In sheared emulsions

K. B. Migler 2002, (Layered droplet microstructures in sheared emulsions finite-size effects), /. Colloid Interface Sci. 255, 391. 

To quantify the increase of a due to pressure, a mean bubble diameter has been estimated using Taylor s stability theory [7] on bubble deformation and break-up in sheared emulsions. According to this theory, bubble size in a sheared emulsion results from a balance between viscosity and surface tension forces. The dimensionless number that describes the ratio of these forces is called the capillary number Q. For large bubble deformations, the maximum stable bubble diameter in a shear flow is expressed as [8] ... 

Flocculation is the mutual aggregation of colliding droplets. In stationary emulsions, droplet collisions arise from Brownian motion (small droplets) and/or from the creaming/sedimentation process (larger droplets). In the latter, the mechanism is often referred to as sedimentation/creaming flocculation. Finally, droplet aggregation can also occur in sheared emulsions. It is important to point out that the droplet size distribution is not altered by the flocculation and creaming/sedimentation destabilization mechanisms. 

Droplet Microstructure and String Stability in Sheared Emulsions Role of Finite-Size Effects... 

Vananroye, A., Pujrvelde, R V., and Moldenaers, P. 2006. Effect of confinement on droplet breakup in sheared emulsions. Langmuir 22 3972-3974. 

Pathak JA et al. Layered droplet microstructures in sheared emulsions Finite-size effects. J Colloid Interface Sci 2002 255(2) 391-402. 

The largest volume of hydrauHc fluids are mineral oils containing additives to meet specific requirements. These fluids comprise over 80% of the world demand (ca 3.6 x 10 L (944 x 10 gal))- In contrast world demand for fire-resistant fluids is only about 5% of the total industrial fluid market. Fire-resistant fluids are classified as high water-base fluids, water-in-oil emulsions, glycols, and phosphate esters. Polyolesters having shear-stable mist suppressant also meet some fire-resistant tests. 

Srinivasan, M. P., and Stroeve, P., Subdrop ejection from double emulsion drops in shear flow. J. Membrane Sci. 26, 231-236 (1986). 

Ulbrecht, J. J., Stroeve, P., and Pradobh, P., Behavior of double emulsions in shear flows. Rheol. Acta 21, 593-597 (1982). 

Our goal is to develop a property-performance relationship for different types of demulsifiers. The important interfacial properties governing water-in-oil emulsion stability are shear viscosity, dynamic tension and dilational elasticity. We have studied the relative importance of these parameters in demulsification. In this paper, some of the results of our study are presented. In particular, we have found that to be effective, a demulsifier must lower the dynamic interfacial tension gradient and its ability to do so depends on the rate of unclustering of the ethylene oxide groups at the oil-water interface. 

For effective demulsification of a water-in-oil emulsion, both shear viscosity as well as dynamic tension gradient of the water-oil interface have to be lowered. The interfacial dilational modulus data indicate that the interfacial relaxation process occurs faster with an effective demulsifier. The electron spin resonance with labeled demulsifiers suggests that demulsifiers form clusters in the bulk oil. The unclustering and rearrangement of the demulsifier at the interface may affect the interfacial relaxation process. 

The rheological properties of a fluid interface may be characterized by four parameters surface shear viscosity and elasticity, and surface dilational viscosity and elasticity. When polymer monolayers are present at such interfaces, viscoelastic behavior has been observed (1,2), but theoretical progress has been slow. The adsorption of amphiphilic polymers at the interface in liquid emulsions stabilizes the particles mainly through osmotic pressure developed upon close approach. This has become known as steric stabilization (3,4.5). In this paper, the dynamic behavior of amphiphilic, hydrophobically modified hydroxyethyl celluloses (HM-HEC), was studied. In previous studies HM-HEC s were found to greatly reduce liquid/liquid interfacial tensions even at very low polymer concentrations, and were extremely effective emulsifiers for organic liquids in water (6). 

Figure 7.8. (a) Photomicrograph of a premix silica-in-octane-in-water emulsion. The octane contains 17% vol of silica particles. Compositions are given in the text, (b) Same sample after being sheared at 3750 s in a Couette geometry device. The scale bar corresponds to 10 pm. 

Preparation of Emulsions. An emulsion is a system in which one liquid is colloidally dispersed in another (see Emulsions). The general method for preparing an oil-in-water emulsion is to combine the oil with a compatible fatty acid, such as an oleic, stearic, or rosin acid, and separately mix a proportionate quantity of an alkali, such as potassium hydroxide, with the water. The alkali solution should then be rapidly stirred to develop as much shear as possible while the oil phase is added. Use of a homogenizer to force the resulting emulsion through a fine orifice under pressure further reduces its oil particle size. Liquid oleic acid is a convenient fatty acid to use in emulsions, as it is readily miscible with most oils. 

It was previously shown that the formation of a stable emulsion of methylene chloride in water was vital for the successful formation of individual microspheres [4,9]. Two main factors played an important role in the emulsification of methylene chloride in water and influenced the microsphere size the interfacial tension of the methylene chloride droplets in the surrounding aqueous phase and the forces of shear within the fluid mass. The former tends to resist the distortion of droplet shape necessary for fragmentation into smaller droplets whereas the latter forces act to distort and ultimately to disrupt the droplets. The relationship between these forces largely determines the final size distribution of the methylene chloride in water emulsion which in turn controls the final size distribution of the solid microspheres formed. 

Figure 3.5 Demonstration of correlation between the stickiness of protein-coated droplet pair encounters in shear flow (left ordinate axis) and viscoelasticity of concentrated emulsions (right ordinate axis) with the strength of protein-protein attraction as indicated by the second virial coefficient A2 determined from static light scattering , percentage capture efficiency (0%) A, complex shear modulus (G ) for emulsions stabilized by asl-casein or (3-casein (pH = 5.5, ionic strength in the range 0.01-0.2 M).

Emulsion viscosities have been measured as a function of water content (10, 20 and 40S), temperature and shear rate in a thermostatted rotating viscometer. The shear rates were varied between 0.277 and 27.7 s"1 with measurements taken at temperatures between 5 and 20° C. Above 20°C, separation of water from the emulsion occurred, rendering viscosity measurements unreliable. The apparent viscosity of the emulsion below 20° C increases drastically with the watercut in the emulsion and decreases with Increasing shear rate (Fig. 5). Emulsions containing more than 20X water were found to behave as pseudo-plastic fluids. 

Produced salt water, if it remains in the oil in sufficient quantities to require a dehydration-desalting facility, is carried in an emulsion state. The emulsion exists as a result of mechanical shearing of produced water and its dispersion into oil by turbulence. Once water is dispersed in droplets, many of them are combined with the various emulsifying agents produced in the crude and resist natural separation because of their size and envelopment by the emulsifying agents. 

When an oil-in-water emulsion is created by the application of shear force to a sizes. In order to create an emulsion of very small droplets, the droplets must be... 

Changing the shape of the dispersed species while flowing also has an impact. Since emulsion droplets and foam bubbles are not rigid spheres, they may deform in shear flow. In the cases of electrostatically interacting species, or those with surfactant or polymeric stabilizing agents at the interface, the species will not be noninteracting, as is assumed in the theory. Thus, Stokes law will not strictly apply and may underestimate or even overestimate the real terminal velocity. 

---