Formation patterns at the air-grain interfaces in spinning granular films at high rotation rates

Experimental and numerical studies are described in which a thin film of air-immersed grains is spun in vertical and tilted containers about their axis. At high rotation rates a steep depression appears around the axis of rotation. Interesting fractal type patterns with dimension D = 1.7 ± 0.05 are observed at the air-grain interfaces in the depression. By utilizing computer simulations, it is shown that the fractal-like patterns may be associated with a sharp deformation of the volume occupied by the particles within the depression hole due to turbulent diffusion.