Sheared emulsions

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

Fig. 7. Diagram of the Couette mixer. The premixed emulsion, initially located in the syringe, is forced by a piston into the gap between the rotor and the stator. The sheared emulsion is collected at the top of the mixer...

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... 

An emulsion can be made by simply mixing oil into water with sufficient mechanical shear. Emulsions of any significant stability, however, contain oil, water, and at least one emulsifying agent. The emulsifying agent may lower interfacial tension thus making it easier to create small... 

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. 

Since emulsion droplets are not rigid spheres, the coefficient of 0 is around 3-6 for many emulsion systems [3-5], More concentrated emulsions are non-Newtonian depends on shear rate and are thixotropic (ri decreasing with... 

Some studies have been made of W/O emulsions the droplets are now aqueous and positively charged [40,41 ]. Albers and Overbeek [40] carried out calculations of the interaction potential not just between two particles or droplets but between one and all nearest neighbors, thus obtaining the variation with particle density or . In their third paper, these authors also estimated the magnitude of the van der Waals long-range attraction from the shear gradient sufficient to detach flocculated droplets (see also Ref. 42). 

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. 

Additionally, mechanical (primarily shear), freeze—thaw, and thermal stabiHty the tendency to form sediment on long-term standing and compatibiHty with other dispersions, salts, surfactants, and pigments of acryHc dispersions are often evaluated. Details on the determination of the properties of emulsion polymers are available (60). 

The Hercules viscometer was originally designed for paper and paperboard coatings, but its use has been extended to paints, adhesives, mineral slurries, emulsions, and starch solutions. The iastmment, noted for being robust and rehable, is particularly well suited for quaUty control and product formulation. It is capable of measuting viscosity over a moderate range 1-10 mPa-s) up to high shear rates (115,000 ). A more recent model is the... 

Preparation of Emulsions. An emulsion is a system ia which one Hquid is coUoidaHy dispersed ia another (see Emulsions). The general method for preparing an oil-ia-water emulsion is to combine the oil with a compatible fatty acid, such as an oleic, stearic, or rosia acid, and separately mix a proportionate quantity of an alkah, such as potassium hydroxide, with the water. The alkah 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 ia emulsions, as it is readily miscible with most oils. 

Many different combinations of surfactant and protective coUoid are used in emulsion polymerizations of vinyl acetate as stabilizers. The properties of the emulsion and the polymeric film depend to a large extent on the identity and quantity of the stabilizers. The choice of stabilizer affects the mean and distribution of particle size which affects the rheology and film formation. The stabilizer system also impacts the stabiUty of the emulsion to mechanical shear, temperature change, and compounding. Characteristics of the coalesced resin affected by the stabilizer include tack, smoothness, opacity, water resistance, and film strength (41,42). 

Emulsion Polymerization. Poly(vinyl acetate) and poly(vinyl acetate) copolymer latexes prepared in the presence of PVA find wide appHcations in adhesives, paints, textile finishes, and coatings. The emulsions show exceUent stabiHty to mechanical shear as weU as to the addition of electrolytes, and possess exceUent machining characteristics. 

---