Drop coalescence, mechanism

Silicones exhibit an apparently low solubility in different oils. In fact, there is actually a slow rate of dissolution that depends on the viscosity of the oil and the concentration of the dispersed drops. The mechanisms of the critical bubble size and the reason a significantly faster coalescence occurs at a lower concentration of silicone can be explained in terms of the higher interfacial mobility, as can be measured by the bubble rise velocities. 

MECHANISMS OF DROP DISPERSION, DROP COALESCENCE, AND PHASE INVERSION... 

The second generation model comprised more refined flow analysis, two mechanisms of dispersion (the fibrillation mechanism and a drop splitting mechanism for low supercritical capillary numbers, with the choice of break-up mechanism based on locally computed microrheological criteria), as well as coalescence effects [Huneault et al., 1995a]. The latter effects were taken into account by determining the coalescence constant in Equation 7.117 from the plot shown in Figure 7.18. Thus, the developed model was self-consistent, fully predictive, without any adjustable parameters. 

Since derivation of this relation considered only the drop-splitting mechanism and neglected coalescence, its validity may be limited to small capillarity numbers, K = 1-2, and low concentrations. 

Twin-screw extruder Extension of the model developed by Shi and Utracki. Better fluid mechanics computations and microrheological drop fracture mechanisms were used. The model takes into account the coalescence. Huneault et al., 1995a... 

In most practical cases, it is not the volume distribution of drop concentration n( V, t) that is of greatest interest, but rather some of its moments or their combinations, which have clear physical meaning and can rather easily be found experimentally. Experimental results can give the answer to a question whether these moments agree with an accepted model of drop interactions, since the choice of the mechanism of drop coalescence is, as a rule, based on a number of assumptions. 

In the case of low interfacial coverage with surfactant, the collision of two emulsion drops (step A—in Fig. 2) usually terminates with their coalescence (step B—>C in Fig. 2). The merging of the two drops occurs when a small critical distance between their surfaces, hj. is reached. Sometimes, depending on the specific conditions (larger drop size, attractive surface forces, smaller surface tension, etc., — see, e.g.. Ref. 2), the approach of the two drops could be accompanied with a deformation in the zone of their contact (step B—>D in Fig. 2) in this way a liquidfilm of almost uniform thickness h is formed in the contact zone. This film could also have a critical thickness h, of rupture in fact, the film rupture is equivalent to drop coalescence (see step D—>C in Fig. 2). The mechanisms of coalescence... 

It is widely recognized that the size of the drops entering the separator is a parameter of great importance, since it will affect both the settling and coalescence mechanics inside the vessel. The drop size at a given point in the process is dependent on the turbulent fluctuations, the history of the fluids up to that point, and the physical properties of the mixtiue. Traditionally, the existence of a specific drop size equilibrium in any turbulent field is assumed. This has been investigated experimentally by several authors, particularly for stirred systems (7). Some authors have also looked more specifically into tube flow (8). 

Chapters 26—29 all discuss hydrodynamic aspects of emulsified systems. The contribution by Danov, Kralchevsky, and Ivanov presents a very fundamental and thorough survey of different phenomena in emulsions related to dynamic and hydrodynamic motions, such as the dynamics of surfactant adsorption mono-layers, which include the Gibbs surface elasticity, and characteristic time of adsorption, mechanisms of droplet-droplet coalescence, hydrodynamic interactions and drop coalescence, interpretation of the Bancroft rule with regard to droplet symmetry, and, finally, kinetics of... 

In particular, the mechanisms effective during coalescence are not known sufficiently yet. Hence, small-scale experiments are often required. As even small concentrations of contaminants have a dominant effect on drop coalescence those experiments have to be conducted with the original liquids. 

FIGURE 11.16. Multiple emulsion degradation can take place by several mechanisms. Important pathways include (a) secondary emulsion coalescence with little change in drop size in the PE, (ft) PE drop coalescence with httle change in secondary emulsion characteristics, and (c) loss of PE internal phase to the final external phase due to diffusion or solubilization. 

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