Stability Limit of Liquid Water in Metastable Equilibrium with Subsaturated Vapors

Abstract

A pure liquid can reach metastable equilibrium with its subsaturated vapor across an appropriate membrane. This situation is analogous to osmotic equilibrium: the reduced chemical potential of the dilute phase (the subsaturated vapor) is compensated by a difference in pressure between the phases. To equilibrate with subsaturated vapor, the liquid phase assumes a pressure that is lower than its standard vapor pressure, such that the liquid phase is metastable with respect to the vapor phase. For sufficiently subsaturated vapors, the liquid phase can even assume negative pressures. The appropriate membrane for this metastable equilibrium must provide the necessary mechanical support to sustain the difference in pressure between the two phases, limit nonhomogeneous mechanisms of cavitation, and resist the entry of the dilutant (gases) into the pure phase (liquid). In this article, we present a study of the limit of stability of liquid water--the degree of subsaturation at which the liquid cavitates--in this metastable state within microscale voids embedded in hydrogel membranes. We refer to these structures as vapor-coupled voids (VCVs). In these VCVs, we observed that liquid water cavitated when placed in equilibrium with vapors of activity aw,vapair<or=0.85 (relative humidity<or=85%). When expressed as a pressure in the liquid based on thermodynamic calculations, the liquid cavitated at pressures P<or=-22 MPa (-220 bar). This limiting pressure is smaller in magnitude than the limit predicted by homogeneous nucleation theory or molecular simulations (Pcav=-140 to -180 MPa). To determine the cause of the disparity between the observed and predicted stability limit, we examine experimentally the likelihood of several nonhomogeneous mechanisms of nucleation: (i) heterogeneous nucleation caused by hydrophobic patches on void walls, (ii) nucleation caused by the presence of dissolved solute, (iii) nucleation caused by the presence of pre-existing vapor nuclei, and (iv) invasion of air through the hydrogel membrane into the voids. We conclude that, of these possibilities, (i) and (ii) cannot be discounted, whereas (iii) and (iv) are unlikely to play a role in determining the stability limit.

Birth and Growth of Cavitation Bubbles within Water under Tension Confined in a Simple Synthetic Tree

“…3, showing that the released pressure amounts to values ranging within the tens of MPa range (30 MPa ±16 MPa). Due to the measurement error in the image analysis, the estimated pressure presents a lot of uncertainty, but is comparable to that measured for cavitation in static hydrogels with the same composition (22 MPa) 3,7 , and to other experimental methods 6 . Note that the pressure released by triggered cavitation is of the same order than with spontaneous cavitation (Fig 3d).…”

“…We use pHEMA hydrogels of the same chemical composition as in 3,7 , but with a periodic array of holes or cavities, created using the novel experimental setup shown in Fig. 1a.…”

Evaporation-induced cavitation in nanofluidic channels

“…Our observations suggest the possibility of air bubbles being nucleated at the nanochannel entrance and transported into the xylem while the meniscus remains pinned at the entrance of the nanochannels (SI Appendix, Fig. S22B), which could be another possible mechanism for heterogeneous liquid cavitation under negative pressure (3,24). Our analysis also suggests that, no matter where the original bubble occurs, the cavitation bubble growth rate in plant xylem likely depends on the water evaporation rate on the surface.…”

“…Our analysis also suggests that, no matter where the original bubble occurs, the cavitation bubble growth rate in plant xylem likely depends on the water evaporation rate on the surface. Direct visual observations of cavitation in nanoscale channels could help gain deeper insight into cavitation in these systems, which is currently studied using indirect acoustic methods (25) or hydraulic conductance measurements (26), or in hydrogel systems (24).…”

From the sap's perspective: The nature of vessel surfaces in angiosperm xylem

“…Gas-saturated aqueous solutions confined in systems that are under subatmospheric or negative pressure are prone to forming gas bubbles through heterogeneous nucleation on surfaces, especially on small surface irregularities, such as in crevices, or surface locations with hydrophobic properties (Blander and Katz, 1975;Creech et al, 2002;Wheeler and Stroock, 2009;Belova et al, 2011;Brennen, 2014). Most attempts to maintain or transport water or aqueous solutions in artificial systems under negative pressure without bubble formation only succeed under highly controlled conditions, when solutions are completely degassed, or when surfaces are treated to be completely hydrophilic and free of crevices that could act as bubble nucleation sites (Creech et al, 2002;Stroock, 2008, 2009;Boatwright et al, 2015).…”