Chaotic granular mixing

Several models for convective mixing of coarse, freely flowing in granular tumblers have been proposed over the past decade. Powders of practical interest, by contrast, are frequently fine and cohesive, and cannot be analyzed with these models. Moreover, even in the freely flowing regime, mixing transverse to the dominant, convective, direction is typically slow and inefficient. In this paper, we examine two chaotic mixing mechanisms, the first of which can be intentionally applied to increase transverse mixing rates severalfold, with new prospects for further improvements in three-dimensional mixing through judicious process design. The second mechanism occurs spontaneously in fine grains, resulting in mixing rates overwhelmingly exceeding what would be possible in freely flowing grains. Finally, we show that the same chaotic mixing mechanisms seen in simple drum mixers are also found to be at work in more complex blender configurations widely used in batch industrial operations. (c) 1999 American Institute of Physics.