Water droplet deformation

On the basis of experimental observations (Fig. 3.27), Senda et al.[335h415] proposed six modes for water droplet deformation, and breakup during impingement on a hot surface coupled with heat transfer and evaporation (Fig. 3.28). Each mode occurs under a specific combination of surface temperatures and impact conditions, as described below. 

Flow properties of macroemulsions are different from those of non-emulsified phases 19,44). When water droplets are dispersed in a non-wetting oil phase, the relative permeability of the formation to the non-wetting phase decreases. Viscous energy must be expended to deform the emulsified water droplets so that they will pass through pore throats. If viscous forces are insufficient to overcome the capillary forces which hold the water droplet within the pore body, flow channels will become blocked with persistent, non-draining water droplets. As a result, the flow of oil to the wellbore will also be blocked. 

The most widely studied deformable systems are emulsions. These can come in many forms, with oil in water (O/W) and water in oil (W/O) the most commonly encountered. However, there are multiple emulsions where oil or water droplets become trapped inside another drop such that they are W/O/W or O/W/O. Silicone oils can become incompatible at certain molecular weights and with different chemical substitutions and this can lead to oil in oil emulsions O/O. At high concentrations, typical of some pharmaceutical creams, cosmetics and foodstuffs the droplets are in contact and deform. Volume fractions in excess of 0.90 can be achieved. The drops are separated by thin surfactant films. Selfbodied systems are multicomponent systems in which the dispersion is a mixture of droplets and precipitated organic species such as a long chain alcohol. The solids can form part of the stabilising layer - these are called Pickering emulsions. 

Experimental data for mass transfer from freely circulating fluid spheres are difficult to obtain because of deformation and because of the presence of surface-active agents which reduce circulation. Shown in Fig. 5.30 are data from three studies on water droplets in isobutanol where the droplets were nearly spherical and were observed to be circulating. The data are in fair agreement with each other and with Eq. (5-39). The effects of shape changes and surface-active agents are discussed in Chapter 7. 

A stabilising effect in the presence of salt was also noted by Aronson and Petko [90]. Addition of various electrolytes was shown to lower the interfacial tension of the system. Thus, there was increased adsorption of emulsifier at oil/water interface and an increased resistance to coalescence. Salt addition also increased HIPE stability during freeze-thaw cycles. Film rupture, due to expansion of the water droplets on freezing, did not occur when aqueous solutions of various electrolytes were used. The salt reduced the rate of ice formation and caused a small amount of aqueous solution to remain unfrozen. The dispersed phase droplets could therefore deform gradually, allowing expansion of the oil films to avoid rupture [114]. 

Kinsella (13, 14) summarized present thinking on foam formation of protein solutions. When an aqueous suspension of protein ingredient (for example, flour, concentrate, or isolate) is agitated by whipping or aeration processes, it will encapsulate air into droplets or bubbles that are surrounded by a liquid film. The film consists of denatured protein that lowers the interfacial tension between air and water, facilitating deformation of the liquid and expansion against its surface tension. 

It should be emphasised that polymeric surfactants prevent the coalescence of water droplets in the multiple emulsion drops, as well as coalescence of the latter drops themselves. This is due to the interfacial rheology of the polymeric surfactant films. As a result of the strong lateral repulsion between the stabilising chains at the interface (PHS chains at the W/O interface and PEO chains at the O/W interface), these films resist deformation under shear and hence produce a viscoelastic film. On approach of the two droplets, this film prevents deformation of the interface so as to prevent coalescence. 

The simple estimation discussed above gives us the critical parameter value Xcr 4-y/nfi. A more accurate calculation that takes into account the deformation of the drop [58, 79] gives Xcr = 1-625. For water in oil, S = 3 10 H/m and for water droplet of 1 cm radius, the critical strength of the electric field E. = 2.67 KV/cm. Therefore an electric field with strength Eg < 3 KV/cm will not cause any noticeable deformation of a drop with radius li 1 cm placed far away from the other drops. 

In 1983, Thingstad and Pengeurd conducted photo-oxidation experiments and found that photo-oxidized oil formed emulsions (11). Nesterova et al. studied emulsion formation and concluded that it was strongly correlated with both the asphaltene and tar content of oil and also the salinity of the water with whieh it was formed (12). Mackay and Nowak studied emulsions and found that stable emulsions had low conductivity and therefore a continuous phase of oil (13,14). Stability was discussed and proposed to be a funetion of oil composition, particularly waxes as asphaltenes. It was proposed that a water droplet could be stabilized by waxes, asphaltenes, or a combination of both. The viscosity of the resulting emulsions was correlated with water content. Later work by the same group reported examination of Russian hypotheses that emulsions are stabilized by colloidal particles which gather at the oil— water interface and may combine to form a near-solid barrier that resists deformation and thus water-water coalescence (15). It was speculated that these particles could be mineral, wax crystals, aggregates of tar and asphaltenes, or mixtures of... 

Adsorbed layers that crumple clearly possess resistance to surface deformations. Such resistance is manifested as surface viscosity. In a free suspension, micrometer-sized water droplets are normally spherical. Using two suction pipet as shown in Fig. 12a, such a droplet (here, formed in 0.1% diluted bitumen) is stretched and then released, thus... 

Figure 13 Resistance to coalescence. Two water droplets are pressed together in 0.1% bitumen in toluene. The compressing force, calculated from droplet shape deformation, is equivalent to about 10,000 g acceleration.

A study was made of the deformation and break-up of water droplets dispersed in an epoxy resin phase under shear in terms of microrheology and the interaction and coalescence dynamics occurring between the water droplets stabilised by the emulsifier molecules examined theoretically. A phase inversion model is proposed to account for the effects of some variables on phase inversion and the structure of the waterborne particles. 29 refs. 

Recently, a honeycomb-patterned cellulose triacetate film was successfully fabricated by casting of water-in-oil (W/0) emulsion on the substrate, and the subsequent deacetylation yielded a honeycomb-patterned cellulose film without deformation as shown in Figure 16-13 (Kasai and Kondo 2004). In the film forming process, when cellulose triacetate started to be precipitated as a honeycomb-patterned film, the honeycomb frames were supposed to be simultaneously stretched by natural drying of water droplets as a mold. This stretching effect of natural dry presumably resulted in an alignment of cellulose molecular chains along the honeycomb frames, similarly to formation of NOC-typed form (Kasai et al., unpublished). 

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