Foam preventatives, definition

By definition osmotic pressure equals the pressure that has to be applied onto a mobile semi-permeable membrane (filter) separating foam and liquid in order to prevent liquid phase from entering the foam. This pressure can be calculated as the difference between pressure of liquid column and pressure of foam column, both having the same height [84]... 

The scaled surface area and its variation with d> are of crucial importance in the definition and evaluation of the osmotic pressure , H, of a foam or emulsion. We introduced the concept in Ref 37, where it was referred to as the compressive pressure , P. It has turned out to be an extremely finitful concept (22,27,38). The term osmotic was chosen, with some hesitation, because of the operational similarity with the more familiar usage in solutions. In foams and emulsions, the role of the solute molecules is played by the drops or bubbles that of the solvent by the continuous phase, although it must be remembered that the nature of the interaetions is entirely different. Thus, the osmotic pressure is denned as the pressure that needs to be applied to a semipermeable, freely movable membrane, separating a fluid/fluid dispersion from its continuous phase, to prevent the latter from entering the former and to reduce thereby the augmented surface free energy (Fig. 4). The membrane is permeable to all the components of the continuous phase but not to the drops or bubbles. As we wish to postpone diseussion of compressibility effects in foams until latter, we assume that the total volume (and therefore the volume of the dispersed phase) is held constant. 

The previous definitions aid in the understanding of the chemistry and engineering of foams and are critical when dealing with the formation and stability of foam structure. Persistent or stable foams consist of a network of thin liquid films, which exhibit complex hydrodynamics. For a drilling foam to remain persistent, or stable, several mechanisms are required to prevent the loss of liquid and gas from the foam and to prevent premature collapse of the foam, when subjected to environmental stresses. 

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