Water at the cavitation limit: Density of the metastable liquid and size of the critical bubble

The ability of a liquid to sustain mechanical tension is a spectacular manifestation of the cohesion of matter. Water is a paradigmatic example, because of its high cohesion due to hydrogen bonds. The knowledge of its limit of rupture by cavitation can bring valuable information about its structure. Up to now, this limit has been obscured by the diversity of experimental results based on different physical measures of the degree of metastability of the liquid. We have built a fiber optic probe hydrophone to provide the missing data on the density of the liquid at the acoustic cavitation limit. Our measurements between 0 and 50°C allow a clear-cut comparison with another successful method where tension is produced in micron-sized inclusions of water in quartz. We also extend previous acoustic measurements of the limiting pressure to 190°C, and we consider a simple modification of classical nucleation theory to describe our data. Applying the nucleation theorem gives the first experimental value for the size of the critical bubble, which lies in the nanometer range. The results suggest the existence of either a stabilizing impurity in the inclusion experiments, or an ubiquitous impurity essential to the physics of water.