Why does cling film (Saran Wrap) cling better to surfaces which are wet or damp than dry? The product we know in Canada as Saran Wrap is a clear plastic film (no idea of the chemical formulation, believe it to be polyethelene). Among its uses are to cover a dish or bowl. If the hard surface is dry, it doesn't always want to cling, although it can be stretched a little (it has practically no elasticity; any appreciable stretch occurs as plastic deformation), and this helps it grip the surface.
However, if the surface is moist (perhaps from condensation), the film seems almost to be attracted to it.
Worst of all is handling this film with wet or damp hands while attempting to cover a dry container... seems the stuff will aggressively hang on to damp skin and not want to hold on to the dry container (be it metal or ceramic) at all.
What is the physics underlying this? Hard to believe it is surface tension; assuming the stuff is made of polyethylene, one would think it would be hydrophobic, not hydrophilic. Also hard to believe it is electrostatic; the phenomenon is the same for metal, ceramic, and skin - very different electrical properties in each case.
 A: To start the ball rolling, I found a thread at http://www.reddit.com/r/askscience/comments/1rhpc1/how_does_clingfilm_saranwrap_stick_to_itself_does/ which contained some interesting information:


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*the formulation of Saran wrap was changed a few years ago when Dow sold the brand to SC Johnson

*the new material is made with polyethylene which doesn't stick very well

*to improve the stickiness, a gelatin like adhesive was added. Quoting from the reddit:



While I only posses very limited knowledge on the subject, I might be able to add something of value to the already great responses in this thread.
  I had the opportunity to work for Glad (makers of Cling-Wrap) and observed that in addition to the various plastics that are added in the extrusion process they add a substance known only to me as "GMO" a greasy sort of substance in order to make the film tacky.
  I was told this substance was similar in composition to gelatin, but I am not sure how much the production staff knew about the "GMO" themselves.

Now I am quite familiar with the behavior of gelatin in the presence of a little bit of moisture: it becomes extremely sticky.
Could it really be that simple?
update a little searching of the patent literature uncovered the following patent application by SC Johnson (number 20030211270):
Paragraph 10:

We have discovered an alternative, cost-effective structure for a food grade film wrap (and a method for making it) containing a pressure sensitive adhesive (PSA) in combination with other film features, that is flexible, preferably transparent, strong yet easily torn off from the roll using a cutter bar, and that is sufficiently tacky to seal around a container or object but not too tacky to cause “blocking”. Furthermore, the food grade film wrap that we have developed can be pressed or sealed into place without excessive amounts of force or without having to stretch it, for example, to activate it, making this product easy to use.

Further down the patent they say that "JonBond 743" is the preferred adhesive - but a quick Google search suggests this was sold to Bostik. There the trail went cold. I am going to contact them to see what they have to say about this...
A: Despite the changes on the chemicals of the film (gelatin and all) it's still mostly an electrostatic effect. The notion that water acts like a good conductor is misleading. Water is NOT a good conductor. In fact it's a rather good insulator. What makes water conductive is the salts and minerals in it. Tap water is conductive while distilled (purified) water is definitely not. So let's see ...


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*You've stated that the film is "attracted" aggressively to your moist hands. That implies "action from a distance" which means that it is most definitely an electrostatic effect due to polarization of the film as well as the water molecules.

*If the film clings to your moist hands when you touch it then surface tension and the van der Waals force joins the game.
That's what I can assume anyway as we I don't have saran-wrap at my house so no experiment for me ;-)
