Homogeneous Nucleation Hypothesis

5 CLINICAL USE OF INJECTED GAS MICROBUBBLES ECHOCARDIOGRAPHY POTENTIAL FOR CANCER DETECTION 

This last-mentioned successful series of control experiments, involving addition of cavitation nuclei to the cardiovascular system, brings to mind at least one feasible medical application for injected (synthetic) surfactant-stabilized microbubbles (cf. Chapters 9 and 10) as concerns evaluation of cardiovascular function. Various types of size-controlled, nontoxic, synthetic micro- 

Earlier studies (ref. 440-442) with ordinary air microbubbles (without any synthetic surfactant coating) have already shown that echocardiographic contrast produced by microbubbles is useful in the qualitative analysis of blood flow and valvular regurgitation. In addition, quantitative studies (ref. 440) have shown a correlation between individual contrast trajectories on M-mode echocardiography and invasive velocity measurements in human beings. Meltzer et al. (ref. 441) have shown that velocities derived from the slopes of contrast trajectories seen on M-mode echocardiography correlate with simultaneous velocities obtained by Doppler techniques. (This correlation is expected because both measures represent the same projection of the microbubble velocity vector, that is, in the direction of the sound beam.) More detailed studies (ref. 442) confirmed that microbubble velocity obtained from either Doppler echocardiography or M-mode contrast trajectory slope analysis correlates well with actual (Doppler-measured) red blood cell velocity. Thus, these early studies have shown that microbubbles travel with intracardiac velocities similar to those of red blood cells. 

Apart from echocardiography, another promising clinical application of synthetic microbubbles is the ultrasonic monitoring of local blood flow in the abdomen (analogous to the earlier use of gas microbubbles to monitor myocardial perfusion (ref. 443)). Such refined ultrasonic blood flow measurements, utilizing injected 

CONCENTRATED GAS-IN-LIQUID EMULSIONS IN ARTIFICIAL MEDIA. I. DEMONSTRATION BY LASER-LIGHT SCATTERING 

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These results stimulated a number of studies, both in industry (Conoco, Esso, Shell Pipeline) and in academia (University of Maryland, M.I.T.). The objective was, primarily, to delineate the mechanism that led to these explosive events. The results of many small-scale experiments, primarily conducted by Shell Pipeline Corporation and M.I.T., led to the hypothesis that the apparent explosion was, in fact, a very rapid vaporization of superheated LNG. Contact of LNG, of an appropriate composition, with water led to the heating of a thin film of the LNG well above its expected boiling temperature. If the temperature reached a value where homogeneous nucleation was possible, then prompt, essentially explosive vaporization resulted. This sequence of events has been termed a rapid phase transition (RPT), although in the earlier literature it was often described by the less appropriate title of vapor explosion. 

Assuming the argument is valid, it would then be possible to contact fused NaCl (or, presumably NaOH, Na2S, or smelts with these constituents) with water and to state that the resulting explosion stemmed from a homogeneous nucleation of a solution of salt in water. Their hypothesis therefore explains qualitatively the effect of variations in smelt composition on explosivity. It also clarifies the result that green liquor normally explodes more violently than pure water since, in the former, there are dissolved salts (of the NaCl type) to enhance the salt effect at the interface. 

Becker-Doiing nucleation hypothesis indicates a much larger number, of the order of 100, for the critical cluster. Klein and Driy, in nucleation studies combining the drop method and homogeneous precipitation, found the rate of nucleation of strontium sulfate to depend on the 27th power of the concentration, indicating a nucleus containing 52 ions. 

This misconception is particularly common in crystallization. The hypothesis of a perfectly mixed system is, for crystallization and precipitation processes, labeled as mixed-suspension, mixed-product removal (MSMPR). With diis model the crystalUzer is modeled with a spatially homogeneous NDF, generally called the crystal-size distribution (CSD). However, the fact that the CSD is constant through the vessel does not mean that the rates of crystal nucleation, molecular growth, aggregation, and breakage are constant. 

The approximate solution of the evolution equation for P (x, y t) based on the hypothesis of low coupling between cells shows that the transition between two homogeneous stationary states occurs via an inhomogeneous transient state. Transient inhomogeneous structures may be also responsible for the stabilisation of an unstable state. Such kinds of phenomena are reminiscent of nucleation processes (Blanche, 1981 Frankowicz, 1984) and might be considered as a nonequilibrium analogue of an equilibrium phenomenon. 

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