Bubble nucleation homogeneous

Generation Spontaneous generation of gas bubbles within a homogeneous liquid is theoreticaUy impossible (Bikerman, Foams Theoiy and Industrial Applications, Reinhold, New York, 1953, p. 10). The appearance of a bubble requires a gas nucleus as avoid in the liquid. The nucleus may be in the form of a small bubble or of a solid carrying adsorbed gas, examples of the latter being dust particles, boiling chips, and a solid wall. A void can result from cavitation, mechan-ic ly or acoustically induced. Blander and Katz [AlChE J., 21, 833 (1975)] have thoroughly reviewed bubble nucleation in liquids. 

Bubble Nucleation in a Liquid Phase The above classical nucleation theory can be easily extended to melt nucleation in another melt. It can also be extended to melt nucleation in a crystal but with one exception. Crystal grains are usually small with surfaces or grain boundaries. Melt nucleation in crystals most likely starts on the surface or grain boundaries, which is similar to heterogeneous nucleation discussed below. Homogeneous nucleation of bubbles in a melt can be treated similarly using the above procedures. Because of special property of gases, the equations are different from those for the nucleation of a condensed phase, and are hence summarized below for convenience. 

The nucleation temperature, which exceeds the boiling point of the species, is the temperature at which bubbles spontaneously appear in the liquid. Bubble nucleation is a rate process, and its description on the basis of a nucleation temperature is a simplification. Homogeneous nucleation temperatures are substantially above the boiling point heterogeneous nucleation—aided, for example, by impurities like dust—may occur at somewhat lower temperatures that nevertheless still exceed the boiling point. 

In theory it is possible to nucleate bubbles either in the bulk phase or at solid surfaces as a result of statistical density fluctuations. In practice, the theoretical and measured fracture pressures of pure liquids are far in excess of those corresponding to the superheats or supersaturations for vapor or gas bubble nucleation experimentally observed in engineering systems (F2, F7, Kl). On the other hand, conditions for homogeneous nucleation become favorable at extremely high superheats in the presence of ionizing radiation (G4, G5). The latter observation led to the introduction of the liquid-hydrogen bubble chamber. A simple explanation of this phenomenon is that the... 

According to the theory proposed by Colton and Suh [54—56], the rate that bubbles nucleate homogeneously (iVhom) is given by... 

FIG. 20 A plot of experimental data from Skripov [6] for the homogeneous boiling of pentane. The part ACB is the normal superheat limit. As the temperature is increased, so the time required for explosive boiling of liquid drops rapidly decreases. In going from 145.6 to 146.0 C, the waiting time drops from 550 to about 0.5 sec. If the superheated liquid droplets are exposed to radiation, very much lower superheats are needed to cause bubble nucleation. Temperatures as low as 129.5 C can cause bubble formation. 

TABLE 8 Homogeneous N2 Bubble Nucleation in Nonaqueous Liquids... 

Several models for foam-based devolatilization are available. The fundamentals are reviewed by Lee [22]. Bubble nucleation during devolatilization of polymer melts has been explained by homogeneous nucleation [23, 24], heterogeneous nucleation [25-27], and a mixed-mode nucleation [28]. Yarin et al. [29] considered a secondary nucleation. 

Typically, physical foaming is a three-step process (1) mixing a blowing gas is dissolved in the polymer to form a homogeneous solution (2) bubble nucleation subsequent pressure release or temperature increase induces phase separation due to the thermodynamic instability, and gas starts to form nuclei and (3) bubble growth and stabilization. 

There are two types of nucleation the homogeneous one and the heterogeneous one. The classic nucleation theory is the approach used to describe bubble nucleation in polymeric foams, although there is a discrepancy attributed to the intervening heterogeneous nucleation. 

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