Knudsen gas provides nanobubble stability

We provide a model for the remarkable stability of surface nanobubbles to bulk dissolution. The key to the solution is that the gas in a nanobubble is of Knudsen type. This leads to the generation of a bulk liquid flow which effectively forces the diffusive gas to remain local. Our model predicts the presence of a vertical water jet immediately above a nanobubble, with an estimated speed of $\sim 3.3\mathrm{m}\mathrm{/}\mathrm{s}$, in good agreement with our experimental atomic force microscopy measurement of $\sim 2.7\mathrm{m}\mathrm{/}\mathrm{s}$. In addition, our model also predicts an upper bound for the size of nanobubbles, which is consistent with the available experimental data.