Water cavitation from ambient to high temperatures

Francesco Magaletti,Mirko Gallo,Carlo Massimo Casciola

doi:10.1038/s41598-021-99863-z

Francesco Magaletti, Mirko Gallo + Show 1 more

Open Access

https://doi.org/10.1038/s41598-021-99863-z

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Abstract

Predicting cavitation has proved a formidable task, particularly for water. Despite the experimental difficulty of controlling the sample purity, there is nowadays substantial consensus on the remarkable tensile strength of water, on the order of −120 MPa at ambient conditions. Recent progress significantly advanced our predictive capability which, however, still considerably depends on elaborate fitting procedures based on the input of external data. Here a self-contained model is discussed which is shown able to accurately reproduce cavitation data for water over the most extended range of temperatures for which accurate experiments are available. The computations are based on a diffuse interface model which, as only inputs, requires a reliable equation of state for the bulk free energy and the interfacial tension. A rare event technique, namely the string method, is used to evaluate the free-energy barrier as the base for determining the nucleation rate and the cavitation pressure. The data allow discussing the role of the Tolman length in determining the nucleation barrier, confirming that, when the size of the cavitation nuclei exceed the thickness of the interfacial layer, the Tolman correction effectively improves the predictions of the plain Classical Nucleation Theory.

Highlights

Predicting cavitation has proved a formidable task, for water
One of the main results of the paper is provided in Fig. 1 which shows the cavitation pressure of ultra-pure water as a function of temperature
In this paper we have shown in detail how a diffuse interface method completed with realistic equations of state for bulk water and for the vapor-liquid surface tension can be exploited in combination with rare event techniques to evaluate the free-energy barrier

Summary

Introduction

Predicting cavitation has proved a formidable task, for water. Despite the experimental difficulty of controlling the sample purity, there is nowadays substantial consensus on the remarkable tensile strength of water, on the order of −120 MPa at ambient conditions. A rare event technique, namely the string method, is used to evaluate the free-energy barrier as the base for determining the nucleation rate and the cavitation pressure. The probability that a fluctuation able to trigger the transition takes place could be extremely rare This implies that the time needed to observe cavitation can be quite long on the atomistic scale. The free-energy barrier and, as a consequence, the nucleation rate strongly depend on temperature and pressure, spanning a range of one hundred orders of magnitude, with the transition becoming quite fast at spinodal conditions. Classical Nucleation Theory (CNT) provides the basic framework to understand the nucleation process It assumes a uniform state inside the spherical bubble up to the dividing surface which separates the vapor from the external uniform liquid. The CNT nucleation barrier is found to be overestimated leading to large discrepancies with the experimentally observed cavitation rates and cavitation p ressures[6,10]

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