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Any discussion of adhesion promotion for plastics is not complete without considering surface energy. Surface energy, in simple terms, indicates the tendency of materials to "bead" on a surface. The Wikipedia explanation is long and complex (and a link is given below), but basically the greater the surface energy, the more likely any given liquid is to spread over the surface. The lower the surface energy, the more likely any given liquid will bead up on the surface. Substances like wax have a low surface energy and that is why when we wax our cars, water beads on it afterwards. Alternatively dish soaps have surfactants that decrease the surface energy of the liquids they are mixed with and this allows the detergent to "wet-out" and spread over the dishware!
While there are excellent pieces of lab equipment on the market to measure surface energy directly, one inexpensive alternative is the dyne pen. Dyne pens are available in sets and commonly available in a range of 30 to 60 with special pens outside of this range also available.
The issue with dyne pens is that the test is very subjective and it is typical to have measurements of +/- 3 or more.
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