Abstract
Air seeded nanobubbles have recently been observed within tree sap under negative pressure. They are stabilized by an as yet unidentified process, although some embolize their vessels in extreme circumstances. Current literature suggests that a varying surface tension helps bubbles survive, but few direct measurements of this quantity have been made. Here, we present calculations of dynamic surface tension for two biologically relevant lipids using molecular dynamics simulations. We find that glycolipid monolayers resist expansion proportionally to the rate of expansion. Their surface tension increases with the tension applied, in a similar way to the viscosity of a non-Newtonian fluid. In contrast, a prototypical phospholipid was equally resistant to all applied tensions, suggesting that the fate of a given nanobubble is dependent on its surface composition. By incorporating our results into a Classical Nucleation Theory (CNT) framework, we predict nanobubble stability with respect to embolism. We find that the metastable radius of glycolipid coated nanobubbles is approximately 35 nm, and that embolism is in this case unlikely when the external pressure is less negative than –1.5 MPa.
Highlights
Nanobubbles further complicate the picture: it was recently discovered that gas-filled bubbles, tens to hundreds of nanometers in radius, can be “air seeded” into the liquid xylem sap at pit membranes (Schenk et al, 2015; Kaack et al, 2019)
DGDG was identified in the sap in a recent landmark publication (Schenk et al, 2021) using electrospray ionization mass spectrometry to characterize the lipid composition of xylem sap of seven angiosperm species
We can infer that the lipid structure has less time to re-organize in response to the interface expanding at more negative pressures, as the area increases faster than the lateral diffusion of lipids can compensate for
Summary
Trees are capable of transporting water at high flow rates against gravity, without the aid of a mechanical pump like the heart in animals. Water vapor transpires from the leaves, generating tension (Pickard, 1981; Zimmermann et al, 1994), which pulls water further upwards in the xylem. This mechanism has come to be known as the cohesion-tension theory (Pockman et al, 1995). More accurately, negative pressure, may disrupt the hydrogen bonds linking water molecules to one another, in much the same way that exposure to vacuum boils liquid water (Vera et al, 2016). Nanobubbles further complicate the picture: it was recently discovered that gas-filled bubbles, tens to hundreds of nanometers in radius, can be “air seeded” into the liquid xylem sap at pit membranes (Schenk et al, 2015; Kaack et al, 2019). One would expect nanobubbles to be unstable with respect to “boiling” at highly negative pressures.
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