Triboelectricity

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Introduction

Story about the Greeks or other ancients and what they knew about triboelectricity?

From the wikipedia:

"The triboelectric effect (also known as 'triboelectric charging') is a type of contact electrification in which certain materials become electrically charged after they come into contact with another different material and are then separated (such as through rubbing). The polarity and strength of the charges produced differ according to the materials, surface roughness, temperature, strain, and other properties."

The first to organize a series was Volta in 1779. Modern lists give:

(Most positively charged) (Small negative charge) Acetate
Human skin Wood Rayon
Leather Lucite Synthetic rubber
Rabbit’s fur Amber Polyester
Glass Sealing wax Styrene (Styrofoam)
Quartz Acrylic Orlon
Mica Polystyrene Plastic wrap
Human hair Rubber balloon Polyurethane
Nylon Resins Polyethylene(like Scotch tape
Wool Hard rubber Polypropylene
Lead Nickel Vinyl (PVC)
Cat\'s fur Copper Silicon
Silk Sulfur Teflon
Aluminum Brass Silicone
Paper (Small positive charge), Silver Rubber
Cotton (No charge) Gold Ebonite
Steel (No charge), Platinum (Most negatively charged )


Volta's undated laboratory table describing systematic observations on triboelectricity. Source: Volta : science and culture in the Age of Enlightenment by Giuliano Pancaldi


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Physical Origin

The basic cause for the triboelectric effect is the formation of bonds between contacting surfaces that result in charge imbalance when the bond is broken by removing the objects from contact. When two objects contact, a chemical bond can be formed between some parts of the two surfaces (adhesion). In some cases, charges may move from one item to the other to minimize electrochemical potential, creating a net charge imbalance. When the objects are then separated, the bonds can be broken in a way such that the net charge imbalance remains. Though the triboelectric effect is usually associated with rubbing, the rubbing itself does not play a crucial role; it merely allows for many contact cycles thus resulting in a larger charge buildup.

Now that the object is electrically charged, contact with a neutral conductor (or any object having significantly differing electric charge) may result in a discharge of the built-up static. A person walking across a carpet may build up a charge of up to several thousand volts, enough to cause a spark in air (which has a dielectric breakdown of ~30,000 V/cm).

Though the discharge energy from this spark is typically small (tens of microjoules for a typical person-doorknob spark), it can be very dangerous near combustibles. Items can be made resistant to the triboelectric effect by giving them a conductive coating; typical anti-static materials include long-chain aliphatic amines and amides, ammonium salts, esters of phosphoric acid, polyethylene glycol esters, polyols, conductive polymers, and Indium Tin Oxide.

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Luigi Galvani

In 1783, according to popular version of the story, Galvani dissected a frog at a table where he had been conducting experiments with static electricity, Galvani's assistant touched an exposed sciatic nerve of the frog with a metal scalpel, which had picked up a charge. At that moment, they saw sparks and the dead frog's leg kick as if in life. The observation made Galvani the first investigator to appreciate the relationship between electricity and animation — or life




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Creating Charges with Friction - Triboelectricity

Anyone who has ever felt the zap of an electric shock after walking across a carpet and then touching a metal door knob has experienced that two objects rubbing together can create electrostatic charges. Whenever two different materials rub against each other it is likely that one will leave with more electrons than it started with, and the other will leave with less. This is called Triboelectricity (tribo means friction). From the study of chemistry we learn that different materials have different desire for electrons. (This is called electronegativity.) Some materials are very greedy and will always steal electrons from things they come in contact with, others are more willing to give up electrons. As the neutrally charged person walks across the wool carpet, his leather soled shoes have less desire for electrons than the wool carpet. As a result, electrons get stolen from the shoe by the carpet. With every step the person becomes more and more positively charged. That charge distributes itself over the body. When the positively charged person gets near the metal door he will actually attract charges from the door which jump in the form of a spark. Notice how only the negative charges (electrons) are free to move. It is important to point out that if he was wearing rubber soled shoes on a wool carpet, his shoes would steal electrons from the carpet. He would become more negatively charged with each step. When he gets near the door the electrons will jump from him to the door. From his point of view it would look and feel the same as it did in the first example. He can't tell whether charges jumped to or from him. If we did a study of many materials and put them in order from those with the least desire for electrons to those with a very strong desire for electrons we would have created a Triboelectric series. For example, if leather were rubbed with wool, the leather becomes positive and the wool negative. Yet if rubber is rubbed with wool, the rubber becomes negative and the wool positive. It is important to note that this series is true only if the samples are clean and dry. The presence of moisture, dirt, or oils may cause some of the items to interact differently.


Risks and corrective methods

The effect is of considerable industrial importance in terms of both safety and potential damage to manufactured goods. The spark produced is fully able to ignite flammable vapours, for example, petrol, ether fumes as well as methane gas. Means have to be found to discharge carts which may carry such liquids in hospitals. Even where only a small charge is produced, this can result in dust particles being attracted to the rubbed surface. In the case of textile manufacture this can lead to a permanent grimy mark where the cloth has been charged. Some electronic devices, most notably CMOS integrated circuits and MOSFET transistors, can be accidentally destroyed by high-voltage static discharge. Such components are usually stored in a conductive foam for protection. Grounding self by touching the workbench, others, or using a special bracelet or anklet is standard practice while handling unconnected integrated circuits. Another way of dissipating charge is by using conducting materials such as carbon black loaded rubber mats in operating theatres.


Keyword in references:

Folding of Electrostatically Charged Beads-on-a-String: An Experimental Realization of a Theoretical Model


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