EFFECTS OF TEMPERATURE AND SURFACE-ROUGHNESS ON TIME-DEPENDENT DEVELOPMENT OF WALL SLIP IN STEADY TORSIONAL FLOW OF CONCENTRATED SUSPENSIONS

A flow visualization technique was applied to investigate the time and temperature-dependent development of wall slip and the rheological behavior of a concentrated suspension, containing 63% by volume solid glass spheres and a poly (butadiene-acrylonitrile-acrylic acid) terpolymer matrix, using steady torsional flow. Flow visualization allowed the concomitant determination of the wall slip velocity and the shear viscosity of the concentrated suspension. The deformation rate, shear stress, and the wall slip velocity values during torsional flow were time dependent and asymptotically reached steady-state values. The characteristic time necessary to reach steady state decreased with increasing shear rate and temperature. Increasing temperature also increased the wall slip velocity. The flow visualization technique was further utilized to determine the yield stress of the suspension directly, which was found to decrease with increasing temperature. Increased surface roughness prevented the wall slip of the concentrated suspension under certain conditions; however, it frequently resulted in the fracture of the suspension samples. Sample fracture became more pronounced with the preshearing of the samples.