(347i) Drying From the Inside: The Dynamics of Cavitation In Model Xylem Networks

The xylem in vascular plants carries liquid from the roots to the leaves during transpiration.  Under common conditions imposed by the sub-saturated state of water in the soil and the atmosphere, the liquid in the xylem is at negative pressures and thus is metastable with respect to its vapor.  To avoid catastrophic failure upon cavitation (boiling), the xylem conduits are segmented; nanoporous membranes that separate neighboring segments inhibit the growth of a vapor bubble beyond the segment in which it formed and allow for the liquid to flow along alternate paths around an empty segment.  We will report on a study of the dynamics and thermodynamics of this process in a microfluidic system that mimics the architecture of xylem.  We will show how the coupling between stochastic cavitation events and hydraulic relaxation of stresses through the network leads to an unusual drying process in which 1) the drying front propagates from the inside out rather than the outside in and 2) the rate of drying can be non-monotonic with respect to time.  We will further explain how this process is modified in the presence of a gradient of stress and a mean flow in the fluid.  We will finish with a discussion of possible insights into plant physiology that emerge from this study and the implications for technologies that could exploit liquids under tension.