Study may explain counterintuitive effect of why hotter systems can cool more quickly

Good luck grappling with this longstanding physics conundrum. Even the meaning of ‘faster’ in this context could be open to question. For example there’s rate of heat loss, then there’s total time taken.

Ever since the days of Aristotle, people have made the counterintuitive observation that hot water sometimes freezes faster than cold water, says Phys.org.

In modern times, the observation has been named the Mpemba effect after Erasto Mpemba, an elementary school student living in what is now Tanzania in the early ’60s. When making ice cream, Mpemba observed that using warmer milk causes the ice cream to freeze faster than when using colder milk.

In the last few decades, the Mpemba effect has been studied and observed in several physical systems besides water, including carbon nanotube resonators and ice-like water cages called clathrate hydrates. Despite these findings, the causes of the effect are not well-understood.

Proposed explanations include the presence of impurities, hydrogen bonding, and supercooling. Even the mere existence of the Mpemba effect remains controversial, as one recent study found insufficient evidence to replicate a meaningful effect.

Now, their interest rekindled by a recent paper proposing a generic mechanism for similar effects, scientists Antonio Lasanta and co-authors from universities in Spain have returned to the question in a new study published in Physical Review Letters.

In their work, the researchers theoretically demonstrate and investigate the Mpemba effect in granular fluids, such as those made of sand or other small particles. Using simulations of granular systems and a simple kinetic theory approach, the researchers were able to determine that the initial conditions in which the system is prepared play a critical role in determining whether or not the system exhibits the Mpemba effect. Their analysis also enabled them to identify the initial conditions required in order for a granular system to exhibit the Mpemba effect.

“Our work shows that the existence of the Mpemba effect is very sensitive to the initial preparation of the fluid or, in other words, to its previous history,” co-author Andrés Santos at the University of Extremadura in Badajoz, Spain, told Phys.org. “In our opinion, this may explain the elusiveness and controversy of the Mpemba effect in water, as a consequence of the lack of control on the detailed initial preparation of the sample.”

As the researchers showed, if a system is not prepared under certain initial conditions, then the colder system cools down more quickly than the warmer one, as expected, and there is no Mpemba effect.

“We theoretically showed, at least in the case of a gas, that a system’s temperature evolution and thus its cooling and/or heating rate do not depend on initial temperature alone, but also on the previous history of the system that control the initial value of the additional variables,” Santos said. “Therefore, it is perfectly possible that an initially heated system cools down quicker than a colder one with a different history.”

Continued here.