Foam Coalescence

Foam Coalescence Coalescence is of two types. The first is the growth of the larger foam bubbles at the expense of the smaller bubbles due to interbubble gas diffusion, which results from the smaller bubbles having somewhat higher internal pressures (Adamson, The Physical Chemlstiy of Suifaces, 4th ed., Wiley, New York, 1982). Small bubbles can even disappear entirely. In principle, the rate at which this type of coalescence proceeds can be estimated [Ranadive and Lemhch,y. Colloid Inteiface Sci., 70, 392 (1979)]. 

To explain the role of the medium capillary pressure upon foam coalescence, consider a flat, cylindrical, stationary foam lamella of thickness, 2h, circa 1000 A, and radius, R (i.e., 50 to 100 /xm), subject to a capillary pressure, P, at the film meniscus or Plateau border, as shown in Figure 3. The liquid pressure at the film meniscus is (P - P ), where P is the gas pressure. g c g... 

The merging of two or more dispersed species into a single one. Coalescence reduces the total number of dispersed species and also the total interfacial area between phases. In emulsions and foams coalescence can lead to the separation of a macrophase, in which case the emulsion or foam is said to break. The coalescence of solid particles is termed sintering . 

Whatever the cause, coalescence within the rising foam furnishes internal reflux that enriches the foam and, therefore, increases the solute concentration in the overflow foam stream. Foam coalescence due to external reflux will be discussed later. 

Foam Destruction. Net foam generation cannot continue unchecked. It is balanced by foam destruction processes. Chambers and Radke (26) enunciated two basic mechanisms of foam coalescence capillary-suction and gas diffusion. Because capillary-suction coalescence is the primary mechanism for lamellae breakage, we focus on it, and only briefly touch upon foam coarsening by gas diffusion. 

Gas Diffusion. The second mechanism for foam coalescence in porous media, gas diffusion, pertains primarily to the stagnant, trapped bubbles. According to the Young—Laplace equation, gas on the concave side of a curved foam film is at a higher pressure and, hence, higher chemical potential than that on the convex side. Driven by this difference in chemical potential, gas dissolves in the liquid film and escapes by diffusion from the concave to the convex side of the film. The rate of escape is proportional to film curvature squared and, therefore, is rapid for small bubbles (16, 26). 

Total amount of blood in foam coalescer after 125 min = 2 grams. 

Some simulation results for foam coalescence and collapse are presented below. 

The foam coalescence process can be separated into two stages drainage and film rupture. 

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