= Angle of repose
{title2=$\theta_r$}
{wiki}
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 $\tan\theta_r$. 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 $\tan\theta=\mu$, but identifying a pile's <angle of repose> with a microscopic <coefficient of friction> ignores rearrangement and collective contact geometry.
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