Note that we regard the stable bulk nanobubbles as gaseous analogues of microemulsions, namely, the gas-in-water nanobubble phase coexisting with the external gas phase. Therefore, we need to consider a bulk nanobubble dispersed in an aqueous solution that coexists with the ambient gas above the surface of the solution (Figure and also Figure ). The whole system is in a two-phase coexistence state: the nanobubble-containing solution phase coexists with the ambient gas. The requirement of chemical equilibrium of the gas is schematically shown in Figure . The chemical potential of gas in a nanobubble, 𝜇 , can be simplified as ideal gas, and