  The cohesion-tension theory expounds the crude sap ascent thanks to the
negative pressure generated by evaporation of water from leaves. Nevertheless,
trees pose multiple challenges and seem to live in unphysical conditions: the
negative pressure increases cavitation; it is possible to obtain a water
equilibrium between connected parts where one is at a positive pressure and the
other one is at negative pressure; no theory is able to satisfactorily account
for the refilling of vessels after embolism events. A theoretical form of our
paper in the Journal of Theoretical Biology is proposed together with new
results: a continuum mechanics model of the disjoining pressure concept refers
to the Derjaguin School of physical chemistry. A comparison between liquid
behaviour both in tight-filled microtubes and in liquid thin-films is offered
when the pressure is negative in liquid bulks and is positive in liquid
thin-films and vapour bulks. In embolized xylem microtubes, when the air-vapour
pocket pressure is greater than the air-vapour bulk pressure, a refilling flow
occurs between the air-vapour domains to empty the air-vapour pockets although
the liquid-bulk pressure remains negative. The model has a limit of validity
taking the maximal size of trees into account. These results drop inkling that
the disjoining pressure is an efficient tool to study biological liquids in
contact with substrates at a nanoscale range.
