Droplet break

The emulsification process in principle consists of the break-up of large droplets into smaller ones due to shear forces (10). The simplest form of shear is experienced in lamellar flow, and the droplet break-up may be visualized according to Figure 4. The phenomenon is governed by two forces, ie, the Laplace pressure, which preserves the droplet, and the stress from the velocity gradient, which causes the deformation. The ratio between the two is called the Weber number. We, where Tj is the viscosity of the continuous phase, G the velocity gradient, r the droplet radius, and y the interfacial tension. 

The influence of the vi.scosity ratio 8 on the flow behavior in a capillary was discussed by Rumscheidt and Mason [lOj. They pointed out that when the viscosity ratio is small, the dispersed droplets are drawn out to great lengths but do not burst, and when the viscosity ratio is of the order of unity, the extended droplets break up into smaller droplets. At very high viscosity ratios, the droplets undergo only very limited deformations. This mechanism can explain our observations and supports our theoretical analysis assumptions, summarized previously as points 2, 3, and 4. 

Droplet size and interfacial area. In the absence of interfacial effects accompanying mass transfer, the droplets break down by impact with elements of packing and finally reach an equilibrium size which is independent of the packing size. Conversely, small droplets gradually coalesce until the equilibrium size is attained. Pratt and his coworkers 5 29 showed that the mean droplet size attained in the tower is well represented by ... 

The first process is the deformation of droplets into an ellipsoid, until the droplet breaks up. The second process is A droplet is, at high speed, extended into a thread, which is subject to instabilities (according to Rayleigh and Tomatika), and which, therefore, breaks up into a row of droplets (see MT 9.1.5). 

As a consequence, the droplet breaks up into a stream of smaller droplets, each one continuing to shrink by evaporation until the Rayleigh stability limit is reached again. The process of droplet fission is repeated several times and it is called uneven fission or droplet jet fission [5,6],... 

Most flame spectrometers use a premix burner, such as that in Figure 21-5, in which fuel, oxidant, and sample are mixed before introduction into the flame. Sample solution is drawn into the pneumatic nebulizer by the rapid flow of oxidant (usually air) past the tip of the sample capillary. Liquid breaks into a fine mist as it leaves the capillary. The spray is directed against a glass bead, upon which the droplets break into smaller particles. The formation of small droplets is termed nebulization. A fine suspension of liquid (or solid) particles in a gas is called an aerosol. The nebulizer creates an aerosol from the liquid sample. The mist, oxi-... 

Charged liquid exiting the capillary forms a cone and then a fine filament and finally breaks into a spray of fine droplets (see Figure 22-17c and the opening of this chapter). A droplet shrinks to —1 p,m by solvent evaporation until the repulsive force of the excess charge equals the cohesive force of surface tension. At that point, the droplet breaks up by... 

Filling of the stripes/fluid instability and droplet break-up at large coverage... 

Two mechanisms were proposed for the strong upward and downward flows [135], The first was based on interplay of the liquid-liquid and solid-liquid surface tensions and the gravitational and inertia forces. The second is correlated with the neck formation, the droplet break-up and the retossing of the fluid. 

M 54] [P 48] CFD simulations for the flow in the separation-layer micro mixer predict a stable, almost irrotational flow pattern in the inlet region, which is in line with the experimental findings of a transparent region mentioned above [39], This pattern is maintained until the droplet end cap. Changes only occur when the droplet breaks up and falls, inducing rotational flow. 

Figure 3.6 shows that, for a given viscosity ratio, ry2/Vi> between the dispersed (rj2) and continuous (t/j) phases, reducing the interfacia] tension increases the Weber number, lowering the energy needed to cause droplet break-up. As discussed in Section 11.2.1, this relationship can be used to predict whether an emulsion will be easier to form if it is a water-in-oil emulsion rather than an oil-in-water emulsion. 

In the microfluid dynamics approaches the continuity and Navier-Stokes equation coupled with methodologies for tracking the disperse/continuous interface are used to describe the droplet formation in quiescent and crossflow continuous conditions. Ohta et al. [54] used a computational fluid dynamics (CFD) approach to analyze the single-droplet-formation process at an orifice under pressure pulse conditions (pulsed sieve-plate column). Abrahamse et al. [55] simulated the process of the droplet break-up in crossflow membrane emulsification using an equal computational fluid dynamics procedure. They calculated the minimum distance between two membrane pores as a function of crossflow velocity and pore size. This minimum distance is important to optimize the space between two pores on the membrane... 

The droplet breaks at D 0.5, dus yrjcR all. This is Taylor s formula for the critical shear rate at which break occurs ... 

The main objective of a fluid curtain is to help mitigate explosions by absorbing energy as droplets break up and by creating an increased total surface area to reduce potential flammable and toxic hazard zones by dilution with air or by chemical reaction with the water or reactive materials contained in the curtain. 

Coalescence. In the case of coalescence, the separating film of the continuous phase between the droplets breaks and an irreversible fusion of emulsion droplets occurs. 

Stirred tank. The simplest system to achieve droplet break-up is with a stirrer in a vessel. In this case, the flow field is not very intense, as the stirrer is usually not very close to the wall of the vessel, and therefore the droplets remain relatively large (>10 pm). The droplet size distribution is usually relatively wide, but will become smaller with longer treatment times. The power density that can be apphed is relatively small, so if one wants to produce droplets smaller than approximately 10 pm, this equipment does not suffice. 

Groeneweg, F., van Dieren, F., Agterof, W. G. M. (1994). Droplet break-up in a stirred water-in-oil emulsion in the presence of emulsifiers. Colloids and Surfaces A Physicochemical and Engineering Aspects 91 207-214. 

Figures 9.43 and 9.44 also illustrate one of the major problems of emulsion membrane systems, i.e., degradation of the emulsion on prolonged contact with the feed solution and high-speed mixing of the product and feed solutions. Prolonged stirring of the emulsion with the feed solution causes the copper concentration in the feed solution to rise as some of the emulsion droplets break. Careful tailoring of the stirring rate and surfactant composition is required in order to minimize premature breaking of the emulsion.

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