Deformation during emulsification

Another role of the surfactant is to initiate interfacial instability, e.g., by creating turbulence and Raykleigh and Kelvin-Helmholtz instabilities. Turbulence eddies tend to disrupt the interface since they create local pressures. Interfacial instabilities may also occur for cylindrical threads of disperse phase during emulsification. Such cylinders undergo deformation and become unstable under certain conditions. The presence of surfactants will accelerate these instabilities as a result of the interfacial tension gradient. 

These are dispersions of liquid drops in an immiscible liquid medium. The most common systems are oil-in-water (O/W) and water-in-oil (W/O). It is also possible to disperse a polar liquid into an immiscible nonpolar liquid, and vice versa these are referred to as oil-in-oil (0/0) emulsions. In order to disperse a liquid into another immiscible liquid, a third component is needed that is referred to as the emulsifier. Emulsifiers are surface-active molecules (surfactants) that adsorb at the liquid/liquid interface, thus lowering the interfacial tension and hence the energy required for emulsification is reduced. The emulsifier plays several other roles (i) it prevents coalescence during emulsification (ii) it enhances the deformation and break-up of the drops into smaller units (iii) it prevents flocculation of the emulsion by providing a repulsive barrier that prevents close approach of the droplets to prevent van der Waals attraction (iv) it reduces or prevents Ostwald ripening (disproportionation) (v) it prevents coalescence of the drops and (vi) it prevents phase inversion. 

FIGURE 18.2 Scheme for particle deformation and breakup during emulsification or foaming. 

The presence of a surfactant means that, during emulsification, the interfacial tension need not be the same everywhere (Figure 6.15). This has two consequences (1) the equilibrium shape of the drop is affected (2) any y-gradient formed will slow down the motion of the liquid inside the drop (this diminishes the amount of energy needed to deform and break-up the drop). 

Interfacial instabilities may also occur for cylindrical threads of disperse phase that form during emulsification or when a liquid is injeded into another from small orifices. Such cylinders undergo deformation [45-48] and become unstable... 

The surfactant plays many roles, one of which is lowering interfacial tension, hence p, hence the stress needed to deform and break up a droplet. It is essential that the surfactant prevents coalescence of newly formed drops. The various processes occurring during emulsification, i.e. droplet break-up, adsorption of surfactant and droplet collisions (which may or may not lead to coalescence), are illustrated in Figure 2.2. Typically, each of these processes occurs numerous times during emulsification, and the timescale for each step is very small, for instance a microsecond. 

Various processes take place during emulsification [45] breakup of droplets, adsorption of sinfactant molecules, and droplet collisions (which may lead to coalescence and larger droplets). These processes may occur simultaneously during emulsification, as the time scale for each step is very small (microseconds). Breaking of drops is feasible if the deforming force exceeds the Laplace pressure, / l (the difference between the pressure inside and outside the droplet), which is the interfacial force that acts against droplet deformation ... 

Emulsions are commonly produced at well-eads during primary and secondary (waterflood) oil production. For these processes the emulsification is usually not attributed to formation in reservoirs, but rather to formation in, or at the face of, the well-ore itself [693]. However, at least in the case of heavy-il production, laboratory [694] and field [695,696] results suggest that water-in-oil emulsions can be formed in the reservoir itself during water and steamflooding. Energy is needed for emulsification, partly because of the increased surface area that is created in forming small droplets and partly because deformation of large drops is needed before smaller... 

Emulsions allow for evaluating fhe influence of stresses acting tm the liquid during the atoinizatitHi process, as inner drops may deform and breakup. Two different kinds of emulsions were prepared by emulsification with a colloid mill. A subsequent dilution step with an aqueous solution of a thickener allowed adjusting the disperse phase cmitent at cmistant inner oil drop size. The emulsifier was always adjusted to the disperse phase content. 

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