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Re: rapid boiling at the very end with liquid nitrogen



This certainly doesn't rate the label of 'enlightenment', but I'll
just add my further observations.

It's not just metals that produce this effect. The same 'rapid
boiling at the end' occurs for bananas, hotdogs, handballs etc.

My testable hypothesis is that we are being fooled by the rate of
boiling at the end of the cooling process. The initial cooling has a
BIG delta T and results in an almost continuous flow of the N2 gas
around the object. (The Leidenfrost effect depends on this thin
continuous film of vapor. See Denker's input.

As the endpoint arrives, this all-encompassing film of vapor is
broken up into the more commonly seen tiny bubbles. This final rush
of individual bubbles sounds and looks more 'disruptive', 'vigorous'
etc. but I'll wager that the actual conversion rate of liquid to
vapor does NOT increase at the endpoint.

Anybody got a handy flowmeter to hook up to a dewar?

OK, ya'll, at 1:08 PM -0500 6/13/03, Michael J. Moloney wrote:
My students and I have seen metal samples cooled to liquid nitrogen
temperature, and for all metals we tried (copper, aluminum, iron, and
lead) we saw a rapid boiling at the end, when we think the metal had
reached the same temperature as the liquid.

I've played around with the ideas of heat exchange, heat capacity and
thermal conductivity but have no satisfactory explanation for this
phenomenon.

Any enlightenment would be greatly appreciated.


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Chuck Britton Education is what is left when
britton@ncssm.edu you have forgotten everything
North Carolina School of Science & Math you learned in school.
(919) 416-2762 Albert Einstein, 1936