Anomalous heating in a colloidal system: Observation of the inverse Mpemba effect

We present the first experimental observation of anomalous heating in colloidal systems, a phenomenon known as the inverse Mpemba effect. In particular, under certain conditions, we observe that an initially cold system heats up faster than an identical warm system when coupled to the same thermal bath. We study the effect in a system consisting of a Brownian particle in a tilted double-well potential set in an asymmetric domain. We also study the forward Mpemba effect, where an initially hot system cools faster than an identical warm system. We observe the forward effect for a wide range of temperatures and domain sizes (region in space explored by the particle). By controlling the relative sizes of the basins of attraction of the stable and metastable potential wells, we can provide a direct kinetic path between the hot initial state and the cold equilibrium. We then observe exponentially faster cooling. This is the first experimental evidence for the strong Mpemba effect.