Phase dispersion droplet breakup

Atomization. A gas or Hquid may be dispersed into another Hquid by the action of shearing or turbulent impact forces that are present in the flow field. The steady-state drop si2e represents a balance between the fluid forces tending to dismpt the drop and the forces of interfacial tension tending to oppose distortion and breakup. When the flow field is laminar the abiHty to disperse is strongly affected by the ratio of viscosities of the two phases. Dispersion, in the sense of droplet formation, does not occur when the viscosity of the dispersed phase significantly exceeds that of the dispersing medium (13). 

Wieringa, J.A. Dieren, F. van Janssen, J.J.M. Agterof, W.G.M., 1996, Droplet breakup mechanisms during emulsification in colloid mills at high dispersed phase volume fraction, Chemical Engineering Research Design, 74, 554-562. 

Equation (A12) is widely used in RE, but it does not account for the specific interactions of the dispersed phase. In this respect current research is focused on drop population balance models, which account for the different rising velocities of the different-size droplets and their interactions, such as droplet breakup and coalescence (173-180). 

Indirectly related to the cell models of this section is the work of Davis and Brenner (1981) on the rheological and shear stability properties of three-phase systems, which consist of an emulsion formed from two immiscible liquid phases (one, a discrete phase wholly dispersed in the other continuous phase) together with a third, solid, particulate phase dispersed within the interior of the discontinuous liquid phase. An elementary analysis of droplet breakup modes that arise during the shear of such three-phase systems reveals that the destabilizing presence of the solid particles may allow the technological production of smaller size emulsion droplets than could otherwise be produced (at the same shear rate). 

Figure 9.7 Photographs of droplet shapes in planar extensional flow for various viscosity ratios M of the dispersed to the continuous phase. The droplets are viewed in the plane normal to the velocity gradient direction. The critical capillary numbers Cac and droplet deformation parameters Dc at breakup are also given. The droplet fluids are silicon oils with viscosities ranging from 5 to 60,000 centistokes, while the continuous fluids are oxidized castor oils both phases are Newtonian. (From Bentley and Leal 1986, with permission from Cambridge University Press.)...

In actual situations several processes occur simultaneously. The details of any particular dispersion processes are also affected by the viscosity of each phase, the shear in the system, the interfacial energy, the pressure of solid particles, and dissolved substances. In nonuniform shear flow (e.g., tubular Poiseuille flow), for example, droplet breakup can be related to the bulk rheological properties of the dispersed and continuous phases and the critical Weber number (We ) as shown in Figure 3 (3). The We is a dimensional group defined by... 

Figures 12.1-12.6 show the radical change in EPR particle morphology from reactor powder to pellets, but the relatively static morphology from pellets to fabricated articles. This is due to the great efficiency of commercial-scale corotating twin-screw pelletization extruders (8). The EPR phase is efficiently dispersed and attains the stationary value of particle size, as described by theoretical treatments of droplet breakup and coalescence (13-15). This droplet breakup and coalescence occurs in the molten state of the viscoelastic iPP and EPR, matrix and dispersed phases, in the extruder under a complex strain held, which is a combination of nonuniform, transient shear and elongational helds. Eurther, a variable temperature prohle is used along the barrel of the extruder causing complex variation in the viscoelastic properties of these components.

The dynamics of droplet breakup in squeezing regime can be qualitatively described in the following way as the tip of the dispersed phase enters the main channel, it starts to fill the main cross section, while the hydraulic resistance to flow in the thin films, between the dispersed phase interface and the walls of the obstructed microchannel, creates an additional pressure drop along the growing droplet. 

The type of emulsion that will be formed is also influenced by the critical Weber number [36-38,40]. Figure 4.6 shows that for a given viscosity ratio, 172/171, between the dispersed (172) and continuous (171) phases, reducing the interfacial tension increases the Weber number, and lowers the energy needed to cause droplet breakup (see [37,40]). For a given flowing system involving a viscous oil, the viscosity ratio will be smaller, and an emulsion is easier to form, if it is a W/0 emulsion rather than an 0/W emulsion. 

It is clear that this phenomenon is phase morphology-dependent. Only in those blends where the minor phase is dispersed into sufficiently fine droplets, this phase has the opportunity to exhibit fractirMiated crystallization. Hence, only at low blend compositions and/or good matching viscosities of both phases (where the capillary number C predicts droplet breakup being dominant above coalescence) the occurrence of coincident crystallization is possible. 

The steady-state deformation of isolated droplets decreases with increasing dispersed phase elasticity for the same imposed capillary number. A linear relationship between critical capillary number for droplet breakup (Kn-i,) and dispersed-phase Weissenberg number (Wi[Pg.934]

Size of Dispersed Dropiets in Fiow. The effects of the properties of blend components and mixing conditions on fineness of the phase structure are well understood quahtatively for binary polymer blends with dispersed structure. It is broadly accepted that the size of dispersed droplets in flowing blends is controlled by the competition between the droplet breakup and coalescence (70,90-94). On the other hand, the droplet breakup and coalescence in blends with viscoelastic components are complex events described only approximately. Moreover, the flow field in mixing devices is also complex, which fiirther complicates correct description of the phase structure development (70,92-94). 

From the above equations it is possible to calculate the size of the largest drop that exists in a fluid undergoing distortion at any shear rate. In these equations, the governing parameters for droplet breakup are the viscosity ratio p (viscosity of the dispersed phase to that of the matrix) the type of flow (elongational, shear, combined, etc.) the capillary number Cfl, which is the ratio between the deforming stress (matrix viscosity x shear rate) imposed by the flow on the droplet and the interfacial forces a/R, where ais the interfacial... 

Experimental work by Sundararaj and Macosko, nicely contrasted the competing effects of droplet breakup and droplet coalescence in both Newtonian and non-Newtonian mixtures. They concluded that the extent of interfacial tension reduction due to the presence of block copolymer was insufficient to be the primary reason for the reduction of the droplet size, and the primary effect of the copolymer was to prevent droplet coalescence through steric stabilization of the droplets. Sundararqj and Macosko also noted that the droplet size as a function of shear rate for a pure blend decreased to a minimum value, then increased at higher shear rates. No data was given for the compatibilized blend and they referred to this shear rate dependence of the dispersed phase size for the pure blend as "anomalous". Sundararaj and Macosko noted that this anomalous behavior has been observed previously by... 

Principal concern of this study was to understand and limit the dispersing characteristics of an emulsion treated by an air-assisted atomizer, where consequently the interfacial tension plays another important role. Dispersion or breakup of the internal emulsion droplets (primary or secondary) during spraying is mainly a function of interfacial tension (determining the energy required to deform and increase the interface) and the viscous flow stresses (depending on the applied velocity field and the viscosity ratio A of dispersed to continuous phase). Therefore, interfacial tension was measured for SEs (SE interface between continuous and disperse fluid phases) and DEs (DE interface between continuous and dispersed... 

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