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Axial Segregation
Avalanche Dynamics
Published Research Papers
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A fascinating property of binary mixtures of granular media is their tendancy to segregate when shaken, rotated or poured. For example, an initially homogenous jar of mixed nuts will segregate from the vibrations during shipping. When opened, the larger Brazil nuts will be on the top of the jar, while the smaller ones reside on the bottom.

Research at the University of Minnesota's granular material laboratory focuses on two unique, self-segregating phenomena: axial segregation of rotated mixtures of glass beads and avalanche stratification of poured mixtures of sand and sugar.

Axial Segregation Petite Movie

Axial Segregation

Rotating cylinders or drum mixers are used for mixing granular materials in addition to other industrial processes including calcinating and drying granular materials. In some cases, however, horizontal drum mixers will also segregate the individual species. For example, when a horizontal cylinder is partially filled with two different sizes of glass beads and rotated about its long axis, the mixture separates into alternating bands of relatively pure single concentrations (see above icon). When the cylinder is rotated at ~15 rpm, segregation occurs within a few minutes. We have found that decreasing the rotation speed to ~5 rpm causes the bands to disappear, restoring the mixed state. This reversible axial segregation has been studied with digital surface analysis and magnetic resonance imaging.

 

Tiny, little avalanching movie.

Avalanche Dynamics

Segregation of granular materials is a significant problem for particle processing industries such as pharmaceutical and agricultural firms. Initially homogeneous, binary mixtures of sand and sugar, when poured against the closed edge of a "quasi-two-dimensional" Hele-Shaw cell (similar to an ant farm) will spontaneously form alternating stratified layers (note the above icon). We have used digital images taken with a CCD camera to determine the wavelength and degree of segregation of the bands as a function of plate separation of the cell and flow rate. These results are compared with several numerical and (increasingly) physical computers simulations of the system.

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