Charging of Solids - Triboelectrification

The most important type of charge separation involves the contact and friction between solids known as triboelectrification. When two solid materials, A and B (see Figure 3.1), contact and possibly rub against each other, electrons could move across the interface.

Metals

        Triboelectrification also happens when the two contacting materials are metals. When two metals contact, a voltage difference is established across the interface, with a magnitude from a couple of tenths to a few volts. 

        If the metals are "well-defined" metals, the contact potential difference can be calculated from the work functions., the energy it takes to remove a loosely bound electron from the metal. It should be stressed, however, that this charge exchange between metals only gives rise to what we normally understand as static electricity when the two metals are separated extremely quickly, such as when a metal powder is blown against a metal.

Insulators

        It is conceivable that only electrons located close to the surface can participate in the charging of highly insulative materials. Similar to metals, for some of these materials it is possible to measure the work function for loosely bound electrons. charging experiments with insulators can only yield quantitatively predictable results if the surfaces are carefully prepared and the experiments are performed in vacuum. And such experiments might disclose very little about what one could expect to find under more-practical conditions.

Contact Electrification:Triboelectric Series

        One of the material parameters influencing the course of a charging process between two solid materials is the permittivity. Scientifically speaking, permittivity is defined as the ratio between corresponding values of the dielectric displacement and the electric field strength. The stronger the forces, the higher the permittivity of the material.

        This is the background for Coehn's law, which states that when two materials are in contact with each other, the one with the highest permittivity becomes positive. This law was originally based on a comparison of known values of permittivity and published triboelectric series

        Table I shows an example of a triboelectric series. Such a series should be used with caution because the order of the materials could vary from series to series.

Most Positive (+)
Air	+++ +
Human Hands, Skin
Asbestos
Rabbit Fur
Glass
Human Hair
Mica
Nylon
Wool
Lead
Cat Fur
Silk
Aluminum
Paper
Cotton
Steel	-  - - -
Wood
Lucite
Sealing Wax
Amber
Rubber Balloon
Hard Rubber
Mylar
Nickel
Copper
Silver
uv Resist
Brass
Synthetic Rubber
Gold, Platinum
Sulfur
Acetate, Rayon
Polyester
Celluloid
Polystyrene
Orlon, Acrylic
Cellophane Tape
Polyvinylidene chloride (Saran)
Polyurethane
Polyethylene
Polypropylene
Polyvinylchloride (Vinyl)
Kel-F (PCTFE)
Silicon
Teflon
Silicone Rubber
Most Negative (-)