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 be havior of a concentrated suspension, containing 63% by volume solid gl ass spheres and a poly (butadiene-acrylonitrile-acrylic acid) terpolym er matrix, using steady torsional flow. Flow visualization allowed the concomitant determination of the wall slip velocity and the shear vis cosity of the concentrated suspension. The deformation rate, shear str ess, and the wall slip velocity values during torsional flow were time dependent and asymptotically reached steady-state values. The charact eristic 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 util ized to determine the yield stress of the suspension directly, which w as found to decrease with increasing temperature. Increased surface ro ughness prevented the wall slip of the concentrated suspension under c ertain conditions; however, it frequently resulted in the fracture of the suspension samples. Sample fracture became more pronounced with th e preshearing of the samples.