Spontaneous chaotic granular mixing

There are several types of instabilities in fluid mechanics that lead to sp ontaneous chaotic mixing and intricate patterns. Classical examples include the Kelvin-Helmholtz instability(1,2) in shear layers, the instability of Taylor-Couette flow between rotating cylinders(3,4) and the Rayleigh-Benard instability in thermal convection(5). More recently, a variety of two- and three-dimensional chaotic mixing phenomena have been observed in other geo metries(6-9). Mixing in granular flows(10,11), unlike that in stirred fluid s, is thought to be diffusive-although periodic forcing has been used to en hance granular mixing(12-13), spontaneous chaotic granular mixing has not p reviously been reported. Here we report the observation of chaotic granular mixing patterns in simple cylindrical tumblers partially filled with fine grains. The patterns form spontaneously when sufficiently fine grains (less than or similar to 300 mu m diameter) are blended. We identify the mechani sm by which the chaotic patterns are produced: a periodic stick-slip behavi our occurs in the shear layer separating static and flowing regions of grai ns. This causes weakly cohesive grains to mix at rates overwhelmingly excee ding those achievable for previously studied(11,14) freely flowing grains.