How Does Water Boil?

Insight into the boiling of water is obtained from molecular dynamics simulations. The process is initiated by the spontaneous formation of small vacuum cavities in liquid water. By themselves, these defects are very short lived. If, however, several cavities occur at close distances, they are likely to merge into larger vacuum holes. At the liquid-vapor interfaces, single or small groups of water molecules tend to leave the liquid surface. Once the system is propagated beyond the transition state, these evaporation events outnumber the competing reintegration into the hydrogen-bonded network.

Figure 1

Volume of our simulation cell containing 256 water molecules as a function of the temperature. The lower solid curve corresponds to the heating experiment, while the upper one to cooling. The dashed line reflects the volume of an ideal gas of an equal number of particles.

Figure 2

Upper series: Breaking of a hydrogen bond leading to the repulsion of two water molecules (highlighted in blue). Such processes may initiate the formation of small vacuum cavities (yellow, lower picture) in liquid water.

Figure 3

Snapshots taken from a representative dynamical pathway showing the boiling of water. The beginning of the phase transition is set to $t=0$. Subsequent merging of spontaneously formed cavities (yellow) in liquid water lead to the formation of successively growing vacuum domains. The overall process was typically observed within 300 to 400 ps.

Figure 4

Snapshots taken from the same trajectory as illustrated in Fig. 3 but for later times. Left: dynamical transition state of the $\mathrm{\text{liquid}}\leftrightarrow \mathrm{\text{vapor}}$ transformation. At this stage heavy perforations are observed in the hydrogen-bonded network, which finally collapses after crossing the transition state leading to gaseous water (right). Isolated water molecules are highlighted in blue.