm, indicating that both hydrophobic and hydrophilic nanoparticles can promote cavitation, and cavitation with hydrophobic nanoparticles is promoted to a greater extent than that with hydrophilic nanoparticles.…”

Nanobubble-controlled nanofluidic transport

“…Surface-pinned nanobubbles (1), nanoscale air pockets residing at liquid-solid interfaces, defy physical (2,3) and thermodynamic (4) predictions of instantaneous dissolution. Experimental recordings attribute long nanobubble lifetimes to liquid oversaturation with gas (5,6), small three-phase contact angles (1), contact line pinning (7), and gas accumulation at three-phase interfaces (8,9). Across these mechanisms, a common feature is the reduction of the gas-phase concentration gradient between the nanobubble surface and the bulk gas-saturated solution.…”