THE RHEOLOGY OF CONCENTRATED DISPERSIONS OF WEAKLY ATTRACTING COLLOIDAL PARTICLES WITH AND WITHOUT WALL SLIP

It is demonstrated that weakly flocculated, concentrated colloidal dis persions show slip flow when sheared between smooth concentric cylinde rs. The precise pattern of behavior seen depends upon the stress and u pon how long the flow is left to establish prior to measurement. With delay times of order hours, slip is not seen until a critical stress i s exceeded (typically about 1 Pa) and, thus, the true low-shear viscos ity can be determined provided care is taken to ensure the stress does not exceed the critical level. With short delay times of order minute s, slip is seen irrespective of how small the stress is and the low-sh ear viscosity can be underestimated by several orders of magnitude. Co mparisons of flow curves obtained using smooth and roughened cylinders show that slip only occurs at the inner cylinder, and also that bulk flow is re-established at higher stresses where the dispersions start to shear thin. The apparent low-shear, relative viscosity measured in the presence of slip appears, to a first approximation, to depend only upon the concentration of particles, and not on particle size, medium viscosity, or the strength of the attractive forces causing the flocc ulation. In consequence the slip coefficient appears to depend primari ly on particle concentration. In contrast, the true low-shear, relativ e viscosity (RLSV) is found to increase exponentially with the interac tion strength. For example, an attractive well in the interparticle po tential of order 10 kT gives rise to RLSV of order 10(6) in the concen trated, submicron dispersions studied here.