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Re: cooling water



On Tue, 2 Jun 1998, Bob Purcell wrote:

A neighbour posed this question regarding ice cube formation:

"Very occasionally the cubes form small bumps or even quite long (>1
cm), thin projections from the surface of the ice cube toward the top
of the freezer. What causes these? Why don't I always see them?"

Coincidentally I just posted to sci.physics on this. See below. (I
couldn't resist adding more stuff to it.

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William J. Beaty SCIENCE HOBBYIST website
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Subject: Re: Ice spikes in freezer?
From: billb@eskimo.com (William Beaty)
Date: 1998/05/29
Message-ID: <6kn8uk$u10$1@eskinews.eskimo.com>
Newsgroups: sci.physics

Lollygagpr (lollygagpr@aol.com) wrote:
: Anyone know why sometimes a cube or two of ice in the freezer will have
: a sharp spike coming out of the cube? In mine they get up to an inch or
: so. Anyone know what I'm taking about? The only thing I can think of is
: that there is some sort of E-field in the fridge that cuases this. Like
: a small stream of water bending around a comb with static electricity
: on it. Anyway, this has been bugging me for a while. Let me know if
: anyone knows the answer. Thanks.

If the power fails and is later restored, then ice cubes may freeze
starting at the bottom and sides, since the tray is in conductive contact
with the cooling coils under the surface. The top of the cube remains
liquid as the walls of ice grow inwards.

If this occurs, then as the water expands upon freezing, the remaining
pocket of liquid water gets squeezed. The fluid-filled hole contracts
faster than it would if the water simply solidified without expanding. If
water contracted upon freezing rather than expanding, then the water level
would fall rather than rising, and a sort of funnel-shaped pocket would be
formed. Indeed, exactly this phenomenon is seen when a cup full of melted
paraffin wax freezes from the walls and inward, as long as the ambient
temperature is warm enough so the wax doesn't "skin over." Homemade
candles usually display this funnel-shaped hole surrounding the wick. I
wonder if similar things happen in the freezing, contracting lava in
Hawaiian volcano craters.

In ice, as the hole contracts, the water level rises and the edges of the
rising pool solidify. The pool rises and shrinks until it vanishes as a
contracting point. The end result would be a small, sharp-tipped
"caustic" pyramid in the center of the cube. If you melt a spot in a
sub-0C (frosty!) icecube by using a soldering iron, then watch it
re-freeze, you'll see the small pyramid appear. This was discussed in
Science News within the past year. The authors noticed this effect when
spatters of chicken gravy on the inside of a freezer were all displaying
little sharp points in the center of each gobbet.

If the walls and top of the ice cube contract too fast, then a sort of
"squeeze-bulb" effect occurs, and water is slowly squirted upwards while
the rim of the pool keeps freezing. The water level rises much faster
than the diameter of the pool contracts. A sort of upside-down stalactite
is the result. If you look carefully, you can often see sheets of bubbles
in the clear ice which outline the old bounadaries of the water pocket.
There will be a row of bubbles in the center of the ice-spine, since
dissolved air comes out of solution as the walls of ice close on the
remaining channel of water.

To maximize the length of ice spines, I suspect that one could employ
something like a vertical tube filled with water. As the chilled tube
causes walls of ice to grow inwards, the remaining water would rapidly
rise out of the small aperature at the top. The temperature of the air
would have to be adjusted just right, so that the edges of the rising pool
of water would freeze quickly enough to prevent the pool from running down
the sides, but slowly enough that the pool would not freeze over and plug
up the tip of the lengthening "ice pipe." Freudian, eh? :)

Sometimes the ice-spines are tilted. This could be because of the fan
in the freezer: if air currents cause one side of the water drop at the
tip of the spine to freeze faster, then the water at the tip of the spine
would expand in the opposite direction. Or perhaps the "wind" just blows
the tip-droplet sideways, so it grows diagonally as the edges of the
"pipe" grow upwards. Do tilted ice-spines still appear if the freezer's
fan is disabled?

As with the solidifying wax, the ambient temperature must not be too low,
otherwise the pool of liquid will skin over and freeze from the top
downwards, quenching the spine-building process. And the bottom and sides
of the ice cube must be well chilled, as when an aluminum icecube tray is
sitting in a freezer which has only recently been turned on.

I keep noticing triangular pyramids on ice cubes, and ice-spires having
triangular crossections. I have no clue as to why these should occur.

I've never heard of anyone filming a time-lapse of this phenomena. Since
it fascinates so many people, I bet such a thing would rapidly end up on
the evening national news. Anyone need some instant fame? :)