Apparent slip in colloidal suspensions

Abstract

In this study, we have carried out experiments to characterize the wall slip of colloidal suspensions of kaolinites. To demonstrate slip, the rheological measurements were carried out with parallel-plate geometry with smooth and rough plates. The asperities of the rough surface penetrated the slip layer and created a nearly no-slip region, whereas the smooth plate showed significantly higher slip, a conclusion drawn by comparing the macroscopic flow curves in both cases. Two slip regimes were identified, namely, (i) an elastic slip regime below the yield stress of the suspension where the material slips like a solid and (ii) a slip regime above the yield stress where the material yields and flows. The slip velocity was quantified using a simple phenomenological slip model that seems to capture slip in both flow regimes. The transition from the first slip regime to the other has been resolved numerically as the material starts yielding first at the edge of the parallel-plate geometry with the yield point propagating inwards as the rotational speed is increased. The numerical method also establishes uniquely the yield stress value, which was found to agree with data obtained from parallel-plate, cone-and-plate, and concentric cylinder geometries.