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Schlagwörter:
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Zusammenfassung:
How and when does water ‘‘fracture’’? In other words, how and when does a small cavity, or nucleus, form that does not heal but grows to macroscopic size, thus becoming a bubble? This question is important in various areas of technology and nature, affecting, for example, the ability of tall trees to draw sap to great heights (1, 2).
The classical answer, developed by Volmer in the 1930s and described in his monograph (3), implies that, in ideal conditions, it is next to impossible to create a bubble in water because the tension (or negative pressure) required is of the order of thousands of atmospheres (1 atm is about 0.1 MPa; for more modern accounts see refs. 4⇓–6). Although this result had some uncertainties as far as precise numerical values were concerned, the order of magnitude—dictated by the strength of the intermolecular hydrogen bonds—seemed robust. However, it was also in flagrant conflict with experience, because cavitation is often encountered at tensions of the order of one or a few atmospheres, as, for example, in the acoustic cleaning baths used by dentists and jewelers. Even more strange is the embarrassingly wide range of nucleation thresholds reported by different investigators.
The way out of these paradoxes was suggested by Harvey et al. (7), who postulated that in ‘‘real life’’ nucleation in water does not occur in the homogeneous liquid, as postulated in the classical theory, but at ‘‘weak spots,’’ such as preexisting small gas pockets trapped on solid walls or on floating motes, hydrophobic nanoparticles, or other impurities. …
