SLIP VELOCITY AND SLIP LAYER THICKNESS IN FLOW OF CONCENTRATED SUSPENSIONS

The rheological characterization of highly filled suspensions consisti ng of a Newtonian matrix (hydroxyl-terminated polybutadiene), mixed wi th two different sizes of aluminum powder (30% and above by volume) an d two different sizes of glass beads (50% and above by volume), was pe rformed using a parallel disk rheometer with emphasis on the wall slip phenomenon. The effects of the solid content, particle size, type of solid particle material, and temperature on slip velocity and slip lay er thickness were investigated. Suspensions of small particles of alum inum (mean diameter of 5.03 mu m) did not show slip at any concentrati on up to the maximum packing fraction. However, suspensions of the oth er particles exhibited slip at the wall, at concentrations close to th eir maximum packing fraction. In these suspensions, the slip velocity increased linearly with the shear stress, and at constant shear stress , the slip velocity increased with increasing temperature. The slip la yer thickness increased proportionally with increasing size of the par ticles for the glass beads. Up to a certain value of (filler content/m aximum packing fraction), phi/phi(m), the slip layer thickness divided by the particle diameter, delta/D-P, was 0, but it suddenly increased and reached a value that was independent of phi/phi(m) and the temper ature. On average, the ratio of delta/D-P was 0.071 for aluminum and 0 .037 for glass beads. (C) 1998 John Wiley & Sons, Inc.