Microbubbles in fresh water

DEMONSTRATION OF FILM-STABILIZED MICROBUBBLES IN FRESH WATER... 

Additional studies collectively suggest that there are approximately 10 times as many microbubbles in the sea as there are in fresh water, at least within the diameter range of 30-100 pm (ref. 50). It appears that some of these measured microbubbles in the... 

There is other acoustical evidence to support the belief of Sirotyuk and other investigators that stable microbubbles serve as cavitation nuclei in fresh water. As noted by Sirotyuk (ref. 25), numerous experiments have disclosed that the cavitation threshold of water is increased by degassing of the liquid or by the... 

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... 

Experiments to distinguish between these two possibilities have often involved measurements of ultrasonic attenuation (ref. 5,9,31,32). The popularity of this approach derives in part from the fact that small impurities in liquids, such as suspended particles, have negligible influence on attenuation in comparison with even a very small concentration of microbubbles (ref. 9). (Microbubbles, in contrast to solid particles, appreciably increase the compressibility of a liquid, introducing forms of viscous losses and nonreversible energy exchanges that do not exist in the case of solid particles.) It is therefore of considerable interest that all fresh tap water samples measured by Turner (ref. 9) showed substantial and persistent abnormal (ultrasonic) attenuation, amounting to a minimum of 44% over that of distilled water it was concluded that this result stemmed from the presence of stabilized micron-sized bubbles. 

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... 

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