The viscosity concept of thermal systems is borrowed to describe theflowability of granular powders in this article with the granular temperatureis defined analogously. Eyrings rate process theory and free volume concept,which have been proved to be very powerful in dealing with many thermallyactivated phenomena in a wide variety of fields, are utilized to deriveviscosity equations of granular powders under a simple shear. The obtainedviscosity equations are examined only with empirical experimental observationsin describing powder flowability, due to the lack of instruments andmethodology for directly determining the viscosity of granular materials. Thecontinuous shear thickening rather than the discontinuous shear thickening arepredicted and found to be dependent on shear rate, the cohesive energy betweenparticles, and the particle volume fraction, though the discontinuous shearthickening may still occur if certain conditions are met during shear, such aslocal particle volume fractions approach to the jamming point created by theshear induced inhomogeneity. A fundamental mechanism on how dry granularpowders flow is proposed on the basis of what are demonstrated from theviscosity equations.The work presented in this article may lay a foundation toscale powder flowability in a more fundamental and consistent manner, at leastproviding an approach to consistently define the viscosity of granular powders.Since the same approaches are employed to derive the viscosity equations ofgranular powders as used to derive viscosity equations of liquids, colloidalsuspensions, and polymeric materials, both athermal and thermal systems arethus unified with a single methodology.
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