Disjoining pressure driven transpiration of water in a simulated tree.

Hypothesis: Transpiration occurs in 100 m tall redwood trees where water is passively pulled against gravity requiring the evaporating liquid meniscus in stomata pores to be under absolute negative pressures of -10 atm or higher. Disjoining pressure can significantly reduce pressure at meniscus in nanopores due to strong surface-liquid molecular interaction. Hence, disjoining pressure should be able to solely govern the transpiration process.

Simulations: Expression of disjoining pressure in a water film is first developed from prior experimental findings. The expression is then implemented in a commercial CFD solver and validated against experimental data for water wicking in nanochannels of height varying from 59 nm to 1 µm. Following the implementation, the transpiration process is simulated in a 3D domain comprising of a nanopore connected to a tube with ground-based water tank, thus mimicking the stomata-xylem-soil pathway in a 100 m tall tree.

Findings: Disjoining pressure is found to induce absolute negative pressures as high as -23.5 atm at the evaporating meniscus and can also sustain high evaporation fluxes in nanopore before the meniscus completely dewets. This is the first report to integrate disjoining pressure into continuum simulations and study the transpiration process in a 100 m tall tree using such simulations.