The electroviscous effect in ethylcellulose latex suspensions. Effect of ionic strength and correlation between theory and experiments

The viscosity or, more precisely, the rheological behavior of colloidal suspensions is most sensitive to the overall properties of both the particles (mainly size and shape) and their double layers (surface charge and double layer thickness). The mere presence of uncharged colloids alters the ̄ow ®eld in such a way that the apparent dynamic viscosity of the suspension, g, is larger than that of the dispersion medium, g0 [1]. Thus, in the case of a dilute suspension of uncharged spheres, the well-known Einstein formula [g = g0(1+2.5/), / being the volume fraction of solids] can be applied. Furthermore, if, as is normally the case, the particles possess a nonzero surface charge and, as a result, an ionic atmosphere or electrical double layer, the viscosity of the suspension is even higher, giving rise to the so-called electroviscous e€ects [2, 3]. In the case of dilute suspensions of rigid particles, only the primary electroviscous e€ect manifests itself: when the liquid dispersion medium is forced around the particle by means of an externally applied shear ®eld, the electrostatic attraction between the particle's surface charge and the counterions in the double layer hinders the liquid ̄ow inside the double layer and provokes a further increase in the bulk suspension viscosity. As / is increased, higher electroviscous e€ects (secondary and tertiary) may appear, besides the fact that the g±/ relationship also ceases to Colloid Polym Sci 278:647±653 (2000) Ó Springer-Verlag 2000 ORIGINAL CONTRIBUTION