Hydroxyl ion stabilization of bulk nanobubbles resulting from microbubble shrinkage

Hypothesis: The shrinkage of microbubbles that are less than about 50 mu m in diameter is a well-known phenomenon that results from the surface tension. It has also been shown recently that hydroxyl ions have an extremely strong affinity for gas-water interfaces including bubble surfaces. A theoretical model is proposed that predicts bulk nanobubble stability in water, based on a force balance that results from the shrinkage of microbubbles. This model was designed to test the hypothesis that the surface tension of a shrinking microbubble can ultimately be balanced by the repulsion of the hydroxyl ions that initially adsorb onto the microbubble surface prior to shrinking. Theory: The present model considers the forces due to ionic repulsion as microbubbles shrink under the surface tension. No special assumptions are required in the present model other than the recently reported strong affinity hydroxyl ions have to gas/water interfaces. The Debye-Huckel theory was used to determine the number of ions on the microbubble surface. Findings: The results of this model predict a stable balance between the surface tension and the electrostatic repulsion of hydroxyl ions for nanobubble diameters less than 1100 nm. This predicted maximum in nanobubble size is shown to be consistent with experimental findings. (C) 2020 Elsevier Inc. All rights reserved.