The Mpemba effect is surprising: hot water sometimes freezes faster than cold. Scientists built a simple model: a bead in a thick, honey-like medium, on a landscape with two dips and a wall at the edge. The bead moves viscously, unable to hop over the central bump. The hot bead, having more energy, darts around wider, hits the wall more often, and after bouncing off, rolls into the target dip, settling down. The cold one moves sluggishly, lagging behind. The wall is key: without it, the effect vanishes in an infinite space. The dips can be any shape — symmetry isn't required.
This simple mechanism helps explain puzzles across sciences — from chemistry to materials science.
as written by the authors arXiv:2604.01543
The original Mpemba effect refers to the idea that a macroscopic hot system cools faster than an initially colder one. In its microscopic and classical versions, the system is modeled as an overdamped particle in an external potential, and the corresponding Mpemba effect has been observed experimentally and explored theoretically. We establish that the existence of the one-dimensional Mpemba effect for a particle in an asymmetric polynomial double-well potential is driven solely by the presence of a hard wall, irrespective of the potential's double-well shape and metastability. The Mpemba effect disappears if we consider an infinitely large system. We then show that the underlying mechanism of the Mpemba effect is governed by the fine structure of the initial statistical population as it probes the tails of the potential, which also explains the Mpemba effect in single-well and symmetric double-well potentials.
The effect was discovered by a Tanzanian schoolboy who noticed that hot ice cream mixture hardens faster than cold.