Why hot water freezes faster than cold water
The fact that heated liquids freeze faster than cold liquids is known as the Mpemba effect, which was observed as far back as the 4th century AD by Aristotle. Researchers from Universidad Carlos III De Madrid (UC3M) are demonstrating how this phenomenon occurs in granular fluids.
Study co-author Antonio Lasanta is from the UC3M Gregorio Millán Barbany University Institute for Modeling and Simulation on Fluid Dynamics, Nanoscience and Industrial Mathematics.
“We can simulate on a computer and make analytical calculations to know how and when the Mpemba effect will occur,” said Lasanta.”In fact, we find not only that the hottest can cool faster but also the opposite effect: the coldest can heat faster, which would be called the inverse Mpemba effect.”
In 1960, a Tanzanian student named Erasto Mpemba explained to his teacher that the hotter mixture of ice cream froze faster than the cold one. This inspired a technical document on the subject, and frequent analysis of the effect began.
Regardless of the widespread interest, there have been conflicting explanations on exactly what that the Mpemba effect is and under what circumstances it happens.
“It is an effect that, historically, has not been addressed in a rigorous manner but merely as an anomaly and a didactic curiosity,” said co-author Antonio Prados.”From our perspective, it was important to study it in a system with the minimum ingredients to be able to control and understand its behavior.”
This approach led the researchers to their own theory of what conditions cause the Mpemba effect to occur.
“The scenario that the effect will most easily occur in is when the velocities of the particles before heating or cooling have a specific disposition–for example, with a high dispersion around the mean value,” said researcher Francisco Vega.
This means that the temperature of the fluid can evolve more rapidly if the state of the particles is prepared before the cooling.
The research team is planning to carry out experiments to confirm their theory. According to the scientists, learning more about this effect may lead to useful applications in everyday life. The findings of the study are published in Physical Review Letters.