Experimental observation of the Mpemba effect in a granular gas

Homogeneous gas of inelastic hard spheres or disks with restitution coefficient $r < 1$, freely cooling or coupled to a stochastic thermostat. Kinetic theory predicts that the granular temperature curves $T(t)$ of two samples cross when their initial temperatures and initial fourth Sonine coefficients $c_{2}$ ($c_{2} = (d/(d+2)) \langle v^{4}\rangle/\langle v^{2}\rangle^{2} - 1$ in d dimensions, which vanishes for a Maxwellian) are suitably chosen. The question is whether this crossing occurs in a laboratory granular gas (vibrated monolayer, levitated or microgravity system) with measured $T(t)$ and $c_{2}(t)$ in quantitative agreement with kinetic theory.

Particle-tracking measurement of $T(t)$ and $c_{2}(t)$ in a homogeneous granular gas showing the predicted crossing with prepared initial $c_{2}$.