(181ai) Plant-Inspired Heat Pipes Exploiting Nanoporous Wicks - Development and Application of Materials Sustaining Fluid Under Tension

Among many different technologies for transferring heat, the heat pipe (HP) – closed-circuit systems in which a working fluid transfers its latent heat as it cycles between an evaporator and a condenser by capillary pumping force – is one of the most efficient systems known today and has offered an attractive alternative in an array of applications¹.  However, two main limitations have constrained the application of conventional HPs: 1) the maximum capillary head developed by the micro-porous wicks in loop HPs of ~1 bar; and 2) the saturation state in the condenser region creates an unbounded liquid-vapor interface that causes undesired transients and additional resistance to heat transfer. 

        The transpiration process in vascular plants operates by a mechanism similar to the recirculation of liquid through the wick of HPs².  Unlike in HPs, though, the water pressure in plants is able to reach large negative values (down to -100 bars) to overcome gravity, viscous drag in the liquid path, and the reduced chemical potential of water in sub-saturated soils.  The secret behind the amazing abilities of plants is a the structure of plants cell walls, which serve as nano-porous membranes, allowing the water pressure to be less than zero (under tension).  Inspired by plants, our research is focused on the development of a nano-porous heat pipe (NPHP) exploiting nano-porous wicks, including a theoretical model of the coupled transport and thermodynamic phenomena and an experimental platform with which to test design concepts for NPHPs. 

        After presenting the design considerations of a NPHP, we will focus on a central experimental challenge in its development: nanoporous membranes that allow for the pure liquids to come to metastable equilibrium at negative pressures with sub-saturated vapors.  We will discuss several membrane structures developed in our laboratory and our experimental methods for characterizing them.  We will then turn to an intriguing open question about water under tension: several experimental studies focusing on cavitation thresholds (the pressure at which the thermodynamically metastable liquid turns into vapor by nucleation) of water under tension have been reported, yet there is no consensus³.  Many experiments using various methods have found the stability limit of water at room temperature is between -20 ~ -30MPa^4-6, while experiments in quartz inclusions appear to reach a stability limit of -140 MPa (with the use of an extrapolated equation of state).  We will describe how measurements based our distinct experimental approach compare with those in the literature and discuss the implications for our understanding of the phase diagram of water and on the practical limitations on the use of water at negative pressure in technologies such as NPHPs.   

1          Faghri, A. Heat Pipe Science and Technology.  (Taylor and Francis, 1995).

2          Nobel, P. S. Physicochemical and Environmental Plant Physiology. 2nd edn,  (Academic Press, 1999).

3          Caupin, F. & Herbert, E. Cavitation in water: a review. Comptes Rendus Physique 7, 1000-1017 (2006).

4          Briggs, L. J. Limiting Negative Pressure of Water. Journal of Applied Physics 21, 721-722 (1950).

5          Wheeler, T. D. & Stroock, A. D. Stability Limit of Liquid Water in Metastable Equilibrium with Subsaturated Vapors. Langmuir 25, 7609-7622, doi:10.1021/la9002725 (2009).

6          Davitt, K., Arvengas, A. & Caupin, F. Water at the cavitation limit: Density of the metastable liquid and size of the critical bubble. EPL 90, 16002 (2010).