(211h) A New Technique to Measure Shear Band Thickness: The Influence of Solid Volume Fraction On Powder Rheology in a Couette Cell with Secondary Axial Flow

The main objective of this study is to propose a newly developed indirect technique of estimating the thickness of shear band for granular matter using solid volume fraction data obtained in a continuous couette cell. We were able to quantitatively measure the solid volume fraction and its fluctuations by using a capacitance probe in a modified Couette shearing cell in which a small secondary axial flow is imposed. It was shown earlier that there is no path from quasi-static to rapid flows by changing the shear rate if the particles are constrained from further dilation. However, by continuously removing a small amount of material in the axial direction, the regime of flow shifts to the intermediate regime where the shear stress-shear rate has a power-law relationship. The decrease in solid fraction indicates that this change of regime happens because the particles gain freedom of movement and they can actually collide. In terms of rheology, we found that the dependency of shear stress to shear rate decreases for particles with an increased propensity for dilation at increasing shear rates (compressible powders). Interestingly, highly deformable (elastic) and odd-shaped materials, i.e., EPDM and a detergent, exhibit the most intriguing behavior in that the materials show little variation in solid fraction during flow but show a very strong dependence of the ratio of shear to normal stress on shear rate (power-law index, n=1.1-1.2).

Our experimental results revealed that while on the outer stationary wall the solid volume fraction remains constant or it increases (i.e. particles pack) by increasing shearing rate, on the inner wall, the opposite is true. Furthermore, we used the capacitance probe to measure solid fraction as a function of position and shear rate in the shear gap and found the thickness of the shearing layer. Using this technique, we found that the shearing layer starts when the solid fraction begins to decrease by increase in shear rate (velocity in radial direction decays as one moves from rotating to stationary wall). Previous studies in a traditional Couette cell predicted the shear band to be 10-14 particle sizes wide close to the rotating wall while our experiments show an increase between 18-24 particle sizes when the axial flow is present. The solid fraction decreases and its fluctuations increase when the material is flowing (axially) in addition to being sheared radially. From the experiments conducted in the Couette device, it appears that the axial flow rate has a minor influence on the solid fraction, i.e., once the material is flowing the solid fraction does not change significantly upon an increase in the axial velocity. We found experimentally that the thickness of the shearing layer increases by substituting round particles by odd shaped ones. Also decreasing the roughness of the rotating wall decreased the thickness of the shear band significantly. Finally, it takes more time for particles trapped outside of the shear band to start dilation than particles inside the shear band.