Dispersion coalescence, mechanism

From Equation 13.1, the existence of a critical value of the Weber number emerges, and then a critical particle size for the dispersed phase below which there is no particle deformation. Furthermore, if only physical interactions exist between components, the remaining dynamic coalescence mechanism could change on further processing steps such as a nonstable postprocessing morphology. 

The morphology development of polymer blends is determined by competing distributive mixing, dispersive mixing and coalescence mechanisms. Figure 1 presents a model that helps visualize these mechanisms which govern morphology development in polymer blends. 

With respect to good adhesion, reduced interfacial tension, fine distribution of TLCP phase, and the use of a compatibilizer can be very effective for this purpose. Remarkably improved mechanical properties (good impact properties as well as tensile properties) can be obtained with optimum amounts of the compatibilizer. Excess amounts of the compatibilizer causes the emulsifying effect to coalesce the dispersed TLCP... 

Most studies on heat- and mass-transfer to or from bubbles in continuous media have primarily been limited to the transfer mechanism for a single moving bubble. Transfer to or from swarms of bubbles moving in an arbitrary fluid field is complex and has only been analyzed theoretically for certain simple cases. To achieve a useful analysis, the assumption is commonly made that the bubbles are of uniform size. This permits calculation of the total interfacial area of the dispersion, the contact time of the bubble, and the transfer coefficient based on the average size. However, it is well known that the bubble-size distribution is not uniform, and the assumption of uniformity may lead to error. Of particular importance is the effect of the coalescence and breakup of bubbles and the effect of these phenomena on the bubble-size distribution. In addition, the interaction between adjacent bubbles in the dispersion should be taken into account in the estimation of the transfer rates... 

Mechanical compatibilization is accomplished by reducing the size of the dispersed phase. The latter is determined by the balance between drop breakup and coalescence process, which in turn is governed by the type and severity of the stress, interfacial tension between the two phases, and the rheological characteristics of the components [9]. The need to reduce potential energy initiates the agglomeration process, which is less severe if the interfacial tension is small. Addition... 

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. 

The archetypal, stagewise extraction device is the mixer-settler. This consists essentially of a well-mixed agitated vessel, in which the two liquid phases are mixed and brought into intimate contact to form a two phase dispersion, which then flows into the settler for the mechanical separation of the two liquid phases by continuous decantation. The settler, in its most basic form, consists of a large empty tank, provided with weirs to allow the separated phases to discharge. The dispersion entering the settler from the mixer forms an emulsion band, from which the dispersed phase droplets coalesce into the two separate liquid phases. The mixer must adequately disperse the two phases, and the hydrodynamic conditions within the mixer are usually such that a close approach to equilibrium is obtained within the mixer. The settler therefore contributes little mass transfer function to the overall extraction device. 

In terms of measuring emulsion microstructure, ultrasonics is complementary to NMRI in that it is sensitive to droplet flocculation [54], which is the aggregation of droplets into clusters, or floes, without the occurrence of droplet fusion, or coalescence, as described earlier. Flocculation is an emulsion destabilization mechanism because it disrupts the uniform dispersion of discrete droplets. Furthermore, flocculation promotes creaming in the emulsion, as large clusters of droplets separate rapidly from the continuous phase, and also promotes coalescence, because droplets inside the clusters are in close contact for long periods of time. Ideally, a full characterization of an emulsion would include NMRI measurements of droplet size distributions, which only depend on the interior dimensions of the droplets and therefore are independent of flocculation, and also ultrasonic spectroscopy, which can characterize flocculation properties. 

The decrease in IT is caused by small shifts of atoms located in a layer of 3 to 5 atomic diameters near the interface. Such shifts can be clearly observed in monociystals (reconstruction and relaxation phenomena) [12], There are mechanisms based on the decrease of A at V = const with the decrease of dispersion A/V. The results of action of these mechanisms are change of particle and pore shape, decrease of the micropore amount and surface roughness, etc. during sintering, coalescence, etc. 

In order to prevent coalescence of the dispersed drops, van Duck(36) and others have devised methods of providing the whole of the continuous phase with a pulsed motion. This may be done, either by some mechanical device, or by the introduction of compressed air. 

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