Slip at the Wall and Extrudate Roughness with Aqueous Solutions of Polyvinyl Alcohol and Sodium Borate

Abstract

Wall slip during flow of viscoelastic fluids through cylindrical tubes and the surface character of the extrudate have been studied using aqueous solutions of polyvinyl alcohol and sodium borate. Experiments with rough and smooth tubes of Plexiglas and aluminum show that the nature of the flow is highly dependent on the characteristics of the tube surface. For example, at apparent wall shear rate γa=7.6 sec−1 in rough and smooth Plexiglas tubes with inside diameter of 0.638 cm, a solution containing 3.5% polyvinyl alcohol and 3.5% Na2B4O7⋅10H2O registered a pressure drop through the rough tube more than twice that measured for the smooth tube. Also, distortion was severe for the extrudate from the rough tube at γa=11.3 sec−1, but comparable distortion from the smooth tube did not appear until γa>30 sec−1. A hot‐film anemometer has been adapted to study slip near the wall of a conduit. With hot‐film and reservoir pressure measurements we have shown that the microscopic nature of a tube wall can promote ...