Liquid–solid interaction at nanoscale and its application in vegetal biology

The water ascent in tall trees is subject to controversy: the vegetal biologists debate on the validity of the cohesion–tension theory which considers strong negative pressures in microtubes of xylem carrying the crude sap. This article aims to point out that liquids are submitted at the walls to intermolecular forces inferring density gradients making heterogeneous liquid layers and therefore disqualifying the Navier–Stokes equations for nanofilms. The crude sap motion takes the disjoining pressure gradient into account and the sap flow dramatically increases such that the watering of nanolayers may be analogous to a microscopic flow. Application to microtubes of xylem avoids the problem of cavitation and enables us to understand why the ascent of sap is possible for very high trees.

Why can the topmost trees reach a height of more than 120 m ?Figure optionsDownload as PowerPoint slideResearch highlights
► Sap is submitted to intermolecular forces making nanolayers at the top of the trees.
► The maximum height of trees is estimated.
► The crude sap pressure remains positive thanks to the disjoining pressure effect.
► The sap flow slipping along xylem tubes is strongly larger than a Poiseuille flow.