Nanobubbles (NBs) in liquid exhibit many intriguing properties such as low buoyance, high mass transfer efficiency and reactivity as compared to large bulk bubbles. However, it remains elusive why or how bulk NBs are stabilized in water and particularly, the states of internal pressures of NBs are difficult to measure due to the lack of proper methodologies or instruments. This study employed the injection of high-pressure gases through a hydrophobized ceramic membrane to produce different gaseous NBs (e.g., N2, O2, H2 and CO2) in water, which is different from cavitation bubbles with potential internal low pressure and non-condensed gases. The results indicate that increasing the injection gas pressure (60–80 psi) and solution temperatures (6–40 oC) both reduced bubble sizes from approximately 400 nm to 200 nm, which are validated by two independent models developed from the Young-Laplace equation and contact mechanics. Particularly, the colloidal force model can explain the effects of surface tension and surface charge repulsion on bubble sizes and internal pressures. The contact mechanics model incorporates the measurement of the tip-bubble interaction forces by atomic force microscope (AFM) to determine the internal pressures and the hardness of NBs (e.g., Young’s modules). Both colloidal force balance model and our contact mechanics model yielded consistent predictions of the internal pressures of various NBs (120 psi-240 psi). The developed methods and model framework will be useful to unravel properties of NBs and support engineering applications of NBs (e.g., aeration or ozonation). Finally, the bulk NBs under sealed storage could be stable for about a week and progressively reduce in concentrations over the next 30-60 days.

Probing internal pressures and long-term stability of nanobubbles in water

ACS Fall 2021

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