Droplet disruption

In the premix emulsification the basic mechanism for the droplet formation is different from the direct emulsification. In fact, in this case the predominant formation mechanism is the droplet disruption within the pore. 

In an attempt to modify the observed droplet behavior, a brief qualitative investigation was carried out with blends of SRC-II heavy distillate and pure heptane. The objective was to enhance droplet disruptive combustion as a means of reducing effective droplet size and hence soot formation. With these fuels visible droplet fragmentation was found to occur throughout the droplet stream. The fragmentation produced new droplets on different trajectories these in turn were terminated by small disruptions, as described above. Three blends were used 60/40, 80/20, and 90/10. Secondary atomization was observed for all three, although the violence of the activity was noticeably reduced as the heptane content of the blend became smaller. This secondary atomization was a completely different process than the... 

Several possible mechanisms have been proposed to explain the influence of US energy on droplet formation and disruption. One assumes the formation of droplets as a consequence of unstable oscillations at the liquid-liquid interface. Such oscillations contribute to droplet disruption only if the diameter of droplets is considerably larger than the oscillation wavelength, which is about 10 xm for oil-water systems. Therefore, this mechanism, which is known as the capillary waves mechanism and rarely used to explain US-assisted emulsification, is only valid for droplets larger than 10 (xm (first steps of the process). 

The effect of the viscosity of the continuous phase was studied theoretically in o/w emulsions containing water-soluble stabilizers and also in w/o emulsions of various oils [32]. In the former, droplets were larger in the absence of a stabilizer than in its presence. However, there was no clear-cut correlation of the viscosity of the continuous phase with droplet size. This can be ascribed to the increased amount of energy dissipated in the immediate vicinity of the droplets relative to the bulk liquid, which may result in more efficient disruption than if the energy dissipation occurs evenly throughout the continuous phase. The addition of a stabiiizer possibly alters and partly suppresses cavitation in the bulk liquid, the cavitation threshoid and viscosity being related similarly as in pure liquids [58]. The energy may subsequently dissipate preferentially at the surface of droplets and result in more efficient use in terms of droplet disruption. 

At a constant energy density, droplet disruption is not expected to depend on the viscosity of the continuous phase. However, coalescence is especially significant at high water contents. The effect may be stronger in the more viscous oils as small droplets will separate more slowly, shortly after disruption, thus increasing contact times and hence the likelihood of coalescence [32]. 

Further research is needed on the influence of the viscosity of the continuous phase on US emulsification with a view to assess the significance of cavitation as a droplet disruption mechanism. 

Droplet Disruption The initial stages of primary homogenization involve the break-up and intermingling of the bulk oil and bulk aqueous phases so that... 

The major droplet disruption occurs in the immediate vicinity of the rotating blades where shear forces are highest, e.g., due to the presence of Taylor vortexes. The effectiveness of droplet disruption depends on the geometry of the mixer and the rotational speed of the blades. Operational parameters include blade and vessel geometries and rotation speed of blades. 

High-Intensity Ultmsonicator. Droplets are disrupted within a field of high-intensity ultrasonic waves. Droplet disruption occurs either due to cavitation or because the frequency of the ultrasonic wave equals the resonance frequencies of the droplets. This causes the droplets to oscillate vigorously. Eventually, the oscillation becomes supercritical and the droplets are disrupted. The effectiveness of sonication, therefore, depends on the nature of the continuous and dispersed phase. The type of oil, as well as the nature of the surfactant, is the limiting factor for the minimal droplet size that can be achieved. 

The breakdown of the stable emulsions and subsequent separation to oil and water (demulsification) are important in nuclear, petroleum, and environmental technologies. The emulsion stability is primarily induced by the use of surfactants and is enhanced by reduced size and narrow size distribution of the emulsion droplets. Disruption to low interfacial activity (hence instability) can be achieved by using demulsification agents, which are, however, costly and environmentally undesirable, as they are irrecoverable. Demulsification can also be achieved by electric and/or centrifugal fields, or by chemical treatment of the emulsion. 

The value of the power density may greatly vary among sites in the apparatus. Near the tip of a stirrer, e would have a much higher value than further away. It means that the effective volume for droplet disruption is much smaller than the total volume of stirred liquid. This has two consequences. First, part of the mechanical energy is dissipated at a level where it cannot disrupt drops (and is thereby wasted). Second, droplet breakup takes a long time, because... 

The span of droplet size distribution linearly decreases with increasing maximum droplet size is the same at both values (Figure 16.17). The broader droplet size distribution at cr = 40 Pa can be explained by (a) partial droplet disruption outside the membrane tube caused by high recirculation rate (v = 3.5 m/sec) or (b) very intensive droplet deformation before detachment from the pore tips. 

According to Darcy s law, the pressure loss for overcoming flow resistances in the pores, Prow, should be proportional to the transmembrane flux, while the expenditure of pressure for droplet disruption, Apdisr. is proportional to the increase in the interfacial area. If Apu = const, then... 

In continuous industrial emulsification, the residence time tres in the zones ofhigh power dissipation is in the order of milliseconds to tenths of a second, also influencing droplet disruption, as determined empirically [15] ... 

Since the exponents of residence time tres and power density Pv are of the same order of magnitude (0.25-0.4), tres and Py may also be mathematically summarized to the volumetric energy density or speciflc disruption energy ,[11,16]. Thus the mean droplet diameter x after droplet disruption in turbulent flow can be calculated by the process function ... 

Droplet disruption due to laminar shear flow has been widely investigated (e.g. [3, 17]). However, it is restricted to a narrow range of viscosity ratios (between disperse and continuous phase Hd/tlc for single droplet disruption or between the disperse phase and the emulsion T d/Tie for emulsions [18] (Figure 20.3). Laminar... 

Figure 20.14 Droplet disruption and droplet formation in emulsification devices different energy input, different efficiency.

After their initial formation, conventional silicas, formed by acidifying silicate solutions, can be significantly influenced by consecutive aging processes. When the fragile, low-densify sihcas as prepared in solution are dried, they often collapse. The surface tension of the water droplets disrupts the silica structiue. Aging processes are important because in the process the walls of the micropores thicken and, hence, become resistant to drying processes. 

The kind and concentration of emulsifier molecules applied have a big influence on the coalescence process during the emulsification process itself and during prolonged shelf-life. In emulsification (droplet disruption processes), the droplets are disrupted into smaller ones. New interfacial area develops, being insufficiently covered by emulsifier molecules. Interfacial active molecules (emulsifiers) are transported by laminar and turbulent flow to the droplet subsurface, diffuse to the interface and adsorb and re-orientate at it. This stabilizes the new droplets formed. However, this process takes some time (milliseconds to minutes), depending on the emulsifier molecular stmcture. Droplets colliding with each other in the meantime will coalesce [2], A detailed study about droplet coalescence... 

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