Granular mixing, segregation, and vibrofluidization
No constitutive equations exist like those for fluids to describe stress in granular media, thus powders or granular materials present a host of challenges. These systems are often considered prototypical complex systems. For instance, the same energy input to mix these systems also causes segregation (like raisins at the bottom of the cereal box).
A simple continuum model of the underlying flow gives insight into the structure of segregation patterns in 2D and 3D flows exhibiting chaotic advection. Mixing and segregation in granular tumblers is perhaps the simplest experimental example of complexity, and we have applied this knowledge in the design of a core laboratory experiment which allows freshmen engineers at Lehigh University to explore this interplay between mixing and segregation.
In contrast, granular media can be treated as a complex fluid that behaves as multiple co-existing phases (gas/liquid/solid) in many systems. We have investigated the rheology of stirred granular media through the transition to vibrofluidization when vertically oscillated. Both through measurement of power to rotate an internal vane and through local high speed density measurements, we can ascertain the state of the bed to understand liquification as a function of effective heating through vibration.