Toward a Grand Challenge in Powder Flows: The Effect of Material Properties, Boundary Conditions and Shear Rate on Fluctuations and Stress Fields in Flowing Powders.

The present research is oriented towards the grand challenge to understand general powder dynamics and the ultimate goal of the work is to develop a quantitative description of active flows of fine powders. The study is centered on the “intermediate” regime of flow where both frictional and inertial effects are important and where fluctuations of strain rate and stress are significant. The main application is in the area of small-size, rough and/or cohesive powders that are industrially relevant. The novelty of the project is to study a relatively large range of materials and flow geometries to gain meaningful insight as opposed to limiting the work to a single “relevant” flow device using a “standard” powder. Extensive previous work mainly in the Physics literature on smooth glass beads and hard, metal spheres have done little to shed light on the behavior of industrially important powders that are usually non-spherical, rough, fine, cohesive and compressible. We report on a series of materials from simple (round beds) to complex (fine, odd-shaped and slightly cohesive), used in a set of judiciously chosen equipment with interchangeable boundary conditions and measure stresses and stress fluctuations as a function of geometry and shear rate. The goal is to develop constitutive equations for powder flows and to use them in continuum-type theoretical models to predict flow patterns, velocity distributions and forces on boundaries such as stationary walls and moving pedals. The approach follows the earlier work of Tardos, (1997), Tardos et al., (2003), and Tardos and Mort, (2005) and applies it to the geometry of the Couette device and to more complex flows such as hopper (funnel) and centripetal (“spheronizer” and high-shear mixer) flows that are relevant to storing, feeding, mixing and granulating powders.