Surface enrichment of ions leads to stability of bulk nanobubbles
Numerous experiments show that bulk nanobubble suspensions are often characterize by a high magnitude of zeta potential. However, the underlying physical mechanism of how the bulk nanobubbles can stably exist remains unclear so far. In this paper, based on theoretical analysis, we report a stability mechanism for charged bulk nanobubbles. The strong affinity of negative charges for nanobubble interface causes charge enrichment, for which the resulting electric field energy gives rise to a local minimum for the free energy cost of bubble formation, leading to thermodynamic metastability of the charged nanobubbles. The excess surface charges mechanically generate a size-dependent force, which balances the Laplace pressure and acts as a restoring force when a nanobubble is thermodynamically perturbed away from its equilibrium state. With this negative feedback mechanism, we discuss the nanobubble stability as a function of surface charge and gas supersaturation. We also compare our theoretical prediction with recent experimental observations, and a good agreement is found. This mechanism provides new fundamental insights into the origin of the unexplained stability for bulk nanobubbles.