A granular material is an assembly of macroscopic grains, such as dry sand, whose collective response depends on contacts, friction and rearrangement. A grain is a solid, but the assembly can support a static pile or flow when contacts reorganize. For ordinary sand, grain motion is set mainly by loading and gravity, rather than equilibrium thermal agitation.
Pouring grains slowly through a funnel produces a pile whose free surface approaches an angle of repose. Tilting a prepared layer can instead reveal a threshold for an avalanche; preparation history can make that threshold differ from the final resting slope. Contacts transmit forces through a nonuniform network, and collisions can dissipate mechanical energy. These mechanisms explain why a granular material cannot generally be assigned the same simple constitutive equation as either an elastic block or a Newtonian fluid. Moisture and cohesion introduce further interactions and must be specified in comparisons.
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.
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