Nucleation, Growth, and Detachment of Bubbles

The last two chapters cover the topics of the production of chlorine and caustic and the phenomena of electrolytic gas evolution. In Chapter 5, Hine et al. describe the engineering aspects of the three processes used in the chlor-alkali industry, and in Chapter 6, Sides reviews the macroscopic phenomena of nucleation, growth, and detachment of bubbles, and the effect of bubbles on the conductivity of and mass transfer in electrolytes. 

The subject of diffusion-controlled bubble growth is, of course, a rather small part of the large subject of bubble dynamics, whose scope is too broad to be included in this review. Specifically excluded are cavitation bubbles, whose collapse is inertia rather than diffusion controlled, the formation and detachment of bubbles from orifices, oscillations of bubbles in a pressure field, and the challenging subject of the mechanism of nucleate boiling heat transfer, in which bubble formation and detachment must certainly play a dominant role. 

The physical process of gas evolution can be divided into three stages nucleation, growth, and detachment. Bubbles nucleate at electrode surfaces from solutions highly supersaturated with product gas and grow by diffusion of dissolved gas to the bubble surface or by coalescence with other bubbles. They detach from the electrode when buoyancy or liquid shearing forces pulling the bubbles away overcome the surface forces binding them. 

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. 

Figure 3.7 Schematic representation of the bubble production cycle (according to [68,81] reprinted with permission from [81] copyright (2002) American Electrochemical Society) (a) a bubble starts growing at a nucleation site b) formation and growth of the bubble (c) equilibrium between the buoyancy force and capillary force is reached (d) bubble detachment.

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... 

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