Fig. 2.

Shape and hydrogen bonding structure of bubbles. (A) Asphericity α as a function of bubble volume from configurations obtained via umbrella sampling. By construction, α is zero for a perfect sphere, and higher values indicate shapes with higher aspect ratios. Also shown are bubbles (not to scale) observed at $p=-150\hspace{0.17em}\text{MPa}$ whose asphericities and volumes are indicated by arrows. (B) Fraction of free OH groups $n_{\text{fOH}}/n_{\text{mol}}$ at the bubble surface as a function of the inverse radius $r^{-1}$ of a sphere with volume v. The arrow indicates the fraction $n_{\text{fOH}}/n_{\text{mol}}$ for a flat interface at $300\hspace{0.17em}\text{K}$ at ambient pressure from ref. 64. Note that we give the fraction of broken hydrogen bonds per molecule, so the highest possible value is 2. The depicted configuration contains a bubble of critical size at a pressure of $p=-150\hspace{0.17em}\text{MPa}$, where the yellow spheres indicate the unoccupied grid points forming the largest bubble.