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This may be one of those things that everybody knew but me (come to think of it, I think I'll add it to the "things I learned today" thread elsewhere).
There are some plastic bags I use which come on a roll. I have been struggling to open them when I remove a bag from the roll, rubbing different portions of them between my fingers for sometimes minutes at a time. My wife saw my struggles and informed me that wetting the fingertips often worked. She then, to my chagrin, demonstrated, opening the bag within seconds. I've repeated the experiment several times and it works every time.
When you rub your fingers on opposite sides of the bag, what you're trying to do is get the two surfaces to move against each other. That means that you need the friction between fingertip and plastic bag to be greater than the plastic-plastic friction. With dry fingers, apparently it is not and the fingers are sliding more than the plastic is. But with wet fingers, the fingertip friction increases and voila!
This simple fact is what is puzzling me. How is moisture increasing friction? I would expect the opposite, since in most situations we encounter, water makes things more slippery. I hypothesize that it has to do with the fact that skin is normally oily, so perhaps the presence of a water layer adds just enough adhesion to whatever the bag is made of (it's recycled plastic, I don't know what type) to do the trick. Thus, I'm guessing this is a chemistry question and posting it in the chemistry forum.
And thus the trick shouldn't work with a non-oily surface, say rubbing between two pieces of aluminum foil. I have yet to check that.
There are some plastic bags I use which come on a roll. I have been struggling to open them when I remove a bag from the roll, rubbing different portions of them between my fingers for sometimes minutes at a time. My wife saw my struggles and informed me that wetting the fingertips often worked. She then, to my chagrin, demonstrated, opening the bag within seconds. I've repeated the experiment several times and it works every time.
When you rub your fingers on opposite sides of the bag, what you're trying to do is get the two surfaces to move against each other. That means that you need the friction between fingertip and plastic bag to be greater than the plastic-plastic friction. With dry fingers, apparently it is not and the fingers are sliding more than the plastic is. But with wet fingers, the fingertip friction increases and voila!
This simple fact is what is puzzling me. How is moisture increasing friction? I would expect the opposite, since in most situations we encounter, water makes things more slippery. I hypothesize that it has to do with the fact that skin is normally oily, so perhaps the presence of a water layer adds just enough adhesion to whatever the bag is made of (it's recycled plastic, I don't know what type) to do the trick. Thus, I'm guessing this is a chemistry question and posting it in the chemistry forum.
And thus the trick shouldn't work with a non-oily surface, say rubbing between two pieces of aluminum foil. I have yet to check that.