The angle of repose characterizes the free-surface slope of a resting granular material, measured relative to horizontal under a specified preparation protocol. A slowly poured dry sand pile often forms an approximately conical surface, with slope . It is a bulk consequence of contact friction, particle shape, packing and any cohesion, rather than a universal constant of one chemical substance.
A simple comparison is to pour a pile through a funnel, measure its height and base radius, and infer the slope from their ratio. A second experiment slowly tilts a layer until it avalanches. The onset angle and the angle after flow stops can differ, so reports should state which quantity was measured. For a single sliding block, static balance suggests , but identifying a pile's angle of repose with a microscopic coefficient of friction ignores rearrangement and collective contact geometry.
The angle of repose is the steepest angle of descent or dip relative to the horizontal plane at which a material, such as gravel, sand, or soil, can rest without sliding. It is a measure of the stability of a pile of granular materials and is influenced by factors such as the shape, size, and moisture content of the particles.
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