Phase continuity factors determining

On the other hand, there are two significant differences between macroemulsions and foams (1) The surfactants in the inter-facial film of a foam cannot dissolve in the dispersed (gas) phase, while in a macroemulsion the solubility of the surfactants in the liquid being dispersed is a major factor determining the stability of the emulsion. (2) In macroemulsions, both oil and water can serve as the continuous phase, i.e., both O/W and W/O emulsions are commonly encountered, while in foams, only the liquid acts as the continuous phase. 

Physical Nature of the Interfacial Film The droplets of dispersed liquid in an emulsion are in constant motion, and therefore there are frequent collisions between them. If, on collision, the interfacial film surrounding the two colliding droplets in a macroemulsion ruptures, the two droplets will coalesce to form a larger one, since this results in a decrease in the free energy of the system. If this process continues, the dispersed phase will separate from the emulsion, and it will break. The mechanical strength of the interfacial film is therefore one of the prime factors determining macroemulsion stability. 

Stability of a macroemulsion is an important factor as this determines its extent of usability for particle preparation or various other applications. Instability is basically coalescence of the dispersed phase droplets or Ostwald ripening (growth of large droplets at the expense of much smaller ones). When this process goes on, the emulsion eventually breaks into two layers. Other processes related to stability but considered less important [3] are (a) creaming or sedimentation, the rate of which is dependent on the difference in density between the continuous and dispersed phases, droplet size, viscosity of the continuous phase and interdroplet interaction and (b) flocculation, dependent on colloidal interactions between the droplets [8, 12]. Several factors determine the stability of macroemulsions these are discussed here in brief. This discussion is largely derived from Rosen [3] and some subsequent investigations [e.g. 6, 7, 13-15]. 

The role of coalescence within a contactor is not always obvious. Sometimes the effect of coalescence can be inferred when the holdup is a factor in determining the Sauter mean diameter (67). If mass transfer occurs from the dispersed (d) to the continuous (e) phase, the approach of two drops can lead to the formation of a local surface tension gradient which promotes the drainage of the intervening film of the continuous phase (75) and thereby enhances coalescence. It has been observed that d-X.o-c mass transfer can lead to the formation of much larger drops than for the reverse mass-transfer direction, c to... 

Flat Sheets. Generally, the interface between a liquid sheet and air can be perturbed by aerodynamic, turbulent, inertial, surface tension, viscous, acoustic, or electrical forces. The stability of the sheet and the growth rate of unstable disturbances are determined by the relative magnitude of these forces. Theoretical and experimental studies 255112561 on disintegration mechanisms of flat sheets showed that the instability and wave formation at the interface between the continuous and discontinuous phases are the maj or factors leading to... 

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