Angle of repose
Angle of repose (or critical angle of repose) of a granular material is the steepest angle of descent or dip of the slope relative to the horizontal plane when material on the slope face is on the verge of sliding. This angle is in the range 0°–90° [1].
References
http://www.kurims.kyoto-u.ac.jp/~kyodo/kokyuroku/contents/pdf/1413-10.pdf Salah A. M. El Shourbagy, Hans-Georg Matuttis. Dependence of the angle of repose of heaps on the particle shape. 理解析研究所講究録 1413 巻 2005 年 75-83.
