When polymer solutions and suspensions flow through confined geometry such as membrane pore, capillary and porous media, the flow rate enhancement in comparison to bulk flow for a given wall shear stress is observed. This flow rate enhancement is termed apparent slip and it was quantified by an effective slip velocity at the wall. It has been explained by several mechanisms; the restriction of polymer chain conformation near the repulsive wall, stress induced diffusion of polymer chain in inhomogeneous stress field and the electrostatic interaction between the polymer chain and the charged wall.
In this work the experimental analysis of apparent slip phenomenon was accomplished by means of slip velocity and slip layer thickness behavior at the wall. In this experiment partially hydrolyzed polyacrylamide(HPAM) of highly flexible polymer and xanthan of rigid polymer were used as polymers and they were flowed through stainless steel capillaries of different diameters within the range of 100μm to 300μm. Apparent slip phenomenon was observed in this flow system and slip velocity and slip layer thickness were obtained as a function of wall shear stress. Both slip velocity and slip layer thickness were revealed as a decreasing function of polymer concentration. It is due to the decrease of diffusion coefficient and flexibility of polymer chains as the concentration is increased. Slip layer thicknesses of HPAM were much higher than the polymer characteristic length and increased with wall shear stress. It can be considered as an evidence that the stress induced diffusion of polymer chains is the dominant factor of apparent slip phenomenon in this flow system. In case of xanthan, however, the slip layer thicknesses were almost constant with wall shear stress, which showed that stress induced diffusion is not the only reason for flow of rigid polymer solution like xanthan.