Experimental investigation of cohesive powder rheology

An annular shear cell-type cohesive powder rheometer has been fabricated, which measures the shear and normal stress states over a range of shear rate and powder concentration. The rheological behavior of a 30-μm spherical silica powder and a 40-μm industrial polymer powder have been experimentally investigated for powder flow in the frictional regime. Although the silica powder appears to be cohesionless, based on Jenike shear cell yield locus measurements, and the industrial powder has strong cohesion, they both demonstrate similar rheological behavior in the frictional flow regime. At low shear rates, the shear stress increases with increasing shear rate until a critical shear rate is reached, at which point the shear stress decreases with increasing shear rate. The shear-to-normal stress relation with shear rate, for frictional flow, is apparently independent of powder concentration, and the data approximately collapse into a single curve for a given powder. The requirements for a constitutive model relating the stress tensor to the rate of deformation for the powder assembly are discussed.