Molecular mechanism for cavitation in water under tension

Abstract

Despite its relevance in biology and engineering, the molecular mechanism driving cavitation in water remains unknown. Using computer simulations, we investigate the structure and dynamics of vapor bubbles emerging from metastable water at negative pressures. We find that in the early stages of cavitation, bubbles are irregularly shaped and become more spherical as they grow. Nevertheless, the free energy of bubble formation can be perfectly reproduced in the framework of classical nucleation theory (CNT) if the curvature dependence of the surface tension is taken into account. Comparison of the observed bubble dynamics to the predictions of the macroscopic Rayleigh-Plesset (RP) equation, augmented with thermal fluctuations, demonstrates that the growth of nanoscale bubbles is governed by viscous forces. Combining the dynamical prefactor determined from the RP equation with CNT based on the Kramers formalism yields an analytical expression for the cavitation rate that reproduces the simulation results very well over a wide range of pressures. Furthermore, our theoretical predictions are in excellent agreement with cavitation rates obtained from inclusion experiments. This suggests that homogeneous nucleation is observed in inclusions, whereas only heterogeneous nucleation on impurities or defects occurs in other experiments.cavitation | water | negative pressure | bubble nucleation | liquid-vapor transition D ue to its pronounced cohesion, water remains stable under tension for long times. Experimentally, strongly negative pressures exceeding −120 MPa (1-6) can be sustained before the system decays into the vapor phase via cavitation, i.e., bubble nucleation. Recently, cavitation in water under tension has drawn research interest due to its importance in biological processes, like water transport in natural (7-10) and synthetic (11, 12) trees, spore propagation of ferns (13), and poration of cell membranes (14, 15). Furthermore, cavitation in water appears to be the driving force behind the sonocrystallization of ice (16,17), and preventing its occurrence remains a challenge in turbine and propeller design (18). Studying the onset of cavitation has also proven to be a valuable tool to locate the line of density maxima in metastable water (4), which contributes to the ongoing effort of explaining the origin of water's anomalies (6,19). Interest in the topic is magnified by the startling discrepancy arising when cavitation in water is investigated using different experimental methods. Although agreement between different methods is excellent in the high-temperature regime, where the liquid is unable to sustain large tension, a significantly higher degree of metastability is reached when studying cavitation in inclusions along an isochoric path (1-5) compared with other techniques (20, 21) at low temperatures (22).Due to the short time scale on which the transition takes place and the small volume of the critical bubble at experimentally feasible conditions, direct observation of cavitation at the microscopic level...

What experiments on pinned nanobubbles can tell about the critical nucleus for bubble nucleation

“…The figure shows that the surface tension for surface nanobubble is also nearly independent on its radius as for bubbles in the bulk solution [26], and is close to the planar value of γ = 0.31 [27] which was calculated from independent MD runs. Note that different from the simple fluid studied here, the curvature effects on surface tension for a more complex fluid, such as water, are non-negligible for small nanobubbles [28]. In contrast, the Tolman length for a LJ fluid has been estimated to be of order 0.3 σ [29], which is why we cannot observe curvature effects for bubbles with radii from 10 σ to 45 σ.…”

Pressure control in interfacial systems: Atomistic simulations of vapor nucleation

“…Our results suggest that simulations using standard barostats, [6][7][8]35 if not performed on a reasonably large system size, might be affected by errors of the order of 10-15 k B T on the barrier height, corresponding to an error of 10 4 -10 6 s 1 σ 3 on the rate.…”

Homogeneous nucleation: Patching the way from the macroscopic to the nanoscopic description

“…Choosing the appropriate one is nontrivial because it is virtually impossible, using the same potential, to get all water properties right in MD simulations-density, surface tension, boiling and freezing points, latent heat, and any other material properties, including their temperature dependence. Despite this and several other difficulties, Menzl et al (13) succeeded in obtaining nucleation thresholds and rates similar to those found in the best available data obtained in water inclusion experiments in quartz, in which a homogeneous nucleation threshold of -140 MPa was found (14). In addition, they were able to determine the values of the parameters appearing in the classical theory, for some of which there was some uncertainty, as mentioned before.…”