Water at Negative Pressure

Abstract

Water samples are sealed in helical Pyrex capillaries which are heated and then cooled to generate negative pressures in the liquid. The helical design of the capillaries enable them to function as combination Berthelot-Bourdon tubes. A vapour bubble is trapped in the capillary with the water when the tube is sealed. On heating the capillary, the water expands until it completely fills the tube. When it is subsequently cooled, the bubble does not reappear because the adhesion to the tube wall prevents the water from contracting at a greater rate than that of the internal tube volume. Consequently, an increasing tension develops in the tube until the water either eventually ruptures (cavitates) or the tension minimum for the experiment is reached where water's thermal expansivity changes sign. Pressure in the capillary is measured by monitoring the torsional movement of the helix with a laser beam reflected from a small mirror suspended underneath. Results are given for H2O, D2O and a H2O/D2O mixture between -20°C and 100°C, and between -250 bar and 500 bar. Freezing the water in the helices has enabled the ice I-water phase transition curve to be extended for the first time into the negative pressure region; -150 bar was reached for D2O and -230 bar was reached for H2O. The locus of points where the thermal expansivity is zero has been extended to -200 bar for H2O, D2O and the HDO mixture. The estimation of water volumes at negative pressure is discussed. An extensive review is given on stretched liquids with particular emphasis on water.