Slip velocity of concentrated suspensions in Couette flow

摘要

We present measurement of wall slip velocity in concentrated suspension of non-colloidal particles. The slip in non-colloidal concentrated suspension mainly arises from wall depletion effect since the non-hydrodynamic effects such as those arising from particle-wall interactions can be small. In this work, we provide a simple methodology for the determination of slip velocity, which requires less experimental work compared to other methods available for slip corrections. The experiments were carried out in a cylindrical Couette geometry of a rheometer. The rheological measurements were carried out first with serrated cup and serrated rotor geometry. Next, the serrated rotor was made smooth by a wax coating while the cup remained serrated. The serrated geometry offers no-slip boundary and the measured viscosity is the true viscosity of suspension, whereas smooth rotor showed significant slip at a higher concentration of particles and the measured viscosity was significantly lower. Comparing the wall shear stresses from the two measurements, we have determined the slip velocity at low shear rates. We have also carried out Stokesian dynamics simulation of simple shear flow of suspension bounded between smooth and serrated walls. The slip velocities from the simulations were calculated by similar analysis as done in the experiments. The simulation and experimental results are in qualitative agreement. It is observed that the wall slip velocity varies linearly with the apparent shear rate. The slip velocity determined from experiments is found to increase with the decrease in viscosity of continuous phase. These observations are in agreement with the previous experimental studies on non-colloidal suspensions.