Liquids under negative pressure, or tension, are metastable systems prone to cavitation—the formation of vapor-filled cavities—which can be found in both technological and biological contexts. Examples range from lithotripsy and ultrasound-induced tissue damage to plant hydraulics and the catapulting of fern spores. Classical nucleation theory predicts that pure bulk water can sustain tensions as low as between -110 and -160 MPa before cavitating, yet experimental studies of highly purified water consistently measure thresholds closer to -20 to -30 MPa. This large discrepancy is widely attributed to the presence of impurities, surface defects, and pre-existing nanobubbles, which act as weak points within the liquid. Biological fluids are particularly vulnerable, as they contain a mixture of water, lipids, proteins, dissolved gases, and solutes. Hydrophobic materials within such mixtures represent the weakest link, cavitating more readily than water due to weaker intermolecular interactions.
This work explores the stability of liquids under tension, bridging the gap between theory and experiment. By modeling organic impurities and surface irregularities, we demonstrate that tensile strength is dictated not by the density of impurities but by the size of the largest one. A single nanodroplet of organic material or a nanopit above a few tens of nm in size can reduce the tensile strength to experimentally observed values. Impurities smaller than 1 nm remain negligible, as the hydrogen-bonded water network can reorganize around them. We further show that lipids, abundant in biological matter, may offer a stabilizing mechanism. Lipid coatings can mask hydrophobic defects, reducing their ability to nucleate cavitation, while still providing greater stability than unprotected surfaces. These findings offer insight into how plants sustain sap transport under high tension and suggest that surfactant-mediated stabilization may play a protective role against cavitation in biological systems.
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