Horizontal Brazil Nut Effect

A new twist on the Brazil-nut effect appears to be a good way to harvest large particles from a granular mixture, according to recent experiments and simulations performed at the University of Texas at Austin. The Brazil-nut effect is an odd but well-known phenomenon in agitated granular mixtures. Depending on the conditions, shaking containers filled with grains of various sizes will cause the larger grains to rise to the top of the mixture (Update 132), or sink to the bottom. The Texas researchers (contact: Sung Joon Moon, 609-258-2977), however, showed that they could also control the horizontal distribution of large grains by using kinks that spontaneously arise in granular layers for sufficiently large container accelerations. A kink separates two regions oscillating with opposite phase: the granular layer on one side of a kink is moving up while the layer on the other side is moving down. Larger particles flow from the two oscillating regions and collect in the kink. The researchers can control the location of a kink by adjusting the driving signal, and harvest the large grains by sweeping the kink to one side of the container. The research shows that trapping results from avalanches that form at the kink as falling fluid-like regions move past rising, effectively solid, regions. The avalanches lead to internal convection rolls that carry the large particles toward a kink. The horizontal Brazil-nut effect may eventually lead to new commercial methods for segregating granular material by size. (S. J. Moon et al., Phys. Rev Lett., upcoming)