EXPERIMENTAL-EVIDENCE OF SLIP DEVELOPMENT IN CAPILLARIES AND A METHODTO CORRECT FOR END EFFECTS IN THE FLOW OF XANTHAN SOLUTIONS

Flow experiments using a capillary rheometer with 0.2% xanthan aqueous solutions were carried out to give experimental evidence of the depen dence of the slip velocity on the geometrical parameters of the rheome ter. The Mooney method was used to evaluate the slip velocity V(s). It was found that V(s) is an increasing function of the wall shear stres s and also of the length to diameter ratio L/D not considered in the M ooney theory. That is, V(s) = V(s)(tau(w), L/D), although, for each ta u(w) value, V(s) becomes independent of L/D at large L/D. In addition, a method to determine the excess pressure drop due to end effects is outlined and experimental results presented. The differences between t his method and the one by Bagley are highlighted.