Drop breakup dilute

Similar experiments were carried out in which drops that were mixtures of /i-decane and various alcohols were injected into dilute solutions of a zwitterionic (amine oxide) surfactant. Here, too, the lamellar phase was the first intermediate phase observed when the system was initially above the PIT. However, with alcohols of intermediate chain length such as /i-heptanol, it formed more rapidly than with oleyl alcohol, and the many, small myelinic figures that developed broke up quickly into tiny droplets in a process resembling an explosion.The high speed of the inversion to hydrophilic conditions was caused by diffusion of n-heptanol into the aqueous phase, which is faster than diffusion of surfactant into the drop. The alcohol also made the lamellar phase more fluid and thereby promoted the rapid breakup of myelinic figures into droplets. Further loss of alcohol caused both the lamellar phase and the remaining microemulsion to become supersaturated in oil, which produced spontaneous emulsification of oil. 

In this part, the breakup of polymer drops wUl be discussed, initially dealing with dilute systems (isolated drops) and subsequently with concentrated dispersions where coalescence is of equal importance. Dispersion in Newtonian systems was discussed in 7.S.2.2. Emulsion microrheology. 

Lyngaae-j0rgensen et al. [1993] developed a predictive model of morphology variation during simple shear flow of dilute polymer blends. The model considers the balance between the rate of breakup and the rate of drop coalescence. It was assumed that (i) The viscosity and elasticity of the dispersed phase are significantly lower than those of the matrix, (ii) Only the cylindrical, large drops (defined by the long and short semiaxes a and df) are able to break and form small drops,... 

The theoretical predictions of drop deformation and breakup are limited to infinitely diluted, monodispersed Newtonian systems. However, it is possible to obtain valid relationships between processing parameters and morphology. Thus it was found that in the system PS/HDPE the viscosity ratio, blend composition, screw configuration, temperature, and screw speed significantly influence the blend morphology [Bordereau et al., 1992]. For more detail on the topic see Chapter 9, Compounding Polymer Blends, in this Handbook. 

There is still another complication. The microrheology has been developed for infinitely diluted systems. Many experimental studies have shown that during the dispersion processes the drop size decreases until an equilibrium value is reached. Its experimental value is usually larger than predicted. The difference, originating in drop coalescence, increases with concentration [Huneault et al., 1993], The coalescence is enhanced by the same factors that favor the breakup, i.e., high shear rates, reduced dispersed-phase viscosity, convergent flow, etc. 

Simple theories are described in which breakup results when disruptive forces in the surrounding fluid exceed cohesive forces, due to interfacial tension and drop viscosity. The results for a single drop are then extended to dilute dispersions in order to predict and correlate data for the DSD. The methodology is extended to more concentrated noncoalescing systems of wider practical importance as well as other dispersion devices. The scope includes a broad range of factors. Although most of the section is devoted to the development of the equilibrium mean drop size and DSD, dispersion kinetics and the time evolution of the DSD are included. 

Effect of Surfactants. For dilute dispersions, the presence of surfactants influences drop size only by reducing interfacial tension. To a first approximation, the drop size may be estimated within the framework developed above using the static interfacial tension in the presence of surfactant. However, drop stretching and breakup occur rapidly. As new interface is created, the rate at which surfactant diffuses to the surface may not be sufficient to maintain a constant interfacial tension. The dynamic a will vary from the static value in the presence of a surfactant to the valne for a clean interface. Phongikaroon (2001) found that for this reason, drop sizes prodnced in a rotor-stator mixer with a surfactant-laden system of known static a were larger than those produced for a clean system of the same o. 

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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