gonzalez at esrf.fr
Mon Nov 6 14:10:42 EST 1995
Hakon Hope <hhope at ucdavis.edu> wrote:
>When a larger object at room temperature is immersed in liquid nitrogen
>at its boiling point, large amounts of gas form. This gas creates an insulating
>layer around the object, so that cooling to liq N2 temperature is relatively slow.
>I have measured the
>cooling rates of crystal-sized objects in three different media:
>(1) Cold nitrogen gas (100 K)
>(2) Liquid nitrogen (77 K)
>(3) Liquid propane (100 K)
>N2 gas: Target temperature (140 K) reached in about 2 s.
>Liquid N2: 140 K reached in about 0.6 s from the time of immersion .
>Liquid propane: 140 K reached in about 1.2 sec.
>The unambiguous conclusion to be drawn from these measurements is
>that liquid N2 yields the highest cool-down rate among the three media tested. Because the rate of heat transfer is proportional to the temperature difference
>there is an a priori advantage to liquid N2. For the small samples used, heat
>transfer is not impeded by bubble formation.
I once tried freezing lysozyme *without cryoprotectant* both by transferring
it directly into a cold N2 air stream at 100k and by plunging it into
lquid N2. The crystal was held by liquid tension in a small fibre loop
(If you freeze a thin coat of water very quickly you can get
"glassy" ice rather than crystal ice -see, for instance,
J.Chem.Phys 97(2) 15 July 92 page 1282. Of course, this vitreous ice does not
cause diffraction rings).
When freezing it in the cold air stream I didn't get crystalline ice. However,
when using liquid nitrogen I did...Obviously in this case the air stream
managed to cool the crystal faster (these crystals are about .5 mm)
How does this tie up with your results? I was thinking that the air flow
around the crystal was more efficient at taking the heat away than the liquid
N2 (because of the "hot air bubble" effect) but this is contradicted by
your experiment. Another explanation is that the liquid N2 cools down the
air inmediately above it, so that the crystal has the chance to cool more
gradually while it is dropping, while the temperature gradient in the
proximity of a coaxial air stream is a lot steeper...
/ \ /\ /\
/ \ / \ / \ Ana Gonzalez (gonzalez at esrf.fr)
/ \/`'`'\ / \
/`,'`,'`,\ \/`,'`,'\ "Don't assume that I'm ever entirely serious
/ \ / \ or entirely joking" - Larry Wall
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