We make a lot of ice in the refrigerator at work. We have a row of those white plastic ice trays stacked two deep, one tray atop another. I’ve noticed that removing the ice from the trays on the top row is very easy; with a gentle twist the cubes fall right out. However, it takes a mighty twist for the trays on the bottom row, and even then the cubes are fractured, with remnants still sticking to the tray.
Can anybody explain this behavior? Speculation is acceptable, but please identify your post as such.
Are the trays filled to the same level? In the set up you describe, I usually see the top one filled just a touch below the dividers, while the lower is overfilled and there’s a solid sheet of ice across the top.
Are the lower trays actually resting on the floor of the freezer compartment?
The freezer coils may be embeded in that floor, so the lower trays could be getting cooled by conduction, and be quite a bit colder than the upper trays.
Some trays are different. I once owned the perfect trays. The compartments were cup-shaped like parabolas. Not square or round. The shape of the parabola is such that the outer shell would freeze first and then as the center froze the things would expand, separate from the sides, and lift themselves right out of the tray. But it’s an invention that never got recognized enough for wide distribution and I never found their likes again. I’ve often been tempted to have them manufactured myself but I know I could never recover the costs for a specialized cheap product that’s hard to explain.
I can’t explain it, but I have witnessed the same thing in my freezer. Specifically I’ve been making very small ice “cubes”, only pouring them about 1/4 inch deep so they freeze quickly and crush nicely in the blender. In one tray they just pop out, in the other they shatter when I twist the tray.
My speculation on the same phenomena is that it has to do with the pushing up of ice cubes. The ones in the top tray freeze so that they scoot up a little bit, while the ones in the bottom tray seem to just expand in place. My guess is that fragments stay behind because this pressure encourages the cubes to stick to small imperfections in the surface.
I think it has to be related to the speed of cooling. Cold air sinks, so I would expect that the ones on the top are freezing faster than the ones on the bottom. I know that the speed of freezing affects crystal size and it might also affect whether the top or bottom of the cube freezes first… but I’m not exactly sure how those combine to cause this.
Next experiments to try: Put in just one tray on the bottom, with nothing on top of it, and see if that makes a difference. Then, try putting an empty tray in the bottom, and fill a tray on top of it.