| This thesis explores wall slip and flow instability issues first by developing and applying experimental means to characterize the onset of macroscopic wall slip in simple shear flow and then relates the critical shear stress at which wall slip is onset to extrudate distortions upon die flow on one hand and to the detachment of the material from the wall in continuous shear roll milling process on the other hand. Furthermore, the characterization of various rheological material functions, including the development of shear and first normal stress difference overshoot and the relaxation modulus, is reexamined together with slip, to suggest that at least for the polymer melts investigated here (a high density polyethylene, a poly (dimethyl siloxane) and an oxetane thermoplastic elastomer) the shear and first normal stress overshoots are artifacts of wall slip and for polymer melts, which exhibit strong wall slip, the relaxation modulus data cannot be collected in the nonlinear region, with important ramifications on the determination of the parameters of nonlinear viscoelastic constitutive equations. Finally, the complications involved in the characterization of wall slip velocities for complex fluids which undergo various structural changes during flow are demonstrated for a bar soap formulation, which exhibits wall slip, as well as changes in the orientation and size distributions of its ingredients during extrusion. |
