Rheology of foams and highly concentrated emulsions: IV. An experimental study of the shear viscosity and yield stress of concentrated emulsions

Abstract The yield stress and shear viscosity have been determined for a series of well-characterized, highly concentrated oil-in-water emulsions (φ > 0.74). The lower end effect in the concentric-cylinder viscometer was eliminated and wall slip was corrected for. The yield stress is given by r 0 = oφ 1 3 Y (φ) R 32 , where σ is the interfacial tension, φ is the volume fraction of the dispersed phase, R32 is the surface-volume mean drop radius, and Y(φ) is a function whose values are found to be significantly lower than those obtained in a previous study. The shear viscosity of all emulsions is accurately described by u e = r 0 Y + 32(φ−0.73)u Ca −1 2 , is the shear rate, μ is the viscosity of the Newtonian continuous phase, and Ca ≡ Ca (− 1 2 VS − 1 3 is the capillary number, whose upper value was about 10−4. The second, viscous term deviates from that predicted by our recent model in the value of the exponent of Ca ( −1 2 vs −1 3 ) and in the appearance of a simple geometric factor that is volume-fraction-dependent. The different exponent may be the result of disjoining-pressure effects in our experimental emulsion systems.