Nucleation of bubbles

The evolution of gases, such as in dre example given above of dre formation of CO(g) in dre U airsfer of sulphur between carbon-saturated iron and a silicate slag, requires dre nucleation of bubbles before dre gas can be eliminated from the melt. The possibility of homogeneous nucleation seems unlikely, and the more probable source of gas bubbles would either be at the container ceramic walls, or on detached solid particles of the containing material which are... 

Judd (1989) interpreted experimental results of Ibrahim and Judd (1985), in which the bubble period first increased and then decreased as subcooling varied over the range 0 < (7 t - Tm) < 15°C (27°F), by means of a comprehensive model incorporating the contributions of nucleate boiling, natural convection, and microlayer evaporation components. The mechanism responsible for the nucleation of bubbles at exactly the frequency required at each level of subcooling is the subject of their continuing research. 

Free Energy of a Cluster For clarity of discussion, crystal nucleation from a melt is used to derive the following relations. For nucleation of liquid droplets, the derivation is similar. For nucleation of bubbles, the formulation is slightly different and is summarized separately below. Let the Gibbs free energy difference between the crystalline and the melt state per mole of the crystalline composition be AGc m = where Hc and /im are the chemical potential (partial... 

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. 

E.A. Hemmingsen, Effects of surfactants and electrolytes on the nucleation of bubbles in gas-supersaturated solutions, Z. Naturforsch. A33 (1978) 164-171. 

Another technique to expedite the transport of the volatile components from the molten polymer is to increase the number and rate of bubbles formed [14], Techniques that have been used to increase the number of bubbles and their rate of formation (nucleation) are the addition of chemical nucleating agents [15] and ultrasound [16]. Nucleation of bubbles in the molten polymer can help expedite the achievement of equilibrium in conventional falling strand devolatilizers. However, this facilitation mechanism cannot get below equilibrium and thus has minimal value. 

The effect of NaCl on bubble nucleation in the presence of hydrophobic surfaces has also been examined. Excess nitrogen gas was dissolved in solution by equilibration under 25 atmospheres of pressure. Immediately following decompression the solution was supersaturated with nitrogen gas. In water and 0.02M NaCl, it was found that bubbles nucleated quickly (<25 sec) at a (hydrophobic) teflon surface. However, a 0.20M solution of NaCl was found to inhibit bubble formation. In ref>eat experiments, bubbles were found to form at the same sites on the hydrophobic surface. It would appear that the microstructure of the surface is important for the nucleation of bubbles. Microscopic surface cracks would present hydrophobic surfaces at very close separations, enabling nucleation to occur more readily. 

Fillers in these systems affect two types of nucleation nucleation of bubble formation and nucleation of crystallization. Nucleation of bubble formation affects the density of foam. Nucleation of crystallization affects the balance of gas formation and phase transition. The timing of both processes is critical. ... 

Processes of heat transfer accompanied by phase change are more complex than simple heat exchange between fluids. A phase change involves the addition or subtraction of considerable quantities of heat at constant or nearly constant temperature. The rate of phase change may be governed by the rate of heat transfer, but it is often influenced by the rate of nucleation of bubbles, drops, or crystals and by the behavior of the new phase after it is formed. This chapter covers condensation of vapors and boiling of liquids. Crystallization is discussed in Chap. 27. 

Nucleation theory has been advanced for vaporization of pure substances1 and for nucleation of bubbles from solutions containing dissolved gas.2 Bubbles of a critical radius and larger grow while bubbles having radii less than this dimension tend to decay. The result of nucleation theory is the prediction of the maximum attainable limit of supersaturation. Two equations are sufficient for this... 

Fabrication of glassy carbon materials is a relatively straightforward, but time consuming process. A preformed polymeric precursor such as phenol-formaldehyde, polyfurfuryl alcohol, polyvinyl alcohol or oxidized polystyrene is slowly heated in an inert atmosphere to a high temperature in excess of 2000 °C. Heating times may be as short as a day or as long as one month. It is not unusual to encounter exothermic temperature regions that must be traversed very slowly (i.e., 1 °C temperature increase per hour) to avoid the nucleation of bubbles. 

Both the nucleation of supercritical anti-solvent bubbles in a polymer+organic solvent-rich phase in the supercritical anti-solvent process (SAS) (or, equivalently, precipitation with a compressed antisolvent PCA) (e.g., [76]) and the nucleation of bubbles of a dissolved supercritical fluid from a saturated and nozzle-expanded solution containing a solute to be precipitated, in the formation of particles from gas-saturated solutions (PGSS) [77] are bubble nucleation problems, to which the above ideas apply. In the latter case, the nucleation of bubbles occurs simultaneously with that of solid particles within the bulk supersaturated solution. 

Holden, B.S.,and Katz, J.L. (1978) The homogeneous nucleation of bubbles in superheated binary liquid mixtures, AIChEJ. 24, 260. 

Figure 11.29 schematically illustrates the described processes that lead to formation of a hydrogen bubble. It also indicates that the adsorbed hydrogen atoms formed electrochemically can react in two ways, which are in competition they may form molecules that dissolve into the electrolyte, eventually forming a bubble, or they may diffuse into the metal as atoms. The presence of adsorbed surfactants and the surface roughness of the electrode can influence the nucleation of bubbles and, as a result, the effective hydrogen pressure. 

The reduction in surface tension is highly significant for the rate of nucleation of bubbles, because the surface tension appears to the third power in the exponent in the expression for the nucleation rate, Eq. (6). With dilute gas solutions, although this effect is important, it may not be dominant. However, as the gas concentration increases, so the importance of the surface tension term is increased. 

Data from Lubetkin and Akhtar [28] are shown for the nucleation of bubbles of ethene from a supersaturated solution in cyclohexane, plotted according to Eq. (6). The Yaxis is in arbitrary units. The two curves are for nucleation on a Pyrex glass surface and a stainless-steel surface. The corresponding advancing contact angles were about 5° and 25°, respectively (Fig. 21). 

At the present, the generally accepted mechanism for nucleation of bubbles (Fig. 6) suggests that gas trapped in small angle crevices of particulate contaminants expands and contracts with the acoustic cycle. Free air bubbles would not be expected to act as nucleation sites on the basis that they are inherently unstable under these conditions and would be expected to dissolve as a result of surface tension. As the bubble volume grows two possibilities arise on the one hand, small gas bubbles may be released into the surrounding liquid and on the other, implosive collapse of the bubble will release a stream of microcavities at which nucleation can occur. 

When the evolving gas flow is relatively small, the nucleation of bubbles may become a limiting factor, particularly in deep liquid layers. This may be a problem when the chemical reaction is reversible (e.g., esterifications). Mechanical stirring or stripping with an inert gas may be helpful (section 4.6.1.3). Another alternative is the use of a liquid film reactor (section 4.6.3.1). 

Finally, we should draw attention to the prevalent use of air-degassed crude oil systems and foam generation by sparging at ambient temperatures and pressures. It is known that solubilities of asphaltenes, resins, PDMS, and PDMS derivatives are likely to be influenced by temperature and dissolution of natural gas. Moreover, sparging represents a poor model for foam generation in gas-oil separators, which involves depressurization and nucleation of bubbles. Use of apparatus designed to replicate the conditions in actual gas-oil separators for basic studies should therefore be encouraged. 

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