Behind the Mpemba paradox.

Area of Science:

• Physics
• Chemistry

Background:

• The Mpemba paradox describes the counterintuitive phenomenon where warmer water can freeze faster than colder water under certain conditions.
• Previous explanations have involved factors like convection, evaporation, and dissolved gases, but a definitive mechanism remains debated.

Purpose of the Study:

• To investigate the role of hydrogen-bond dynamics in the Mpemba paradox.
• To elucidate the physical mechanisms underlying anomalous heat transfer in water during cooling.

Main Methods:

• Analysis of hydrogen-bond stretching and shortening via Coulomb coupling during heating and cooling cycles.
• Evaluation of thermal diffusivity influenced by the ultra-low mass density of the water's surface layer (skin).

Main Results:

• Anomalous relaxation of hydrogen bonds is identified as the primary driver of the Mpemba paradox.
• Heating causes O:H non-bonds to stretch and H-O bonds to shorten due to Coulomb coupling.
• Cooling reverses this, with heat emission rates dependent on initial heat storage, further influenced by enhanced thermal diffusivity from the surface layer.

Conclusions:

• The Mpemba paradox is fundamentally a result of hydrogen-bond anomalous relaxation dynamics.
• Coulomb coupling and altered thermal diffusivity in the surface layer are key factors governing the observed phenomenon.