  A liquid subjected to negative pressure is thermodynamically metastable.
Confined within a small volume, negative pressure can build up until cavities
form spontaneously. The critical negative pressure for cavitation in water has
been theoretically predicted to be in the range of -100 to -200 MPa at room
temperature, whereas values around -30 MPa have been obtained by many
experiments. The discrepancy has yet to be resolved. In this study we perform
molecular dynamics simulations to study cavitation of water under volume
controlled stretching. It is found that liquid water exhibits a nonlinear
elastic compressibility (or stretchability) under hydrostatic tension and
remains stable within the confined volume until spontaneous cavitation occurs
at a critical strain. Subsequently, as the volume-controlled stretching
continues, the cavity grows stably and the hydrostatic tension decreases
continuously until the box volume is large enough for another transition to
form a water droplet. A modified nucleation theory is proposed to predict the
critical condition for cavitation. In particular, a strong dependence of the
critical strain and stress for cavitation on the initial liquid volume is
predicted by the modified nucleation theory, which may offer a possible
explanation for the discrepancies in the values of the critical negative
pressure obtained from experiments.
