Droplet Coalescence in Emulsions

This expression is valid for a partially mobile interface analogous expressions hold for fully mobile and immobile interfaces (Minale et al. 1997). Here, he is the critical thickness of the hquid gap between droplets at which coalescence occurs. Theoretically, one expects he (AHa/STrr), where Ah is the Hamaker constant (Chesters 1991). A value he = 0.2pm gives the solid line in Fig. 9-10, this value of he is an order of magnitude larger than predicted, no doubt because he is used as a fitting parameter to accomodate rough approximations used in the theory. 

100 r r sYlT for blends of 10% polyisobutylene in polydimethylsilox-ane with M = 0.47. The solid 

1 1 j 1 ] 1 1 3 1 -i. 1 1 1 J J 1 i —1 1 t 1 1 when the shear rate is suddenly reduced. (From Grizzuti and Bifulco 1997, reprinted with permission from Steinkopff Publishers.) 

For M 1, this gives dc 2Qhc that is, a few microns. This result is consistent with the measurements of Minale et al. (1997). 

---

Formation of spray in small drops (i.e. 30 microns, with a narrow size distribution) using a rotary disc or nozzle, to enhance the product-air interface. The preparation of the feed (dry matter content, composition, temperature, mixing) must facilitate pumping and spraying with minimal modification (partition of constituents, droplet coalescence in emulsions). 

Hi denotes the number of single particles per unit volume rij is the number of aggregates of k particles (k=2,3,...) per unit volume ay (i, j = 1, 2, 3,. ..) are rate constants of flocculation (coagulation see Figure 4.65) qi denotes the flux of aggregates of size k which are products of other processes, different from the flocculation itself (say, the reverse process of aggregate disassembly or the droplet coalescence in emulsions see Equations 4.346 and 4.350)... 

Partial Coalescence in Emulsions Comprising Partially Crystallized Droplets 167... 

The method employed to obtain the results of Figure 13.17, i.e., determination of average droplet size as a function of time, is quite suitable to establish the rate of coalescence in emulsions. It may even be better to... 

As mentioned earlier, it is not uncommon for emulsion droplets to have diameters well above the accepted colloid size range. However, even in this case colloid principles are of fundamental importance when one considers the problem of droplet coalescence in the destruction of emulsions (de-emulsijication). Here the medium between two approaching droplets is progressively thinned, and before coalescence can occur it is reduced to a film of colloidal thickness (see Chapter 12). 

Coalescence and flocculation are strongly related to one another. Reducing the rate of drop coalescence is one of the main purposes of surfactants in emulsion formulations. The reduction in the interfacial tension on addition of surfactant itself increases the stability of the emulsion with respect to its component phases. However, the role of surfactants is much greater than this and they act to reduce droplet coalescence in several additional ways. 

There appear to be two stages in the collapse of emulsions flocculation, in which some clustering of emulsion droplets takes place, and coalescence, in which the number of distinct droplets decreases (see Refs. 31-33). Coalescence rates very likely depend primarily on the film-film surface chemical repulsion and on the degree of irreversibility of film desorption, as discussed. However, if emulsions are centrifuged, a compressed polyhedral structure similar to that of foams results [32-34]—see Section XIV-8—and coalescence may now take on mechanisms more related to those operative in the thinning of foams. 

The charge on a droplet surface produces a repulsive barrier to coalescence into the London-van der Waals primary attractive minimum (see Section VI-4). If the droplet size is appropriate, a secondary minimum exists outside the repulsive barrier as illustrated by DLVO calculations shown in Fig. XIV-6 (see also Refs. 36-38). Here the influence of pH on the repulsive barrier between n-hexadecane drops is shown in Fig. XIV-6a, while the secondary minimum is enlarged in Fig. XIV-6b [39]. The inset to the figures contains t,. the coalescence time. Emulsion particles may flocculate into the secondary minimum without further coalescence. 

Emulsions Almost eveiy shear rate parameter affects liquid-liquid emulsion formation. Some of the efrecds are dependent upon whether the emulsion is both dispersing and coalescing in the tank, or whether there are sufficient stabilizers present to maintain the smallest droplet size produced for long periods of time. Blend time and the standard deviation of circulation times affect the length of time it takes for a particle to be exposed to the various levels of shear work and thus the time it takes to achieve the ultimate small paiTicle size desired. 

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. 

Studies of flow-induced coalescence are possible with the methods described here. Effects of flow conditions and emulsion properties, such as shear rate, initial droplet size, viscosity and type of surfactant can be investigated in detail. Recently developed, fast (3-10 s) [82, 83] PFG NMR methods of measuring droplet size distributions have provided nearly real-time droplet distribution curves during evolving flows such as emulsification [83], Studies of other destabilization mechanisms in emulsions such as creaming and flocculation can also be performed. 

For parenteral emulsions, the formulation scientist must be particularly aware of changes in particle size distribution of the oil phase. Droplet coalescence results in increased droplet size. As a general rule, average droplet size should be less than 1 pm. Droplet sizes of more than 6 pm can cause blockage of capillaries (capillary emboli). 

In the preseparation chamber, the less dense oil droplets rise, collide, and fuse with adjacent droplets. According to Stoke s law, the larger the diameter of a particle, the faster is its rate of rise. Thus, as small droplets coalesce to form larger droplets, their upward vertical velocity increases. Coalescing tubes or plates are designed to enhance the separation of oil-water emulsions. The emulsion free water is directed away from the tubes or plates and enters the separation section. Some separators are built with an outlet zone for the discharge of clarified water. 

An emulsion is a dispersed system of two immiscible phases. Emulsions are present in several food systems. In general, the disperse phase in an emulsion is normally in globules 0.1-10 microns in diameter. Emulsions are commonly classed as either oil in water (O/W) or water in oil (W/O). In sugar confectionery, O/W emulsions are most usually encountered, or perhaps more accurately, oil in sugar syrup. One of the most important properties of an emulsion is its stability, normally referred to as its emulsion stability. Emulsions normally break by one of three processes creaming (or sedimentation), flocculation or droplet coalescence. Creaming and sedimentation originate in density differences between the two phases. Emulsions often break by a mixture of the processes. The time it takes for an emulsion to break can vary from seconds to years. Emulsions are not normally inherently stable since they are not a thermodynamic state of matter. A stable emulsion normally needs some material to make the emulsion stable. Food law complicates this issue since various substances are listed as emulsifiers and stabilisers. Unfortunately, some natural substances that are extremely effective as emulsifiers in practice are not emulsifiers in law. An examination of those materials that do stabilise emulsions allows them to be classified as follows ... 

Fibrous bed coalescers generally have a fixed filter element constructed of fiberglass or other material that acts to coalesce (bring together) the oil droplets and to break emulsions. The coalesced oil droplets released from the filter are readily separated downstream by gravity. Coalescence in a fibrous bed coalescer involves three steps ... 

Figure 6.8. Two-stage coalescence in double emulsions. Sequence of side view pictures, taken at a frequency of 5000 frames/s, of a water droplet coalescing on the globnle surface. Cfi = 30 CMC of SDS C = 2 wt% of SMO in dodecane. (Reproduced with permission from [18].)...

---