Why are ice cubes on the bottom tray always so much harder to get out than the top tray?

It’s hotter at the top ? !

I have the same problem, so I hope someone has a good explanation and a good solution!

Here’s my hypothesis:

I agree with @sandystrachan that it’s because of the temperature differential between the two levels of the chamber.

What causes the cubes to jam in the tray is the fact that when water freezes it expands, which causes the sides of the cube to push outward on the sides of the tray, jamming it in. Obviously, the water is just as frozen in the top tray as in the bottom, and both will be pushing somewhat against the sides of the tray. But I suspect that the rate of freezing will affect how hard this push is.

In an extremely cold environment, the outside of the cube will solidify a long time before the interior will. As the sides freeze, they will expand, of course, but some of the pressure of that expansion will be directed toward the still liquid center, pushing it upward. As the freezing continues, the outer shell thickens and prevents further expansion outward, so the expansion will be concentrated on the last area of the surface to freeze, namely the center top (which is why ice cubes tend to have a bump in the top center; sometimes this effect is so dramatic that ice spikes over an inch tall are produced).

Now, in a warmer environment, not only does the cube take longer to freeze, but the difference in temperature between the inside and outside of the cube during freezing will be smaller than in the very cold environment. This means that when the cube does freeze, the freezing will happen at more nearly the same time all through the cube. The result is that when all of this expansion happens, there is no thick exterior case of ice to contain the sideways expansion, so it happens nearly as much outward as upward.

It’s unlikely that there’s a tremendous difference in the temperature of the two levels of your freezer, but even a bit of difference would change the freezing rate of the cubes and trigger this effect. I guess that if this hypothesis is right, you’d tend to have a bigger central bump on the cubes of the tray that releases easily, whereas the surface of the stubborn cubes would be a bit flatter, but I don’t know how visible the difference would really be.

@Harp very well thought out and that also makes me wonder if the tray on top RESTING ON TOP of the water of the tray on the bottom might also factor in some way?

I wonder if it has more to do with the bottom trays being in the frizzer longer (people tend to grab and use the top ones most). Something along the lines of mild thawing or temp changes could be causing the ice crystals on the outsides to break apart and reform. I know cubes that have been in there for a while tend to be more brittle and shatter when you take them out of the tray. I dunno, just an idea.

@MissAnthrope No, it happens when I put bottom and top trays in at the same time. I put them in one on top of the other, so I doubt if there would be a significant temperature differential. In any case, if there were, the lower tray would be the colder one, since colder air sinks.

The only way it might work as @Harp suggests, is if, in putting the trays one on top of the other, the top one covers over the bottom one, retaining more heat. Still, if I switch them around (I think I did this once), it seems like the former bottom one also gets easier to dislodge the cubes.

I wonder if sublimation has something to do with it?

Here’s a post that offers several theories and a set of experiments to try.

I think that the idea that the order in which the water freezes could explain the adhesion issue. If it freezes top down, then the top water will expand, and raise the surface water upwards, leaving more room for the water underneath to expand.

If the water freezes bottom up, then the (admittedly minimal) weight of the water above will keep the ice from freezing upwards as much, forcing it to freeze outwards, and creating greater adhesion to the tray walls.

Another thing that post points out is that when you twist to try to get the bottom tray ice out, it fractures a lot more than the upper tray ice will. I’m not sure what that indicates, other than somehow the molecular structure of the bottom ice is subtly different?

And one poster in this discussion offers another sublimation explanation:

I think I’ve got this one. It’s not temperature differential, or the defrost/thaw cycle. The top tray is exposed to the air flow in the freezer, the bottom tray is not. The ice in the top tray sublimes at a much faster rate than the ice in the bottom tray. As the cubes get smaller, they tend to lose adhesion to the tray, and will just fall out. So give us your guess.

I think this is a mechanical problem. When you fill the trays, the ones on top go in last. You slop a little water out of each tray as you place it.

Water from the top trays spill into the bottom trays overfilling them. Overfilled trays are harder to open.

It would be interesting to do a few experiments:

If you put a tray in the top level with nothing below it, or put one in the lower level with nothing above it, does the same thing happen?

If you put a folded dishtowel or other light insulation under the bottom tray as it freezes, does that affect the way it releases?

uh oh, sounds like @Harp is assigning science homework
(ha ha ha ha) back to school for the kid is affecting me

If you have a tray on top, the freezing water in the bottom trey is being held down. As @Harp stated, the top center of the cube is the last to freeze, the same part that the top tray rests on.As it finishes freezing, it pushes against the sides harder, wedging it into the tray.