A new approach to explaining nano-bubbles paradoxical longevity

Abstract

It has been established experimentally that nano-bubbles exist in water for weeks, even months. This contradicts the theory that predicts life span for such small bubbles being on the scale of milliseconds. Nano-bubbles are charged due to adsorption of OH- ions, which creates electric double layers (EDL) at their interfaces. Suggestion has been made that EDL could be responsible for nano-bubbles longevity, but the mechanism remains unknown. We propose such a mechanism here. There is one overlooked feature of the EDL – water molecules in the first layer at interface are oriented, which is supported by many experiments and theory overviewed here. EDL electric field has a gradient component, which exerts force on these oriented molecules dipole moments. We show that it is practically equal to the Young-Laplace force but opposite in direction. It ensures balance of normal forces at interface and creates a pressure gradient that prevents leakage diffusion flux from the stable bubble. In addition, oriented water dipole moments repel each other. This effect is similar to the repulsion of surfactant polar groups, which are known to counterbalance surface tension in tangential direction and stabilize bubbles. Oriented water molecules should cause the same effect. Balance of forces acting on the bubble interface leads to the equation for the stable bubble size that agrees with experimental trends.