Exponentially faster cooling in a colloidal system

• Published in: Nature (London) Vol. 584; no. 7819; pp. 64 - 68
• Main Authors: Kumar, Avinash, Bechhoefer, John
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 06.08.2020
• Subjects: 639/766/530/2804; 639/766/530/951; Humanities and Social Sciences; multidisciplinary; Science; Science (multidisciplinary)
• ISSN: 0028-0836; 1476-4687; 1476-4687
• DOI: 10.1038/s41586-020-2560-x

As the temperature of a cooling object decreases as it relaxes to thermal equilibrium, it is intuitively assumed that a hot object should take longer to cool than a warm one. Yet, some 2,300 years ago, Aristotle observed that “to cool hot water quickly, begin by putting it in the sun” 1 , 2 . In the 1960s, this counterintuitive phenomenon was rediscovered as the statement that “hot water can freeze faster than cold water” and has become known as the Mpemba effect 3 ; it has since been the subject of much experimental investigation 4 – 8 and some controversy 8 , 9 . Although many specific mechanisms have been proposed 6 , 7 , 10 – 16 , no general consensus exists as to the underlying cause. Here we demonstrate the Mpemba effect in a controlled setting—the thermal quench of a colloidal system immersed in water, which serves as a heat bath. Our results are reproducible and agree quantitatively with calculations based on a recently proposed theoretical framework 17 . By carefully choosing parameters, we observe cooling that is exponentially faster than that observed using typical parameters, in accord with the recently predicted strong Mpemba effect 18 . Our experiments outline the generic conditions needed to accelerate heat removal and relaxation to thermal equilibrium and support the idea that the Mpemba effect is not simply a scientific curiosity concerning how water freezes into ice—one of the many anomalous features of water 19 —but rather the prototype for a wide range of anomalous relaxation phenomena of broad technological importance. A colloidal system is used to demonstrate the Mpemba effect and obtain the parameters responsible for its anomalous relaxation dynamics, which are manipulated to achieve exponentially faster cooling than typical.