Thermodynamics of negative pressures in liquids

Nature uses negative pressures in the most resourceful and efficient ways. Yet, negative pressure states are still sometimes considered inaccessible by part of the scientific community. In this paper we show that any condensed phase can exist in absolute negative pressure regimes, while the same is not true for gas phases. We also demonstrate that such states are not merely possible but have, in spite of their metastability, been observed experimentally on numerous occasions. Moreover, physical properties of several substances and mixtures have already been determined in the stretched liquid phase at absolute negative pressures. Nevertheless, conceiving of and succeeding in an experiment that produces high tension in a liquid are rather difficult. Thus, equations of state and computer simulations are powerful tools for studying metastable liquids. By using a simple equation of state we show: how negative pressure regimes can be attained; the maximum intrinsic tension a liquid can sustain; and below which temperature a liquid can be found in this state. Experimental and theoretical work on liquids at negative pressures is reviewed. Furthermore, the similarities and differences between negative temperature and negative pressure states are demonstrated. Due to water's non-trivial behavior as well as its technological and scientific importance, it has been the most studied substance in metastable phenomena. We will thus devote particular attention to some of the rich features of its metastable phase diagram. Water belongs to a class of substances that presents density anomalies. We also show how the negative pressure region of the phase diagram proves to be paramount in understanding the unusual behavior of this class of substances.