Mechanistic insights into surface tension reduction: The potential contribution of nanobubbles

It is currently pure conjecture as to whether bulk nanobubbles (BNBs) can lower a liquid’s surface tension. In the present study, we propose a hypothesis that aims at identifying the underlying mechanism. Here, nanobubbles are probably governed by the diffusion-limited adsorption model, assuming that surface excess near the interface causes the emergence of an instantaneous equilibrium state. The minuscule time constraint for the nanobubble uptake at the free surface is ruled out by applying the asymptotic limit for the dynamic interfacial tension where t $\to 0$ and the net reduction in surface tension ($\Delta \gamma$), is directly proportional to the concentration of nanobubbles (C_NB) and inversely proportional to the diameter of the nanobubbles (D_NB). Nanobubbles were generated by coupling the pathway of the compression-decompression technique and salting-out effect. For all cases, surface tension was estimated and subsequently correlated with the nanobubble properties, and a mechanism was proposed that elucidates a sudden drop in the surface tension at certain salt concentration ranges. It was observed that the sudden decrease in the surface tension was fostered by the presence of charged nanobubbles, likely to be adsorbed on the free surface along with the electrostatic attractive force, both presumably responsible for the surface excess. The presence of a gaseous nanoscale domain has been confirmed by atomic force microscopic (AFM) images depicting the bubble diameter approximately consistent with the mean size measured through nanoparticle tracking analysis (NTA).