Contact electrification

Particles carrying charges of opposite polarity due to contact electrification will be attracted to opposite electrodes when passing through an electric field and thus can be separated from each other. 

Types The primaiy cause of electrostatic charging is contact electrification, which takes place when two different materials are brought into contact and separated. Other causes include induction charging, the formation of sprays, and impingement of charged mist on an ungrounded conductor. 

Contact Electrification This form of charging involves the contact and separation of solid-sohd, sohd-hquid, or hquid-liqmd surfaces. Pure gases do not cause charging unless they cany droplets or dust particles. 

Charging by contact electrification is an active mechanism whenever dissimilar particles make and break contact with each other, or whenever they slide over a chute or an electrode. This charging mechanism is most frequently used to charge selectively and obtain an electrostatic separation of two species of dielectric materials as realized in a free fall electrostatic separator. 

R8. Rose, G. S., and Ward, S. G., Contact electrification across metal-dielectric and dielectric-dielectric interfaces, Brit. J. Appl. Phys. 8, 121 (1957). 

An aerosol is a dispersion of discrete particles in a stream of gas. Starch and cellulose aerosols are potential fire hazards in granaries where friction between the moving, micronized particles causes electrification, whereupon separate accumulations of positive and negative charges may discharge as an electric spark and ignite the combustible solute (contact electrification synonymous with triboelectrification Ross and Morrison, 1988). 

Since Coulomb forces decay much more slowly than van der Waals forces, an EFM experiment can be performed even without any feedback, if the sample roughness is low compared to the tip-sample distance. Terris et al. used this technique for the first time to investigate a contact electrification of a metal-insulator system [355-357]. Later, further improved experiments on contact electrification with single charge sensitivity were performed by... 

Contact electrification. A second static electrification process is contact electrification. Here electrons migrate from clean, dry surfaces of dis-... 

Chains, Stokes law with, 68-70 Chan, T., 120 Chapman, S. J., 168-169 Characteristic charge, 187 Characteristic length and diffusion, 155 Charging mechanisms, 179 collisions with ions, 185 contact electrification, 182-183 corona discharge, 195-198 diffusion, 185-189,195 electric, 179-183 equilibrium with, 200-201 steady-state theory of, 201-207 transient approach to, 207-208 field charging, 185,189-195 flame ionization, 184-185 and force, 179-180 frictional, 184... 

General Principles Electrostatic separation (of particles), also commonly known as high-tension separation, is a method of separation based on the differential attraction or repulsion of charged particles under the influence of an electrical field. Applying an electrostatic charge to the particles is a necessary step before particle separation can be accomplished. Various techniques can be used for charging. These include contact electrification, conductive induction, and ion bombardment. 

Contact Electrification fFig. 19-55aJ When dissimilar materials touch each other, there is an opportunity for the transfer of electric charges. The extent of charge transfer can be such that a significant surface charge of opposite sign is developed when the materials are... 

FIG. 19-55 Schematic representation of charging mechanisms. (A) Contact electrification. (B) Conductive induction. (C) Ion bombardment. Cond. = conductor particle diel. = dielectric particle = high-voltage dc electrode 0 = ions from corona discharge at high-voltage electrode. 

Electrostatic-Separation Machines The first electrostatic machines to be used commercially employed the principle of contact electrification. These were free-fall devices incorporating large vertical plates between which an electrostatic field was maintained. Tribo-electric separation (contact charging) has experienced an increase in applications due to advances in mechanical self-cleaning and electrical design as well as the development of efficient precharging techniques. 

Completing a circuit and measuring the current that flows on contact or separation can quantify the transfer of electrons [2]. A typical result on contact electrification is shown in Figure 24.3. The time constants associated with the current peaks can be adjusted by inserting a series resistance in the measurement circuit. In real time, the equilibration of surface state electrons occurs instantaneously. In these experiments, the plasma polymer of tetrafluoroethylene (TFE) was deposited on two different substrate, nylon film and polished brass, and the contact and separation currents were measured with a (uncoated) brass probe... 

Table 24.2 Triboelectric Series Based on Contact Electrification/Separation Current...

Homewood K. Do dirty surfaces matter in contact electrification. J Electrostal 1981 10 299-304. 

---