Microbubbles acoustical measurements

Separate from his earlier-described acoustical measurements, Medwin (ref. 31) conducted a direct photographic determination of microbubble populations in coastal ocean waters. His results indicated the presence of several million microbubbles per cubic meter (i.e., >103/liter) within the diameter range of 20 to 100 pm, both at the surface and at a depth of 10 feet in Monterey Marina (ref. 31,33). However, as a result of his technique of illumination, Medwin suggested that his count probably included many non-bubbles (ref. 31). 

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). 

Ultrasound contrast agents contain microbubbles of air, nitrogen or fluorocarbon gas coated with a thin shell of material such as albumin, galactose, or lipid. They have an markedly different acoustic impedance compared with blood or tissue. The contrast is usually injected intravenously and must measure less than 7 pm if they are to cross the lungs and reach the arterial circulation. They remain intact for only a short time. 

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