Abstract
Abstract The rheological properties of a series of oil-in-water-in-oil (O/W/O) and water-in-oil-in-water (W/O/W) types emulsion liquid membrane systems were measured using a controlled-stress rheometer. The effect of storage time (aging) on the rheological properties was also determined. The fresh O/W/O type emulsion liquid membranes exhibited a power-law non-Newtonian behaviour at high values of the primary emulsion concentration. Also, the viscosity increased with the increase in the total dispersed phase concentration. Upon aging, the viscosity of the O/W/O type emulsion liquid membrane decreased significantly. The photomicrographs indicated a decrease in the diameter of the multiple droplets due to expulsion of internal phase droplets to the external phase. The fresh W/O/W type emulsion liquid membrane (primary emulsion volume fraction of 0.692) exhibited a highly nonlinear behaviour. With the increase in the shear stress, the viscosity first decreased substantially. However, at some critical shear stress, a sharp increase in viscosity was observed indicating shear-thickening effect. The shear-thickening effect observed in W/O/W type systems was due to phase inversion; the W/O/W type emulsion liquid membrane system inverted to water-in-oil (W/O) emulsion at high shear stresses. Upon aging, the viscosity of the W/O/W type emulsion liquid membrane initially increased with an increase in storage time and then decreased with a further increase in storage time. The photomicrographs indicated swelling of dispersed droplets initially with the increase in storage time. The droplet size, however, decreased significantly with further increase in storage time. The changes in the rheological properties with aging are explained in terms of droplet size changes that occur during aging.
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