Experimental observation of the Mpemba effect in a granular gas · revision history
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Accepted · first revision · record
- Title
- Experimental observation of the Mpemba effect in a granular gas
- Plain
- A granular gas (many grains bouncing and losing energy in each collision) is predicted to show a hotter sample cooling below a target temperature before a cooler one, if the hot sample's speed distribution has the right shape. No experiment has yet confirmed this.
- Precise
- 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.
- Settled by
- Particle-tracking measurement of $T(t)$ and $c_{2}(t)$ in a homogeneous granular gas showing the predicted crossing with prepared initial $c_{2}$.
- Kind
- well-posed
- Answer type
- yes-no
- References
- (empty)
- Literature status
- open
- Status note
- Predicted by Lasanta et al. (2017) and studied in many kinetic-theory and simulation papers through 2026 (for example sheared gases, arXiv:2605.17881); no experimental observation was found.
- Posed since
- 2017