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Ice Formation
August 19, 2024
In today's
bigger is better world, you don't order a large
coffee, you order a 20
fluid ounce Venti coffee. From 1987 through 2004,
McDonald's restaurants had a
supersize option for larger than large
portions of its
French fries and
soft drinks. The
demise of this supersize
upselling was
precipitated by the 2004
documentary,
Super Size Me, by
director,
Morgan Spurlock (1970-2024).[1] Spurlock
dined at McDonald's restaurants three times a
day for thirty days,
consuming an
average of 5,000
kcal per day, far in excess of the
recommended 2,400-2,800 kcal per day for
adult men. His documentary highlighted the rise in
obesity in the United States, as shown in the figure.
Adult obesity in the United States, 1962 to 2014, as measured by the Centers for Disease Control and Prevention (CDC). (Left image and right image by Delphi234, both from Wikimedia Commons. Click for larger image.)
The
prefix,
super, has been used to describe
supercooling, the
unexpected cooling without a
phase change when
liquids can be cooled below their
freezing points without
solidifying. I was introduced to the
concept of supercooling in
elementary school while
listening to my
shortwave radio
and
hearing an extended
discourse on the
topic by an
amateur radio operator.
Stream of Consciousness radio chatter didn't start with the
cellphone/ It's existed on the
amateur radio bands since their inception.
As I wrote in an earlier
article (Supercooled Water, December 22, 2011),
Water has so many unusual
properties that there's a
website dedicated to them.[2] The unusual properties of water most probably are a result of water
molecule being small, and the
forces holding these molecules together in a
liquid or
solid arise from
hydrogen bonding. There are presently twenty confirmed
solid phases of
water ice. The most common ice, the one that
plagues our
winter driving, is
hexagonal ice Ih, and the others exist at different
temperatures and
pressures.
Supercooled water is a
hazard to
aviation, since supercooled water
droplets often existing in
cumulus and
stratus clouds will instantly freeze on
aircraft surfaces and
plug the
Pitot tubes that
indicate airspeed. It's easy to create supercooled water in the
laboratory. You just need to
purify the water to remove contained
minerals. The mineral
crystals act as
nucleation sites, an
effect behind the
idea that
bearded chemists have better
luck with
crystallizing reaction products. High purity water will only supercool to -48.3
°C (-54.9
°F). All of this
molecular dynamics is reflected in thermodynamics, since the
heat capacity and
compressibility of supercooled water would become
infinite at that temperature.[3]
Ice nucleation on surfaces is the topic of a recent
article in
Quanta Magazine by
Elise Cutts, a
geobiologists who is now a
science journalist in
Graz, Austria. One of my
graduate school classmates did his
thesis experiments on the
deposition of thin
silver films on
salt crystals (halite). The
arrangement of atoms at the halite
surface matches that of silver, and this facilitated the deposition of the silver films. The same is true for surfaces that nucleate ice.[4] Those whose arrangement of surface atoms
mimic that of ice are good at ice nucleation.[4]
Snowflake and a Structural model of an hexagonal close packed crystal. The six-fold radial symmetry of some snowflakes arises from the hexagonal crystalline structure of ordinary ice (ice Ih). (Left, a Wikimedia Commons image by Janek Lass. Right image created using Inkscape and also uploaded to Wikimedia Commons. Click for larger image.)
Valeria Molinero and
colleagues at the
University of Utah (Salt Lake City, Utah) have developed a
model for
prediction of the nucleation temperature of ice on a given surface.[4] Model
parameters include the
shapes of surface
defects, and appropriately sized and shaped surface
bumps and
depressions can
squeeze water molecules into
configurations that make it easier or harder for ice to form.[4] Molinero says that knowing what surface features nucleate ice will allow control of ice formation.[4]
Bacteria and
fungi are
efficient natural ice nucleators because of the way their
proteins act as ice
templates.[4] The best such natural ice nucleators the
Pseudomonas syringae bacterium, which is used to
make artificial snow.[4] Larger protein molecules are usually better at ice nucleation, but small fungal proteins are good at ice nucleation when they clump into larger
aggregates.[4] Molinero states that
atmospheric models of cloud formation can be improved if the ice nucleation process is better understood.[4]
References:
- Super Size Me, 2004, Morgan Spurlock, Director, on the Internet Movie Database.
- Martin Chaplin, "Anomalous properties of water," London South Bank University Web Site.
- Emily B. Moore and Valeria Molinero, "Structural transformation in supercooled water controls the crystallization rate of ice," Nature, vol. 479, no. 7374 (November 24, 2011), pp. 506-508.
- Elise Cutts, "The Enduring Mystery of How Water Freezes." Quanta Magazine, June 17, 2024.
- Debdas Dhabal, Rajat Kumar, and Valeria Molinero, "Liquid–liquid transition and ice crystallization in a machine-learned coarse-grained water model," Proc. Natl. Acad. Sci., vol. 121, no. 20(May 6, 2024), Article no. e2322853121, https://doi.org/10.1073/pnas.2322853121.
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