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Ask Slashdot: Physics of Water

bwayne314 (1854406) writes | more than 3 years ago

1

bwayne314 (1854406) writes "We have been having a debate at the lab about mechanics of water and leaky bottles that I am hoping you slashdotters might be able to put to rest.

One group claims that capping a bottle or tube with water residue on the seal will cause some water molecules to remain in the seal and allow liquid from the container to leak out (under certain conditions, like shipping) despite being tightly closed. They call this phenomenon, "creating a river" and opening and drying of the the seal on such a bottle would prevent a leak.

The other faction thinks that any bottle that leaks, simply has a poorly fitting seal and that closing a wet bottle with a proper seal should push any water residue either into or out of the bottle. The distinction here is that there should not be a difference between capping a bottle that is dry vs. capping one that is wet.

So what happens on a molecular level in this situation and who is correct?"

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1 comment

Well... (1)

Quantus347 (1220456) | more than 3 years ago | (#36037988)

I used to work for a major perfume/cologne bottling manufacturer doing engineering troubleshooting on their packaging process, and leakage was a constant battle. That was an alcohol based fluid which has a much lower specific gravity than water and was thus more able to leak through tiny routes. So I can tell you that no, it wont just create a channel through the seal without some structural defect allowing it. When you tighten the cap down, it compresses the seal to a particular pressure, and in the process should squeeze out any fluid still present on the sealing contact surface. They only way for fluid to stay is if there is some sort of divot in the seal or sealing shoulder that traps the fluid, which would leave a pressurized bubble that would pop out as soon as it could. And if the cap is twist applied even that won't happen, as the twisting action will drive any such bubbles out. The only way for a wet seal to "create a River" as they are calling it would be if the seal were already woefully inadequate for the application. It should also be noted that pulling the cap off will often create a small bump of vacuum that can pull fluid back onto the seal surface from its edges even if it wasn't there while the cap was seated.

At that point the only way for more fluid to seep in and through the seal area would be to overcome the sealing pressure and "lift" the seal enough to allow more fluid to pass through the seal region. That can only happen by a)raising the internal pressure of the package through squeezing, heating, hard freezing, etc, b)lowing the external pressure of the package as will often happen during air shipping, or c)loosening the seal itself. Outside of those conditions the most fluid that could be seen to "leak" would be no more than could be trapped in the seal's contact surface area. However, if there is trapped fluid, and one of the above conditions are met, the bubble will make it slightly easier for the fluid to leak, but only enough to relieve the pressure difference. And in that case the wet cap is a catalyst, but not your root cause; your root cause is still a seal that is inadequate to the task/conditions.

If it is a sporadic problem, you will first want to confirm that the capping force and the component dimensions are both consistent and within your specified tolerances. Also see if you can make any correlation between the leaks and any specific component lots, which would indicate the issue is on your supplier's end. If this is a more widespread problem you will need to consider strengthening the seal. The easiest way is simply to tighten the cap down more. Barring that, inserting a thicker seal will accomplish the same thing, but will most likely require a change at the component manufacture level. Increasing the seal surface area could also work. This can be done by making a wider surface on the seal and the bottle side. You can also soften the seal material to allow it to "wrap" down around the lip of the bottle a bit more, but I would be very careful with that, since you are in essence making a weaker seal and would likely need to increase your tightening pressure to compensate. Another option (depending on your process and materials) would be to create a minor vacuum state within the package, either by filling in a low pressure environment or by filling the package with the fluid pre-heated (like in food jars, etc). This would strengthen the seal pressure, and insure that in the even of a leak air will com in rather than fluid going out.

If you don't already, using a basic Vacuum test would give you some good information. Simple trapped bubbles will not allow more fluid past (and be removed, but it would be too slow to do for every bottle), but will bring to light shortcomings in the seal, either from design or bad components. Food coloring or some such can sometimes be helpful to identify the specific path of the leak, too.
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