Anomalous cooling without metastability

Since the temperature of an object that cools decreases as it relaxes to thermal equilibrium, one would naively expect a hot object to take longer to cool than a warm one. However, as long observed, sometimes hot water can cool and begin to freeze faster than cold water. Previous confirmed observations of this anomalous thermal relaxation (the "Mpemba effect") have all involved either a true phase transition (such as water freezing to ice) or at least a metastable state. The typical mechanism is that the system is quickly trapped in a long-lived intermediate state. Specially chosen initial conditions whose dynamical trajectories in state space can avoid this intermediate state can then relax exponentially faster than the typical state, which is caught in the long-lived intermediate state. In previous work, we have shown that it is possible to reliably and reproducibly observe the Mpemba effect in a system consisting of a single micron-sized, colloidal particle diffusing in water and moving in a tilted double-well potential. The higher well of the potential corresponds to a coarse-grained metastable state. Here we show that anomalous relaxation can be observed in a potential that has only a single local minimum. We discuss possible mechanisms explaining this effect.