To: <dishhead@iquest.net>
Cc: <mchen@ugastro.berkeley.edu>
Subject: ice spikes 


The explanation from Henry Bloomfield is basically correct but it's 
missing a few details. Although the rate of freezing has something to 
do with the spikes, it does not have to happen very slowly, just not 
very fast. The type of spikes you have are specific to the fact that 
you are using a plastic ice cube tray and it's the pressure from the 
plastic tray that forces the water to rise up out of the a small hole 
as the cube freezes. Metal trays will not produce the spikes.  Different 
spikes can also form under different conditions as this phenomenon has 
been observed in frozen puddles and plastic flower pots with frozen water 
outdoors.  There have been some that are a half a foot high reported on 
the mountain tops in hawaii.  The spikes you see in your freezer should 
typically have a triangular cross-section, with an occasional rectangular 
cross-section, unless they have become rounded after melting.  The angle 
and cross-section of the spike is determined by frozen sheets of ice forming 
within the ice cube. These ice sheets actually begin to crystallize within 
the ice cube before the top surface or side walls of the cube begin to freeze.  
If you time it right, about an hour and a half to two hours after putting in a 
fresh ice tray, you can check the cubes to see that ice sheets within the cube 
begin to form before anything else begins to freeze.  These ice sheets form 
random planes that are suspended within the cube and must not be disturbed 
for spikes to form.  That is why there is better chance of spikes forming 
when the freezer is not disturbed. If three of these sheets of ice should 
form a triangle, they have the ability to form a funnel.  Once this funnel 
is formed and is near the surface, the surface can lock the funnel in place 
by freezing.  If the conditions are not right, then the funnel will close 
and form a triangular divot on the surface of the ice cube. If the conditions 
are right, this funnel will maintain a liquid center and provide the most 
efficient path for the latent heat of the cube to escape during the freezing 
process.  The shape of this funnel is usually triangular, as that is the most 
likely configuration.  The shape of the funnel beneath the frozen surface of 
the cube is also what determines the angle of the spike.  If the funnel is 
vertical, then the spike will be vertical, but if the funnel is slanted, then 
the spike will form with an angle as it is pushed out of the hole.  Once the 
surface of the cube is completely frozen leaving a triangular hole, the pressure 
from the plastic tray as the ice expands will force the water within the cube 
that is still liquid to come out of the hole.  If the rate of the water coming 
out of the hole is close to the rate of ice freezing, then the edges of the 
triangular funnel will freeze leaving the center liquid to bulge upwards.  
The edges of the bulging water then freezes at a rate that conicides with the 
rising water.  If the freezing is too fast, then the bulges freezes over and 
forms a little pyramid shape.  When the freezing rate matches that of the rising 
water, a tube is formed to allow the water to flow upwards until it runs out of 
liquid water or eventually tapers off and the tube closes due to freezing.  
If you can catch a spike during formation before it closes off, you'll be able 
to see that the spike is formed out of a hollow tube and as soon as you open the 
freezer and remove the tray, the water level inside to tube will begin to drop 
due to a decrease in the rate of freezing.  A fresh ice cube spike will have a 
completely triangular cross-section and will only be rounded due to melting of 
the top portion.  Hope you're still getting these ice cube spikes. Btw, nice 
observatory.  I teach intro astro/physics at uc berkeley.
 
miles chen