Lifetime of superheated water in a micrometric synthetic fluid inclusion

Lifetime of superheated water in a micrometric synthetic fluid inclusion

A synthetic pure water fluid inclusion presenting a wide temperature range of metastability (Th – Tn ≈ 50°C; temperature of homogenization Th = 144°C and nucleation temperature of Tn = 89°C) was selected to make a kinetic study of the lifetime of an isolated microvolume of superheated water. The occluded liquid was placed in the metastable field by isochoric cooling and the duration of the metastable state was measured repetitively for 7 fixed temperatures above Tn. Statistically, measured metastability lifetimes for the 7 data sets follow the exponential Reliability distribution, i.e., the probability of non nucleation within time t equals e t inclusion, according to the equation τ(s) = 22.1 × e1.046×∆T , (∆T = T – Tn). Hence we conclude that liquid water in water-filled reservoirs with an average pore size ≈ 104 µm3 can remain superheated over. geological timelengths (1013s), when placed in the metastable field at 24°C Any liquid can experience three thermodynamic states with regard to the phase diagram: stable, metastable, and unstable. When metastable with respect to its vapour, the so- called superheated liquid persists over the more stable vapour owing to the nucleation barrier related to the cost to create the liquid-vapour interface. Practically speaking, a superheated liquid undergoes any P-T conditions located between the saturation and the spinodal curves (Fig. 48). It should be noted that the term “superheating” does not refer to a particular range of temperature, and goes down to temperatures in the melting area. This superheat metastability gives to the liquid a certain “overstability feature” with respect to vapour. Indeed, geologists have long observed that liquid water displays such overstability in certain low and high temperature contexts. For instance, the soil capillary water (Pettenati et al., 2008) or the water state in very arid environments like the Mars surface (Meslin et al., 2006; Jouglet et al., 2007) are natural examples of low T superheated liquid states whereas certain continental and submarine geysers (Ramboz and Danis, 1990) or the deep crustal rocks (Shmulovich and Graham, 2004) can also generate high T superheated solutions.

Durée de vie de la métastabilité

The shape of the pure water spinodal has long been a matter of debate in the physics community. To date, three competing scenarios are proposed, one with the retracing shape towards positive pressure at low temperatures, the two others with a monotonous decreasing shape. The first model relates to the stability-limit conjecture (Speedy, 1982) based on experimental data on supercooled water. It can be demonstrated (Debenedetti and D’Antonio, 1986) that the retracing shape corresponds to the intersection of the spinodal curve with the Temperature of Maximal Density (TMD) line. The second proposed scenario derives from molecular simulation (Poole et al., 1992) and predicts a constant positive slope, hence a spinodal monotonously extending towards negative pressures. That thermodynamically implies that this is the TMD the slope of which changes from negative to positive slope. The experimental data previously mentioned are here interpreted as related to the presence of a second critical point at low temperature and positive pressure (Poole et al., 1992; Mishima. and Stanley, 1998). The third proposition is the singularity-free hypothesis (Sastry et al.,1996) , associating a non-retracing spinodal with thermodynamic divergences. In this model, all the polyamorphic transitions of liquid water at low temperature are just relaxation phenomena and not real phase transitions, referring explicitly in that respect to the percolation model (Stanley and Teixeira, 1980). It is worth noting that most equations of state (EOS) like the van der Waals equation or the IAPWS-95 EOS result in the retracing behaviour when extrapolated in the metastable field (Fig. 48). Obviously, this is not at all an argument in support of the latter conception. However, we want to highlight that our experimental investigations are not concerned for the time being with the low temperature superheating region, so that our calculations are not influenced by this debate. The problem of the extrapolative capability of the IAPWS-95 equation remains but seems to be satisfying enough as already discussed elsewhere (Span and Wagner, 1993).

 

Cours gratuitTélécharger le document complet

Télécharger aussi :

Laisser un commentaire

Votre adresse e-mail ne sera pas publiée. Les champs obligatoires sont indiqués avec *