Dynamic Surface Tension Enhances the Stability of Nanobubbles in Xylem Sap

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.

Automated summary

Air seeded nanobubbles have been observed within tree sap under negative pressure, with some potentially embolizing vessels in extreme circumstances. Calculations of surface tension for biologically relevant lipids using molecular dynamics simulations suggest that glycolipids and phospholipids exhibit different resistance to expansion. Incorporating these results into Classical Nucleation Theory framework, it is predicted that glycolipid coated nanobubbles have a metastable radius of approximately 35 nm, with embolism being unlikely when the external pressure is less negative than -1.5 MPa.