(108d) Transport of Liquid Water under Tension in a Synthetic Tree

A commonplace and yet remarkable phenomenon is the transport of water through trees: liquid water travels from sub-saturated soil, across the membranes of roots, and through meters of micron-scale ducts (xylem) in the trunk, branches and leaves, in order to maintain hydration of the tree as water evaporates from the leaves during CO₂/O₂ exchange with the atmosphere. This transport process is termed transpiration. Noteworthy characteristics of transpiration include: 1) water is maintained throughout the tree in the thermodynamically meta-stable state (superheated) associated with 10-100 atmospheres of hydrostatic tension, 2) the ducts for water transport through the tree are kept open by a balance between the mechanical strength of membranes and ducts and the hydrostatic tension in the water, and 3) the transport of water through the tree is a passive operation that requires no energy input; water simply travels along a gradient in chemical potential. The transport of superheated water that occurs in trees on a daily basis has never been achieved in man-made systems. We will report on the development of a synthetic system that replicates key aspects of transpiration in trees. Our system, a Synthetic Tree, consists of a closed network of micro-channels embedded directly within a slab of hydrogel (poly(2-hydroxyethyl methacrylate)). One half of the network (channels and surrounding hydrogel material) mimics the root system; the other half mimics a leaf. A single channel that connects the two halves of the network mimics a trunk. In our experiments, we first fill the network with liquid by submersion in degassed water. We then monitor deformation of the gel and flux of water as we expose the system to air at sub-saturated partial pressures of water vapor (i.e., relative humidity = RH < 100%). We will present the following results based on this simple system: 1) liquid water within the network can be maintained in metastable equilibrium with sub-saturated air for extended periods of time (> months). Based on thermodynamics and established equations of state of water, we deduce that the liquid water is under tension (> 10 atm). 2) The hydrogel balances the stresses generated by the tension in the liquid water, and thus resists collapse of the microchannels. We will present an analysis of the elastic response of the hydrogel that confirms our deduction of tension based on thermodynamics. 3) The Synthetic Tree can be used to transport water under tension between two sub-saturated atmospheres (e.g., 98% RH at the root and 75% RH at the leaf). With these results as a basis, we will discuss design considerations for Synthetic Tress related to limiting the cavitation of metastable water, resisting the mechanical collapse of the network defined in the hydrogel material, and maximizing the flux of water for a given driving force. Finally, we will discuss the relevance of these results to applications such as passive pumps for microfluidics, wick materials for heat pipes, and environmentally responsive materials for controlled evaporative cooling.