RHEOLOGICAL STUDY OF W/O/W EMULSION BY A CONE-AND-PLATE VISCOMETER - NEGATIVE THIXOTROPY AND SHEAR-INDUCED PHASE INVERSION

The rheological behaviour of a w/o/w emulsion was examined by a cone-and-plate viscometer. Negative thixotropic flow patterns were observed at lower shear rates. This negative thixotropic behavior was more pronounced and the apparent viscosity increased under increased shear rate, prolonged shearing time, or repeated shear. Further shearing, by raising the shear rate or prolonging the shearing time, rapidly increased the shear stress of the emulsion and induced phase inversion. This phase-inverted emulsion was of the w/o type and in a semi-solid state. The energy dissipated by the flow of the emulsion until the occurrence of phase inversion, the kinetic energy of the emulsion at the phase-inversion point, and the impulse applied to the emulsion by the cone of the viscometer were calculated in order to determine the hydrodynamic parameter determining the phase inversion. The impulse applied by the rotating cone was the parameter determining the phase inversion of the emulsion from a w/o/w to a w/o type characterized by being in the semi-solid state. The negative thixotropic flow and the phase inversion of the w/o/w emulsion to the w/o type induced by shearing were explained as being due to the increase in the volume fraction of the oil droplets by entrapment of water molecules and by coalescence of the oil droplets.