Mathew Tavendale, of King's High School, asks :-

What makes gladwrap stick to itself?

Jim Mitchell, a chemist with the Food and Applied Chemistry division of Environmental Science Research, Auckland, responded.

Adhesion occurs when two surfaces are brought together and are able to make very intimate contact. The strength of an adhesive bond is dependent on the efficiency of contact of the two surfaces, the strength of the attractive forces that can occur between molecules on the two surfaces when they are in intimate contact and the strength of the materials making up the two surfaces.

Gladwrap is polyethylene film containing a liquid "cling additive" (glycerol mono-oleate, commonly called GMO). The GMO forms a very thin liquid layer on the surface of the plastic, enabling very efficient contact between two layers of Gladwrap (or between Gladwrap and a dish). The intermolecular forces between the two plastic film surfaces and the intermediate liquid GMO are sufficient to give an adhesive bond (or more correctly two bonds, one at each liquid-plastic interface). The overall bond between the Gladwrap layers is easily broken by peeling back the film, as the liquid GMO has very low strength and the peeling operation puts considerable force on the GMO layer.

The situation is somewhat similar to two sheets of glass stuck together with a thin film of water, although in this case the bond seems stronger because the glass is not flexible and cannot be peeled away.