A Science Question

While living in Prague I probably drank more tap water than anyone else I knew, but I drank plenty of bottled water as well. Since I was a) environmentally aware and b) lazy, I ended up with a large collection of bottles waiting to go to the recycling bin up the road. After a while I began to observe a pattern, and as I packed up the apartment to move back to the states I took a brief timeout to document the phenomenon. Please observe exhibit A:

shrinking bottles

The bottle on the left is the youngest, the oldest is on the right. The burning question for today is, “why are the bottles shrinking over time?” All the bottles were stored with lids on, some with dribbles of water in them. Sometimes the water had been carbonated, other times not. Bottles do not puff back up when the apartment is particularly warm, nor can I find any other thermal explanation that would not even out over time. Age (on the scale of months) is definitively a factor.

So what’s going on? Is there a chemical reaction with the plastic that is reducing the number of gas molecules inside? Is there a sort of one-way membrane effect going on that lets air molecules out but not back in? Most likely the pressure is lower in the bottles than outside, due to the plastic wanting to revert to its original shape — unless there’s something about plastic that makes it want to shrivel up like that.

Any chemists out there want to hazard a guess? Physicists? Mechanical engineeers? UFO conspiracy specialists? Science Fiction writers? I’ll listen to any theory you care to offer.

13 thoughts on “A Science Question

  1. Not chemical action, but physical. If you expose a plastic bottle to heat, it does naturally shrivel up. If you expose it to less heat, it shrivels up more slowly. If you don’t expose it to any more heat than room temperature, but you leave it there for a very long time, it shrivels up very, very slowly. If it’s lying on its side at the bottom of a bin with other bottles on top of it, it might shrivel up a bit faster.

  2. I’m trying to reconstruct if there were any bottles without lids on them that demonstrated the shriveling. I’m pretty sure all shriveled bottles were lidded. If it were simple shriveling I would assume that uncapped bottles would shrivel faster. I almost always capped the bottles though, so the sample size is much smaller. None of the bottles in question had anything stacked on them; they were standing upright on my counter.

  3. The composition of the bottles may interact with their shape and design. Presumably, the bottles are filled very early in their life, and they are designed to keep their shape when full of water, but with the use of a minimal amount of plastic material.

    Perhaps when the bottles are formed, their rib-like patterns and curving sides are designed to have a bit of compressive tension, which would be offset by the pressure of the water within the bottle when it is filled.

    This would allow the bottle to hold its shape when full, while allowing the designers to use a minimal amount of material. Then, when the bottle is empty there would be no water pressure to resist the tendency of the bottle to compress upon itself.

    Is there anyone who might be able to test this theory?

  4. So far I like Timm’s answer best. Not because it’s more likely to be correct, mind you. The “natural tendency to collapse” theories seem contrary to the lid on/lid off behaviors. A sealed bottle would be less prone to collapsing, but spotty evidence suggests the opposite. More testing will be required.

    Suddenly I feel the need for a Web cam.

  5. I also like the bacteria consuming O2 of other parts of air. If the bottles are sealed, something would have to be reducing the internal air pressure.
    Or: Heating might cause the air to expand, leaking out of the bottle by pushing the lid open just enough for a slow flow. When the bottle cools off, the reduced air pressure sucks the lid tighter and seals, preventing air from retuning. Over time cycles could add up the effect on the bottle.

  6. The trouble I have with the cap leaking theory (which is probably the best theory so far) is that even after the bottles are shriveled to the point at which they never puff up no matter how warm it gets, they continue to shrink.

    My own personal wild guess is similar to the bacteria but instead it is a chemical reaction between the plastic and the air in the bottle (probably O2) that causes some of the gas to combine with plastic molecules. The volume of the air in the bottle is reduced as a result. If that were the case, then at some point the shrinkage of the bottle would stop when all of the particular air molecules were used up.

    I am more tempted than ever to run a controlled experiment.

  7. I have discovered archival bottles that Pat has left rolling around vehicles for months with water still in them. They don’t shrink when they’re full. This evidence would support both Pat’s and Gerald’s theories.

    Also, even when still factory-sealed, the water in these bottles takes on a flavor of the motor oil that was once spilled on the floor mat the bottles were rolling around on. This would suggest that either the cap-to-bottle interface isn’t perfect, or that the bottles themselves are ever-so-slightly porous — they hold water, but very, very gradually, they will let smaller molecules, such as air, seep through. That would give more weight to Gerald’s theory.

    • I have a bottle of water that is still factory sealed that is just over ten years old (don’t ask) and it mimics the same phenomenon. I don’t think it has anything to do with how much water is in the bottle, if it is capped, ect. This has been a head scratcher for me as well though.

  8. Did you try taking a cap off one of the bottles? That could help determine if it’s the material breaking down, or a change in the pressure.

    Oh, CA, why are you drinking the old, abandoned water in Pat’s car?

  9. I also have new sealed bottles and opened and used both timed. They have same phenomena of shrinking over time. Guess inner bottle bacteria has little chance of consuming something inside because the water in is just pure zero calorie and may all them being dieting or sleeping. My principle of guess is someting is going out but don’t return.
    And, one presumption is water dismantled in hydrogin and oxygen and escape at their talent.

    Oh, I didn’t see with empty and capped bottle but almost fully filled and seal capped PET bottles.

    I’ve been thinking this many years too. ~

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