This paper encompasses classic trends as well as recent advances in the understanding of emulsion inversion phenomena. The generalized formulation issue is first discussed from hydrophilic−lipophilic balance to the most recent concepts. The so-called standard inversion line on the formulation−composition map exhibits several branches, referred to as transitional and catastrophic inversions, that bound normal and abnormal emulsion regions. Dynamic inversion is also discussed with its hysteresis zones, where both types of emulsions may be attained, depending upon the system's previous history of the formulation−composition map. Recent findings are reported concerning the effect of variables with practical relevance (i.e., stirring energy, viscosity of phases, surfactant concentration, and partitioning) on the standard and dynamic inversion patterns. State-of-the-art emulsion inversion modeling is briefly discussed.

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Attractive emulsions near the colloidal glass transition are investigated by rheometry and optical microscopy under shear. We find that (a) the apparent viscosity η drops with increasing shear rate, then remains approximately constant in a range of shear rates, then continues to decay; (b) the first normal stress difference N1 transitions sharply from nearly zero to negative in the region of constant shear viscosity; (c) correspondingly, cylindrical flocs form, align along the vorticity and undergo a log-rolling movement. An analysis of the interplay between steric constraints, attractive forces, and composition explains this behavior, which seems universal to several other complex systems.PACS numbers: 83.80. Iz, 83.50.Ax, 47.55.Dz Emulsions are relatively stable dispersions of drops of a liquid into another liquid in which the former is partially or totally immiscible. Stability is conferred by other components, usually surfactants or finely divided solids, which adsorb at the liquid/liquid interfaces and retard coalescence and other destabilizing mechanisms.Emulsions can be regarded as repulsive or attractive, depending of the prevailing interaction forces between drops. In repulsive emulsions, droplets repel each other at any center-to-center distance. On the other hand, attractive emulsions exhibit a potential well at a given distance that exceeds the energy associated to random thermal fluctuations. Consequently, drops in attractive emulsions form flocculates and gel-like structures, whereas droplets in repulsive emulsions do not. This Letter describes for the first time unusual rheological and microstructural features in emulsions under shear, such as alternating changes in the sign of the first normal stress difference with increasing shear rate and formation of domains of drops that align perpendicularly to the direction of shearing. It is shown that these features result from the interplay between composition and attractive forces between droplets. Significant similarities between these trends and those reported for different systems, such as liquid crystalline polymers, colloidal suspensions and polymeric emulsions, are also considered.Experiments were performed on emulsions of bidistilled water dispersed in a lubricant oil base provided by Exxon Chemicals ( ρ oil = 871 kg/m 3 , η oil = 91 mPa · s at 25 o C). The emulsions were stabilized using the nonionic surfactant SPAN 80 (sorbitan monooleate, Sigma) at a concentration of 5 wt.%. Emulsification was carried out by mixing in 1-inch internal diameter cylindrical plastic container and blending with a two-blade paddle for 10 minutes at 1500 rpm. Droplets in this emulsions form flocs mainly due to micellar depletion attractions, * mp@rice.edu, † alejandr@rice.edu, ‡ amontesi@rice.edu because the concentration of surfactant is well above the critical micellar concentration (< 1 wt.%).Rheological measurements were carried out in a straincontrolled ARES rheometer using several geometries [1]. Microscopic observations were performed using a customized rh...

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The effect of systematic changes in the structure of chemicals commonly used in demulsification operations (alkylphenol polyalkoxylated resins and polyurethanes) on the stability and properties of brine-in-crude oil emulsions was assessed experimentally. The relative rates of water separation were characterized via bottle tests and rheometry. Equilibrium interfacial tensions were also measured. Transient changes in drop size distributions were quantified using nuclear magnetic resonance. The phenolic resins promoted coalescence of droplets. Optimum performance was obtained with resins exhibiting intermediate hydrophilicity in a manner consistent with the condition of least emulsion stability for conventional oil-water-surfactant systems. In contrast, polyurethanes promoted flocculation but only slow coalescence. The performance of polyurethanes improved with increase of molecular weight. Phenolic resins and polyurethanes acted synergistically when added simultaneously, rendering water separation rates significantly higher than those observed when they were used individually. Polyurethanes aided sedimentation of water at moderate concentrations (ca. 200 ppm) by "bridging" nearby droplets, but they retarded coalescence when added at significantly higher concentrations, even when phenolic resins were present. Plausible mechanisms for demulsification that are consistent with these findings are proposed and discussed.