A new method to determine the yield stress of a fluid from velocity profiles in a capillary

A new method to determine the yield stress of a fluid from velocity profiles in capillary flow is presented in this work. The method is based on the calculation of the first derivative of the velocity profiles. For this, the velocity profiles of a model yield stress fluid, 0.2 wt.% Carbopol gel, in a capillary were obtained by using a two dimensional particle image velocimetry system. It is shown that the yield stress value may be reliably determined by using only the velocity profiles and the measured wall shear stresses. This fact is corroborated by independent measurements of the yield stress with a stress controlled vane rheometer. On the other hand, the main details of the flow kinematics of yield-stress fluids were also registered and described in this work. Finally, it was found that the gel slips at the wall with a slip velocity that increases in a power-law way with the shear stress.