An experimental investigation of the Mpemba effect

Abstract

The Mpemba effect is not well defined, as it does not distinguish between an initially hotter water sample reaching 0 °C before an initially colder sample or an initially colder sample supercooling and entering its liquid-ice phase transition after an initially hotter sample. These two definitions are explored by measuring the cooling curves of deionized water samples in lidded tins on an expanded polystyrene tray; deionized water in lidded tins on a baking sheet; deionized water in unlidded plastic beakers in an expanded polystyrene tray; tap water in lidded tins on an expanded polystyrene tray; and deionized water in lidded tins in an expanded polystyrene tray. From 176 measurements, 46 and 58 instances of the Mpemba effect as defined in the two aforementioned ways are observed. The temperature measurements align well with Newton's law of cooling, allowing for temperature exponential decay rates to be extracted. It can be concluded that Mpemba effect observations arise from two paths: fluctuations in the convective air currents that dominate the cooling process and variations in supercooling among water samples. The largest temperature differences that exhibited the Mpemba effect were those with tap water, which indicates ions may also be relevant for both interfacial energy transfer and supercooling.