(208d) Thermodynamic Properties of Water Under Tension: Measuring the Pull of H2?

Given the right conditions, liquids can exist at negative pressure, a metastable state where attractive intermolecular forces prevent spontaneous cavitation. The fluid then is said to be under tension, analogous to a stretched rubber band. This tensile state occurs under many situations of interest, such as liquids in nanoporous media and soils, flow in plants under drought conditions, cavitation near propeller blades, and the suction in octopi suckers. Yet, measurements of thermodynamic properties of liquids at negative pressure are limited due to experimental difficulties in creating, maintaining, and measuring this stretched state. Particularly, the properties of arguably the most important liquid in science—water—under tension are as yet scarcely explored. Experimental measurements of these properties are vital to predict the behavior of water under tension, understand the origin of the many anomalies in water, and ascertain which theoretical models and simulations best describe this liquid in metastable conditions.

In this paper, we explore the behavior of water under tension, presenting measurements of its thermodynamic properties in this state, and comparing our measurements with existing theoretical models. For these measurements, we developed a novel microfabricated sensor to manipulate and study a macroscopic volume of metastable water. Our sensor has three main components: (1) a millimeter-sized water reservoir, (2) a nanoporous membrane connecting the reservoir to ambient vapor, and (3) a piezoresistive pressure sensor in mechanical contact with the water reservoir. With this system, we equilibrate the water reservoir with a sub-saturated vapor phase of known activity, and measure the resulting (negative) pressure of the water reservoir with the piezoresistive sensor. All measurements are performed isothermally. Because we have 3 state variables (temperature, pressure and activity) we are thus able to fully specify the thermodynamic state of water. We finish by discussing the implications of these measurements for theoretical models of water, and our plans for future studies using this technology.