Experimental evidence of slip development in capillaries and a method to correct for end effects in the flow of xanthan solutionsa)

Flow experiments using a capillary rheometer with 0.2% xanthan aqueous solutions were carried out to give experimental evidence of the dependence of the slip velocity on the geometrical parameters of the rheometer. The Mooney method was used to evaluate the slip velocity Vs. It was found that Vs is an increasing function of the wall shear stress and also of the length to diameter ratio L/D not considered in the Mooney theory. That is, Vs=Vs(τw,L/D), although, for each τw value, Vs 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 this method and the one by Bagley are highlighted.