Characterising wall-slip behaviour of carbopol gels in a fully-developed poiseuille flow

Abstract

In this paper, we examine the wall-slip behaviour of Carbopol gel, as a simple yield-stress fluid, in a Poiseuille flow by analysing its velocity distribution across a glass capillary tube and near the wall. Optical coherence tomography (OCT) is used to image the flow of the gel seeded by buoyant tracing particles, and particle tracking velocimetry (PTV) is applied to the flow images to obtain the velocity profiles near the wall. In addition, the dynamics of the flow is measured through a viscometric unit integrated with the flow cell. Our velocimetric and viscometric measurements reveal apparent slip of Carbopol gels on the capillary wall, in a wide range of wall shear stresses below and above the yield stress of the material. Our results confirm the presence of a fully plugged flow within the precision of our velocimetric measurement, where the wall stress lies below the yielding point. Above the yielding point, the presence of wall slip is confirmed over a wide range of shear rates. Our results indicate that the slip velocity increases with the wall shear stress in both regimes. We relate the slip velocity to the excess shear stress on the wall, i.e. the wall shear stress minus the shear stress at the onset of slip, using a power-law scaling, with an exponent independent of the microstructure of the fluid. The effect of microstructure of the gel on the wall slip is studied by either varying the concentration of the gel or the stirring rate during preparation. We find that the higher the concentration of the gel or the higher the stirring rate is, the lower the slip velocity is. Moreover, our experiments indicate that increasing the viscosity of the solvent leads to a decrease in the slip velocity.