13 April 2021

Cracked, the cold case of why boiling water freezes faster

Microbubbles and the Mpemba effect: Professor Will Zimmerman appears in the Times and comments on a physical chemistry mystery.

Professor William Zimmerman

I have been exploring what I thought was a 50+ year old mystery in physical chemistry. It turns out it is older than that. Aristotle noticed the phenomenon. Da Vinci commented on it. But a Tanzanian high school student (Erasto Mpemba) with a tenacity rarely seen, noticed it in 1964. And kept pointing it out. Eventually, an esteemed diplomat and Physics Professor, Sir Denis Osborne, championed his caused and they published the seminal paper in 1969.

Briefly, Mpemba worked in an cafe after school. One of his jobs was to make the ice cream. He noticed that if he boiled the milk, the hot boiled milk froze before cold milk. No one believed him. Mpemba and Osborne (1969) published a set of experiments that are deceptively simple, conducted by high school students, where they boiled tap water, and then let it sit to cool to a given temperature, before putting the hot water into the freezer, then observed the time to the onset of freezing (first solidification). They timed how long it took. The fastest was with initial temperature 95degC (and the hottest temperature tested). The slowest had initial temperature 18.5degC.

High school students for the last fifty years have tried these experiments. Debate has raged about the Mpemba Effect, as there is no agreed explanation. In 2012, the Royal Society of Chemistry held an international competition, with over 22000 entrants, for the best "solution". No one solved it. Mpemba and Osborne attended the award ceremony to Bregovic (2012), who was judged to have written the best entry with some very nicely done experiments, and collation of the ideas.

Tom Whipple, science writer of the Times, wrote up a nice piece about it in January 2013, which is the first I heard of the controversy. I read it during a stopover on a five hour drive to the south of England (the traffic is that bad!) and puzzled over the pieces that Bregovic was credited with. And it dawned on me where the microbubbles are generated, and how they are responsible for the much faster heat transfer.

I went and bought the Times that evening, and in the next month developed the equations of my new theory. I presented them at Cambridge University in 2015. But a new theory is only half the scientific story at best. Testing a new theory requires a prediction, and that usually means conducting new experiments. Probably like most University labs, conducting new experiments was banned in the last year, with only high priority exceptions. It dawned on me last December that I should analyse experiments already published on this topic. I did, and a prediction that my theory makes about microbubbles and dissolved gases, correlates beautifully with Mpemba and Osborne's 1969 paper. No one else has a theory that correlates any physical property to these experiments. So I have an explanation. It's microbubbles that do it.

Read the paper: 'Towards a microbubble condenser: Dispersed microbubble mediation of additional heat transfer in aqueous solutions due to phase change dynamics in airlift vessels.'

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