Abstract

Hypothesis: Bulk nanobubbles have been proposed to improve gas exchange in a variety of applications, such as in water treatment, theragnostics, and microfluidic surface cleaning. However, there is currently no consensus regarding the mechanism responsible for their reportedly long lifetimes, which contradicts classical understanding of diffusive bubble dynamics. Recently, there has been increasing support for an electrostatic stability mechanism, following from experiments that observe negatively charged zeta potentials around nanobubbles. Simulations: We use high-fidelity Molecular Dynamics simulations to model bulk nanobubbles under mechanical equilibrium in a sodium iodide electrolyte solution, to investigate ionic adsorption on the liquid-gas interface, and resulting zeta potential. We critically examine the hypothesised electrostatic stress underpinning this previously suggested stability mechanism, which is theorised to stabilise the nanobubbles against dissolution by counteracting the otherwise dominant effects of surface tension, however, has been too difficult to directly measure in experiments. Findings: Ions adsorb onto the liquid-gas interface, confirming an Electric Double Layer (EDL) distribution around the nanobubble with an estimated 𝜁0 ∼ −10 mV zeta potential, in accordance with experiments. However, we find no significant electrostatic stress exerted on the nanobubble surface, as any ion charge density in the EDL is completely neutralised by the rearrangement of the water molecules. As a result, the internal gas pressure is still well predicted by the standard Laplace pressure equation (with a fitted Tolman length correction 𝜆 = 0.31 nm), challenging an essential assumption underlying the previously proposed theories, and we instead speculate on alternative mechanisms for electrostatic-based stability.