| Abstract: | Aqueous dispersion of nearly monodisperse charged polymer spheres of 250 nm diameter have been investigated in steady and oszillatory shear flow. Polydisperse latices were included for comparison. By using different types of rotational rheometers and a capillary viscometer viscosity measurements could be performed over a shear stress range from 10-3 Pa to 105 Pa. At shear stresses below 10-2 Pa the viscosity is independent of the shear rate but strongly increases with qrowing volume fraction ?. In the stress range of 0.1 to 10 Pa a transition to a lower viscosity level is observed. For ??0.2 this transition is negligible and the dispersion behaves Newtonian. The viscosity decrease (shear-thinning behaviour) becomes significant at higher volume fractions and reaches a magnitude of more than 3 decades for ? = 0.50. Polydispersity increases the viscosity level at small stresses. The latter is also increased by growing particle charge (higher pH-value) but reduced by the addition of NaCl (screening of the charge). For volume fractions ? ?0.4 shear-thickening is found up to a viscosity maximum. The maqnitude of shear-thickening can be reduced by addition of NaOH (increase of particle charge) and broadening of the particle size distribution. The concentrated dispersions behave viscoelastic. They show an instantanious elastic deformation at the onset of shear creep tests and exhibit recoverable strains after unloading. In oscillatory shear the storage modulus approaches a constant value with decreasing angular frequency indicating the formation of a three-dimensional lattice structure. |
