Bubble nucleation detachment, importance

Bubble Departure Diameter. At a certain point in the bubble growth process, the bubble detaches from the surface and the cycle begins again. Clearly the release diameter of the bubble is an important factor in understanding nucleate boiling. For pool boiling, Carey [4] gives... 

Hot Spot Growth Under a Bubble. When bubbles grow and detach from a nucleation center on a solid surface, evaporation of the liquid layer commonly occurs, separating the bubble from the solid surface. This microlayer evaporation process is particularly important at low pressures When a small zone under the bubble becomes dry as a result of this process, its temperature increases, and this increase can, under certain conditions, be sufficient to prevent rewetting of the surface on bubble departure, leading to a permanent hot spot and onset of the critical phenomenon. 

Electrolytic gas evolution is a complicated and important problem in many industrial processes. The details of bubble formation and the effects of the bubbles presented in this review are but the microscopic aspects of a phenomenon that affects the macroscopic behavior of electrochemical cells. The knowledge summarized here applies in part to the even larger world of boiling liquids in which bubbles also nucleate, grow, coalesce with each other, and detach. In general, the sudden appearance of a phase much less dense than its parent phase will always be an important phenomenon for research because that phase will profoundly affect the process in which it appears. 

Finally, Eq. (18) specifically apphes to the case of mercury as the second liquid phase, and this may be important when attempting to identify the locus of the nucleation event for electrolytic bubbles produced at hquid mercury electrodes. Some generalizations can be made based on the relative values of the various interfacial tensions (1) If a + ab> then it requires less energy to form a bubble homogeneously in A than at the interface thus the nucleation will be homogeneous. (2) If Va b + ab> the bubbles are most stable in hquid B, and tend to detach from the interface. (3) If both conditions Ta < B + 7ab and Tb < a + Tab hold, then bubbles are most stable at the interface, and the nucleation will be heterogeneous. 

When the discussion focuses on the detachment of a succession of bubbles from a source, the composition of the remaining gas in the reservoir will change with time. This point is taken up later. Finally, as was the case for nucleation, the presence of the gas increases the internal pressure sustainable in the bubble, and this may be important to the stability of trapped bubbles. 

The data on boihng of concentrated polymeric solutions " demonstrate that in such sy stems thermodynamic, diffusional and rheological factors are of primary importance. The diagram of the hquid-vapor phase equilibrium is characterized by a decrease in the derivative dp/dT with the polymer concentration (dp/dT 0 at k 0). This leads to increase in both the nucleation energy and the detachment size of a bubble (see the equation [7.2.59]) and, consequently, to reduction of the bubbles generation frequency. Note that in reality the critical work, for a polymeric liquid may exceed the value predicted by the formula [7.2.59] because of manifestation of the elasticity of macromolecules. 

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