Foam persistence

Foam persistence increases with rise in BW TDS because the bubbles are stabilized by the combined repelling forces of electrical charges at the steam-water interface that result from the high concentration of dissolved salts. The repulsion effect of similar charges prevents bubble thinning, bubble rupture and coalescence mechanisms from taking place. 

Foaming capability relates to both foam formation and foam persistence. Surface tension lowering is necessary, but not sufficient. Other important factors include surface elasticity, surface viscosity, and disjoining pressure [303], Considering stabil-... 

In summary then, the factors promoting foaming in aqueous surfactant solutions are (1) low equilibrium surface tension, (2) moderate rate of attaining equilibrium surface tension, (3) large surface concentration of surfactant, (4) high bulk viscosity, (5) moderate surface viscosity, and (6) electrical repulsion between the two sides of the foam lamella. The first three promote film elasticity the last three promote foam persistence. 

Foaming Special procedures are used in many industries to measure this property. A convenient method is to shake a measured quantity of liquid under arbitrarily established conditions and measure the height of foam and also the length of time the foam persists. The test is applied to the original liquid and to the carbon-treated filtrates. As it may be difficult to determine the relation of height and persistency of foam to concentration, the data are best plotted as in Figure 14 2. 

Proteins in solution reduce the surface tension of the water and consequently aid in formation of food foams. Measure foam persistence, foaming power, and foam stiffness—whipped toppings, frozen desserts, chiffon mixes, angel food cakes, meringues... 

For liquid-gas combinations which tend to foam excessively, high gas velocities may lead to a condition of priming, which is also an inoperative situation. Here, the foam persists throughout the space between trays, and a great deal of liquid is carried by the gas from one tray to the tray above. This is an exaggerated condition of entrainment. The liquid so carried recirculates between trays, and the added liquidhandling load increases the gas pressure drop sufficiently to lead to flooding. 

The understanding of foam persistence, or stability, and bubble coalescence is an analytic dilemma. Unquestionably, this is a dynamic... 

The ability to control foam stability or coalescence rate of bubbles is important in many industrial applications. Foams can persist for a few minutes to several days depending on storage conditions. To effectively utilize foams in any of these situations, it is important to have some control over their stability. Therefore it is very important to deepen our understanding of the mechanisms involved in foam persistence and decay. Most works in this area have been rather empirical and many experimental data are rendered useless because important parameters such as bubble size have not been measured. In this chapter we attempt to summarize the quantitative analysis on foam film stability in aqueous systems in terms of surface tension measurements. 

As we have seen, crude oil foams are usually transient (see, e.g., reference [31]). The absence of significant positive contributions to disjoining pressures means that foam collapse is inevitable when the constituent films drain to their so-called critical thickness. This means that the process of film drainage largely determines foam persistence. In tnrn, that process is usually dominated by the tangential stress boundary condition, Eqnation 1.1, which equates the viscous shear stress to the surface tension gradient in the draining foam film. 

---