The Mpemba effect in the Descartes protocol: A time-delayed Newton's law of cooling approach

TL;DR

This paper introduces the Descartes protocol, a three-reservoir thermal scheme, to analyze the Mpemba effect using a time-delayed Newton's law of cooling, providing exact conditions and approximations for the effect.

Contribution

It develops an analytically tractable framework for the Mpemba effect in multi-step thermal protocols, extending previous two-reservoir models with new bounds and approximations.

Findings

Exact conditions for the Mpemba effect are derived for instantaneous quenches.

The maximal effect occurs at a specific warm temperature and when the waiting time equals the delay time.

The Descartes protocol yields a smaller maximal effect compared to the two-reservoir protocol.

Abstract

We investigate the direct and inverse Mpemba effects within the framework of the time-delayed Newton's law of cooling by introducing and analyzing the Descartes protocol, a three-reservoir thermal scheme in which each sample undergoes a single-step quench at different times. This protocol enables a transparent separation of the roles of the delay time $\tau$, the waiting time $t_{\text{w}}$, and the normalized warm temperature $\omega$, thus providing a flexible setting to characterize anomalous thermal relaxation. For instantaneous quenches, exact conditions for the existence of the Mpemba effect are obtained as bounds on $\omega$ for given $\tau$ and $t_{\text{w}}$. Within those bounds, the effect becomes maximal at a specific value $\omega=\widetilde{\omega}(t_{\text{w}})$, and its magnitude is quantified by the extremal value of the temperature-difference function at this optimum. Accurate and compact approximations for both $\widetilde{\omega}(t_{\text{w}})$ and the maximal magnitude $\text{Mp}(t_{\text{w}})$ are derived, showing in particular that the absolute maximum at fixed $\tau$ is reached for $t_{\text{w}}=\tau$. A comparison with a previously studied two-reservoir protocol reveals that, despite its additional control parameter, the Descartes protocol yields a smaller maximal magnitude of the effect. The analysis is extended to finite-rate quenches, where strict equality of bath conditions prevents a genuine Mpemba effect, although an approximate one survives when the bath time scale is sufficiently short. The developed framework offers a unified and analytically tractable approach that can be readily applied to other multi-step thermal protocols.