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 of water is termed transpiration. To travel from the roots of a tree to the highest leaves, liquid water that is initially at atmospheric pressure must be pulled across hydrodynamic resistances imposed by the 30-100 micron xylem ducts and against gravity. This requires the water to be placed at 10's of bars of negative pressure. In the plant science community, it is hypothesized that capillary pressures generated in nano-scale pores within the leaf allow trees to generate the required high negative pressures. We have developed a microfluidic pump (the synthetic tree) that instead relies on osmotic gradients across a hydrogel material to transduce the chemical potential driving force behind transpiration. I will present the thermodynamic, solid mechanical and transport considerations for the hydrogel material that acts as a pump in this system. I will then present experimental results that illustrate the success of the design as measured by the ability of the synthetic tree to use negative pressures to drive the transport of liquid water. Finally, I will discuss what further developments are required to improve the pumping ability of the synthetic tree and make it a viable contributor to other wick-based technologies, such as heat pipes and water management in air-breathing electrode fuel cells.

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