Rheology of powders and nanopowders through the use of a couette four-vane rheometer : flowability, cohesion energy, agglomerates and dustiness
Abstract
A new four-vane powder rheometer is proposed to study powder and nanopowder rheology at low and high shear rates. As shear rates increase, the powder flow will evolve from a Newtonian regime (low shear rates) to a Coulombic regime (moderate shear rates) and then to a kinetic regime (high shearrates) at which the powder may become self-fluidized because of intense particles collisions. Viscosity measurements of non cohesive glass beads at low shear rates (Coulombic regime) do not strongly depend upon the particle size and can reach values much larger than commonly used viscosities in CFDs models. Since, in the kinetic regime, the shear stress is a strong function of the particle size, agglomerate particle size of cohesive powders can be directly inferred from rheograms. Cohesive carbon black and silica nanopowders have been tested and compared to non-cohesive glass beads microparticles, chosen as reference. For non-cohesive glass bead micropowders, the "agglomerate" diameter average values found from rheological measurements are of the same order of magnitude as the primary diameters of the powder. This indicates that the kinetic theory of granular flows describes well these high shear-rate regimes and that such theory can be used to estimate agglomerate diameters. It is also found that estimated agglomerate sizes of nanometric cohesive materials can reach sizes of hundred micrometers depending upon their cohesion strength. Such agglomerate size measurements have been corroborated with those made by microscopy.