Effect of interparticle forces on shear thickening of oxide suspensions

The rheological behavior of concentrated nearly monodisperse, nearly spheri cal silica, and narrow particle size distribution alumina suspensions was m easured in Couette geometry. Attractive forces between the particles (suspe nsions flocculated at or near the isoelectric point) produce high viscositi es. Repulsive forces between the particles produce dispersed suspensions an d decrease the viscosity at low and intermediate shear rates. The viscosity of the dispersed suspensions shear thickens (abruptly increases and approa ches that of the flocculated suspensions) at a critical shear rate land the refore stress) provided the volume fraction of solids of the suspension is sufficiently high. As the pH is adjusted farther from the isoelectric point the critical shear rate land stress) increase. At pH away from the isoelec tric point the addition of salt decreases the critical shear rate land stre ss). Thus shear thickening is not only dependent upon hydrodynamic interact ions but also depends on surface forces, in particular repulsive interparti cle forces. Increasing the magnitude and range of the repulsive forces incr eases the shear stress land rate) at which shear thickening starts. Reducin g the magnitude of the repulsive force allows the particles to be more easi ly forced into clusters resulting in shear thickening. (C) 2000 The Society of Rheology. [S0148-6055(00)01604-7].