Elongated particles in viscous fluids follow Jeffery orbits — periodic rotations whose character depends on the particle's aspect ratio. Whether dense granular flows of rod-shaped particles follow similar orbits has been unclear.
Researchers sheared frictionless granular rods long enough and found that sufficiently elongated particles reach a quasi-equilibrium state. Their orientational statistics are quantitatively described by classical liquid crystal theory — the same equations that govern thermally-driven molecular liquid crystals. The collision noise from shear substitutes for thermal noise. Athermal granular matter mimics thermal equilibrium.
The mimicry breaks at two limits. At low aspect ratios, the equilibrium theory incorrectly predicts an isotropic (random) state — the real granular system shows ordering that equilibrium theory misses. And when inter-particle friction is introduced, the system shifts from steric screening (shape-based interactions) to frictional gearing (contact-based interactions). The rotational dynamics become fundamentally different from Jeffery orbits.
The friction-driven breakdown is quantified by an effective Ericksen number — the ratio of non-equilibrium rotational driving to steric ordering. When friction pushes this number above a threshold, the system is driven far from equilibrium and the equilibrium analogy fails completely.
The through-claim: a driven system can look like an equilibrium system as long as the driving mechanism produces the same statistics as thermal fluctuations. But the agreement is a coincidence of outcomes, not a shared mechanism. When a new interaction (friction) breaks the coincidence, the system reveals it was never in equilibrium — it was in a state that happened to produce equilibrium-like measurements. The map matched the territory by accident, and the first perturbation exposed the mismatch.