The effect of electrolytes, solid particles, interfacial tension and shear rate on the orthokinetic coalescence efficiency of aqueous emulsion droplets suspended in oil was investigated. It was found that, for a given shear rate, lowering the interfacial tension lowers the coalescence efficiency. The effect of electrolytes is mainly to lower the interfacial tension of the water/oil interface, electrostatic forces playing a minor role. AlCl3 is more effective in lowering the interfacial tension than KCl, resulting in more stable emulsions in the presence of AlCl3. This trend is opposite to that in oil/water emulsions. The emulsions were found to be most stable at intermediate shear rates. At low and high shear rates the emulsion stability is a minimum. The dependence of the coalescence efficiency on the shear rate is a complicated function of the ratio of hydrodynamic forces to the attractive (van der Waals) and the repulsive forces, mainly arising from drop deformation during collision. Similar to oil/water emulsions, the presence of solid particles at the water/oil interface plays an important role in the stability of the emulsions. It was found that the emulsions are more stable in the presence of colloidal gold particles.
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