= Granular material
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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.
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