Hydrodynamics of Foams. Syneresis and Stability

Preliminary remarks. Foam is a fluid multiphase continuous medium possessing an internal structure. The disperse phase consists of gaseous bubbles capsulated into multilayer elastic envelopes consisting of some adsorption layers of surfactants and submersed into a continuous phase. The capsules are in a close contact with one another so that they are locally deformed and form a structure with an anisotropic distribution of the continuous phase around each cell. 

In contrast with two-phase bubble-containing fluids, aerosols, and emulsions, foam has a least three phases. Along with gas and the free continuous liquid phase, foam contains the so-called skeleton phase, which includes adsorption layers of surfactants and the liquid between these layers inside the capsule envelope. The volume fraction of the skeleton phase is extremely small even compared with the volume fraction of the free liquid. Nevertheless, this phase determines the foam individuality and its structure and rheological properties. It is the frame of reference with respect to which the diffusion motion of gas and the hydrodynamic motion of the free liquid can occur under the action of external forces and internal inhomogeneities. At the same time, the elements of the skeleton phase themselves can undergo strain and relative displacements as well as mass exchange with the other phases (solvent evaporation and condensation and surfactant adsorption and desorption). 

The evolution of a foam system, that is, the spatial redistribution of the substance takes place in regions of very complicated geometric and topological 

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