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 inte rest, by contrast, are frequently fine and cohesive, and cannot be analyzed with these models. Moreover, even in the freely flowing regime, mixing tra nsverse to the dominant, convective, direction is typically slow and ineffi cient. In this paper, we examine two chaotic mixing mechanisms, the first o f which can be intentionally applied to increase transverse mixing rates se veralfold, with new prospects for further improvements in three-dimensional mixing through judicious process design. The second mechanism occurs spont aneously 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 t o be at work in more complex blender configurations widely used in batch in dustrial operations. (C) 1999 American Institute of Physics. [S1054-1500(99 )02803-7].