Circular kinks on the surface of granular material rotated in a tilted spinning bucket

We find that circular kinks form on the surface of granular material when t he axis of rotation is tilted more than the angle of internal friction of t he material. Radius of the kinks is measured as a function of the spinning speed and the tilting angle. Stability consideration of the surface results in an explanation that the kink is a boundary between the inner unstable a nd outer stable regions. A simple cellular automata model also displays kin ks at the stability boundary. [S0031-9007(99)09341-2].