Gas nuclei

It has also been found that the presence of particulate matter, and more especially the occurrence of trapped vapour-gas nuclei in the crevices and recesses of these particles, also lowers the cavitation threshold. The way in which nucleation occurs at these sites (and from similar sites on the vessel walls) is shown in Fig. 2.10. 

The detailed work of Bernd (ref. 15-17) and other investigators has also shown that the tensile strength of water is set by the gas nuclei (i.e., microbubbles) present in the water. (Accordingly, the earlier-mentioned definition of the tensile strength of a liquid can be restated as the minimum tensile stress at which the gas nuclei in the liquid start to explode . This property is also often referred to as the cavitation susceptibility (ref. 57).) Using specially constructed sonar transducers, the behavior of gas nuclei was followed by Bernd by measuring tensile strength. Surface... 

During the last four decades, measurements of weak nuclei (i.e., gas nuclei) in liquids have become especially important in view of their influence on cavitation inception (e.g., ref. 57-59). Understandably, the gas nuclei concentration is closely coupled with the free gas content of the liquid (ref. 58, 60). (A distinction is made commonly in the engineering literature between free gas content and dissolved gas content. The free gas content is that portion of gas which has the normal physical properties of bulk gas. In practical situations, the free gas concentration within the liquid is usually several orders of magnitude lower than the dissolved gas concentration (ref. 58).) Many investigators have developed instruments to detect this free gas content (ref. 58,60) and the freestream gas nuclei concentration associated with it (e.g., ref. 58-60). 

Sirotyuk (ref. 25) found that the complete removal of solid particles from a sample of water increased the tensile strength by at most 30 percent, indicating that most of the gas nuclei present in high purity water are not associated with solid particles. Bernd (ref. 15,16) observed that gas phases stabilized in crevices are not usually truly stable, but instead tend to dissolve slowly. This instability is due to imperfections in the geometry of the liquid/gas interface, which is almost never exactly flat (ref. 114). Medwin (ref. 31,32) attributed the excess ultrasonic attenuation and backscatter measured in his ocean experiments to free microbubbles rather than to particulate bodies this distinction was based on the fact that marine microbubbles in resonance, but prior to ultrasonic cavitation (ref. 4), have acoustical scattering and absorption cross sections that are several orders of magnitude greater than those of particulate bodies (see Section 1.1.2). 

In line with discussions included in previous sections, ultrasonic experiments carried out on fresh water by different investigators indicate that the stabilization of gas microbubbles, acting as gas nuclei for ultrasonic cavitation, is always attributable to the presence of surface-active substances in the water (ref. 15-17,25). As a starting point, one should consider that laboratory tests with various tap waters, distilled waters, and salt solutions have shown that no water sample was ever encountered that did not contain at least traces of surface-active material (ref. 46). Sirotyuk (ref. 25) estimates that the content of surface-active substances in ordinary distilled water amounts to 10 7 mole/liter, and in tap water it is 10"6 mole/liter or higher. These values indicate the appreciable content of such substances in both cases (ref. 122), although they differ by roughly an order of magnitude in absolute value. It is essentially impossible to completely remove... 

In addition to acoustical methods, which take advantage of the fact that gas nuclei (i.e., stable microbubbles) are elastic bodies and thus absorb sound energy (ref. 4,5,9,25,26,31,32,50), another class of methods for detecting these gas microbubbles that has been employed repeatedly is based on their optical behavior. Specifically, most of these optical methods involve detection of these long-lived microbubbles in water from the light scattered by them (ref. 5,26,59,60,127). 

More recent light-scattering studies (ref. 26,59,60) of microbubble populations in fresh water, using laser-light sources, have yielded very similar results. For example, Keller s laser-scattered-light technique (ref. 26) provided precise measurements of the size and number of freestream gas nuclei (i.e., long-lived microbubbles) in a cavitation tunnel from microbubble spectra... 

In addition, the maximum of this gas nuclei distribution occurred in the diameter range of 20-40 pm (for sea water samples taken both inside and outside the seaway, i.e., shipping lane). Weitendorf points out that this diameter range (20-40 pm) consists almost exclusively of microbubbles (ref. 60,132). (Contrariwise, the range of nuclei with diameters of 20 pm and below, which as noted above were disregarded in Weitendorfs experiments, consists of suspended particles as well as microbubbles (ref. 60,133,134).)... 

McDonough and Hemmingsen (ref. 419) confirm that for bubbles to develop in vertebrates from such low gas supersaturations, some mechanism or structure must promote the initial in vivo bubble nucleations. They cite, as one initial possibility, the popular, general hypothesis that animals contain a reservoir of microscopic gaseous nuclei in the body fluids or tissues, which expand into bubbles when the organism is decompressed (ref. 2). These authors point out that results consistent with this hypothesis have been obtained with shrimp (ref. 429) and rats (ref. 430), where the application of relatively high hydrostatic pressure before decompression apparently reduced the incidence of bubble formation, presumably by forcing potential gas nuclei into solution before they could serve as bubble precursors (ref. 419). 

However, findings that do not support this mechanism include the observations by Harvey et al. (ref. 431) that bubbles do not form in mammalian blood or isolated frog tissues decompressed to 0.031 atm. Steelhead trout fingerlings also are not affected by exposure to 0.16 atm (ref. 432). McDonough and Hemmingsen (ref. 419) argue that in these experiments, the reduction in external pressure should have caused the gas nuclei... 

The gut bubbles in adult brine shrimp did appear, however, to form from gas nuclei (ref. 419) these presumably were incidentally ingested by the animals during filter feeding. Thus a slow compression schedule increased the number of bubbles by apparently preserving nuclei during compression to the equilibration pressure. At each pressure level, gas could diffuse into the gas nuclei, tending to stabilize them against collapse when further compressed. Prepressurization had the opposite effect, as it would tend to reduce the number of bubbles as a result of presumed dissolution of many gas nuclei (ref. 419 see also Sections 1.3.1 and 1.4.3). 

D.C. Pease and L.R. Blinks, Cavitation from solid surfaces in the absence of gas nuclei, J. Phys. Colloid Chem. 51 (1947) 556-567. 

L.H. Bemd, Cavitation, tensile strength, and the surface films of gas nuclei, in R.D. Cooper and S.W. DorofF (Eds.), Sixth Symposium on Naval Hydrodynamics, Office of Naval Research, Wash. D.C., 1966, pp. 77-114. 

D. K. Barnes, Bubble formation in animals, II, Gas nuclei and their distribution in blood and tissues, J. Cell. Comp. Physiol. 24 (1944) 23-34. 

Based on the Ga quadrupolar interaction and the orientation dependence of the (hyperfine broadened) central linewidth, Koschnick et al argue that their ENDOR signal is due to an interstitial Ga atom which is the residual donor in their film [59], Glaser and co-workers argue that the Ga nuclei observed in their resonance... 

Table 2 Summary of properties for magnetically active quadruploar ga nuclei...

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