Re: Why do ice cubes 'stick' only in the bottommost of two stacked cube trays?

There are, unfortunately, certain questions in life that have been woefully neglected in the academic literature; and yours is one of them. A previous poster asked a similar question: (see http://www.madsci.org/posts/archives/1999-08/935819128.Ch.r.html)

In that answer it was postulated that the cubes had been stored long enough that sublimation may have changed the cubes in the top tray, relative to the bottom, or by taking both trays simultaneously out of the freezer, and allowing the bottom tray to sit in the room a bit longer than the top tray before removing the ice, a water layer might form and increase adhesion of the bottom tray ice.

Now, I don't wish to criticize my fellow Mad Scientist, but the simple fact is that we are indeed Mad Scientists, and therefore, we sometimes think other's ideas are simply mad. I mean, really, would you blame one insane person for calling another insane person "insane." So, I would suggest a different approach for explaining the differences in ice cube adhesion, hopefully without offending my fellow mad scientist.

Here is what we apparently know: Both trays are filled with (presumably) the same temperature water, placed in the same freezer on top of each other, and cool down together. Heat is transferred out of the trays along the exterior of the trays, but the heat transfer between the two trays is probably pretty poor because we can't get much air circulation between the trays. We also have been informed that either tray, when placed on the bottom of the two, will hold onto the ice more than the top tray (so the results are apparently not characteristic of one tray and not the other). So, your powers of observation have potentially eliminated a variable such as if one tray was sprayed with a mold release agent such as Pam.

I suspect, although I don't know, that you see the same or very similar results whether you remove the ice that you put into the freezer last night or if the ice has been sitting several days (which would eliminate the sublimation effect).

I suspect that the difference between the two trays may be related to the rate at which the trays cool and freeze. When the trays are stacked on top of each other, the bottom tray keeps the bottom of the upper tray "warm" and the top tray keeps the top of the bottom tray "warm." Adhesion is a sticky subject and not perhaps as well understood (at least by me) as some other physical phenomena. I just suspect that perhaps the rate at which the ice forms (or the water remains in contact with the tray) might be significant. (It may also be related to whether the ice freezes from the top down, or from the bottom up.)

So, if I were you and I wanted to figure out what was happening (and at the same time reproducibly make good, whole ice cubes), I would try running a couple of experiments. Assuming that your results so far have been consistent, and that each tray behaves as the other, depending only upon which position it is in to produce ice that either releases from the tray or sticks to the tray, let's move forward. First thing, though, let's label each tray with a piece of tape or a Sharpie to make sure the assumption of equal performance is valid.

Experiment #1: How does each tray perform by itself in the freezer, filled with water and allowed to freeze overnight or for a certain number of hours? Fill one tray with water; place in freezer (by default in the bottom position) and run the test. Try removing the ice without running the tray under water, and see if the cubes drop free or stick. My money is on the ice sticking with either tray.

Yes, I know that you will only have half the ice as if you filled both trays but we must make some sacrifices for science. Otherwise we would still be beating together random rocks to make fire.

Experiment #2: Fill an empty tray with water. Wrap the outside with a waterproof barrier, such as a bag, waxed paper, etc., and then wrap the tray in a towel. Then put the tray in the freezer and freeze as before. What we are trying to do here is to reduce the rate at which the ice freezes to see if it has an effect upon adhesion. Now, picky people will argue that the tray is now elevated from the strictly bottom position by the presence of the towel, and that we may interfere with air flow and cooling rate, but the net result still should be that we are decreasing the rate that we are cooling the tray.

After freezing, remove the towel from the tray, and try to pop the ice cubes from the tray. Did they fall out, or did they cling for dear life to the tray?

Experiment #3: Fold the towel until it is about the same thickness as the tray and put it into the freezer, followed by your newly filled ice cube tray. The bottom of the tray will be kept warmer a bit longer by the towel, but not as long as if a tray containing water was actually put under it. After freezing, test for ice adhesion to the tray.

Experiment #4: Find 4 small wooden or plastic blocks, about the same thickness as the ice cube tray. Put them on the bottom of the freezer, and put your filled ice cube tray on top of the blocks. Now you will have good air flow around the tray, but the tray will be in the top, or "good" position. I bet that when you remove the tray from the freezer, the ice is difficult to remove, even though it is in the top tray position.

Experiment #5: My working hypothesis is that adhesion develops during the cooling/freezing process, and that things you do after freezing (time in the freezer, time in the room prior to ejecting ice) have little effect. You have also provided another useful detail. When you run your ice cube tray under the water to eject the ice, you get a bunch of broken ice. That is likely a result of thermal shock caused by your ice being pretty cold. I'll bet that ice cream stored in the freezer is difficult to scoop out too. The real kicker is where you state that even after running the ice tray under water, you still have shards of ice stuck in the tray. Any water layer that formed while you had the tray under the tap refroze as soon as the tap water was removed because the ice is so cold.

The low operating temperature of your freezer leads me to believe that it is pretty aggressive in trying to freeze your ice as soon as possible. So, for a final experiment, let's try slowing down the rate at which you freeze your ice. Fill both trays and wrap them both in a towel. Let them freeze and see if both trays eject the ice similarly.

Now, being the good scientist you are, you will keep a record of your results. You might duplicate your efforts, and randomly substitute one tray for the other. In the middle of your experiment, you might simply fill both trays with water and put them back into the freezer to verify that they still behave the same way as before you started your series of experiments.

My hypothesis is based upon the rate of cooling and freezing, and I might be completely whacked. We don't know if the bottom tray ends up at the same temperature as the top tray; if the bottom tray is 5 degrees cooler than the top tray, that might instead be responsible for the difference in performance. In some northern countries, when the outside temperature gets cold enough, the ice is no longer slippery.

As I mentioned before, it may also be related to whether the ice freezes from the top down or from the bottom up. For the ice in the top tray, the top surface will freeze first. As the ice underneath it freezes, it expands, which would tend to dislodge the ice above it, essentially dislodging the ice as it freezes. The ice in the bottom tray is more likely to freeze from the bottom up, which will not cause the cube to move relative to the tray as it freezes. So, the direction of freezing could also explain adhesion in the bottom tray, and easy ice cube release in the top tray. By putting a towel on top of the top tray, you might be able to get the bottom of the top tray to freeze first, making the ice stick.

Depending upon the results to the suggested experiments, other experiments or approaches might become apparent. But, here is the most important item.

You would be doing us all a big favor if you keep track of your results, and then post them on this site! You would be recognized as one of the world experts on ice cube adhesion phenomena. Simply submit your results as a question (or whatever format the moderator might suggest), and be sure to reference this answer.

If we knew that wrapping a towel around both trays would make the ice easy to remove, or some other approach that you might discover, we could advance the cause of science while producing perfectly formed ice cubes. That would be a worthwhile effort for all of us.

I look forward to hearing your results!