Re: Re> Spikes growing from

["Jim Peters" <jim.peters@ac.hillsdale.edu>]

        Reply to:   RE> Re> Spikes growing from ice cubes

Bill and Leigh,
     Some results from the study of metallic solidification may help to
understanding ice freezing.  When a liquid metal is pored into a mold the
metal freezes from the wall of the mold inward.  Different parts of the wall
start solidification or nucleation, at the same time.  Each solid "icicle" of
solid growing into the liquid is a crystal with long range atomic order. 
Crystals which happen to have  their crystallographic direction of fastest
growth perpendicular to the mold surface and parallel to the direction of heat
flow are favored under certain conditions.  Then we get long columns of
crystals growing into the liquid.  Eventually they meet other crystals and
growth stops.  Thus castings may have this columnar growth leading to
non-random orientations of their grains.  (Grains are regions of a specific
crystallographic orientation.)
     This suggests a possible explanation for the spikes in ice.  Ice is known
to exhibit large grains, on a frozen lake for instance, sometimes growing to
many inches in size, much larger than the frequently tiny crystals in metal
castings.  Suppose the water in an ice cube is freezing from the bottom up.  A
grain nucleated early, near the center of the liquid, with its direction of
fastest growth up, may reach the top before the surrounding liquid has had a
chance to freeze.  If it grows out of the liquid into the air perhaps liquid
molecules will transport along the surface of the spike to the tip where they
are more likely to attach than the sides.  The heat of fusion released on
freezing may under just the right conditions allow the spike to be covered by
a thin film of liquid in equilibrium, so that growth continues above the
original liquid surface.  This would explain the ice spikes leaning sideways
at large angles that you noted below. It would just be that this was the only
orientation that nucleated.  These ideas would also explain the prism shapes
sometimes observed.  Are these shapes hexagonal?  Ice is hexagonal and may
have its direction of fastest growth along its hexagonal direction in some
circumstances and along the hexagonal directions in other circumstances. 
     Has anyone ever seen an ice tube?  If they existed it would seem that if
you caught the spike tube in the act of growing it would be easy to see and
preserve just by pouring the liquid out of its center.  If they don't exist,
perhaps the explanation above will suffice. I would love to see some real data
or conclusive discussion. 

Jim Peters
Hillsdale College
jim.peters@ac.hillsdale.edu
------------------------
 
> Anyone here heard any more about upside-down icicles growing in the
> icecube tray?  I"ve been kicking around a theory, and today I saw the
> effect for the first time.  The departmental fridge got a weekend
> cleaning, and today there were two spikes in the icecube trays; one over
> 1in tall
> . . . 
> The classic report of this effect is when a refridgerator suffers a long
> power failure.  The freezer section warms up and the ice melts.  When
> power is restored, the icecube trays are against a chilled plate, so
> freezing can take place starting with the bottom.
>
> There are reports of ice spikes which lean sideways at extreme angles. 
> This might be caused by air motion disturbing the rate of freezing at one
> side of the meniscus?  Also, ice spikes are frequently prisms rather than
> perfect cylinders.  Their cross-sectional shape might be programmed by the
> shape of the open patch of water at the top of the cube at the onset of
> spike growth? 
>
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