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ACentury ofDevelopmentin Applied Electrostatics

G.S. Peter Castle, Guest Author

To most electrical engi-neers, the term electro-statics brings to mindsome hazy visions ofearly electrical experi-

mentation involving an ancient Greek,a dead cat, and a piece of amber. Manyof us recreated this quaint demonstra-tion of triboelectric charging and itssubsequent attraction in high schoolphysics class. Perhaps some, in moreenlightened schools, substituted mod-ern materials such as plastics and syn-thetic fibers. In university electricalengineering programs, electrostatics isusually ignored or relegated to math-ematical simplifications involvingquasi-electrostatic approximationsof important applications in electro-magnetics. Meanwhile, a serious workinvolves the state-of-the-art study oftopics such as electrodynamics and dig-ital electronics. These fields have domi-nated so many important advances inthe 20th century.

Given this, many engineers maybe surprised to hear that electrostaticprocesses are the basis of many impor-tant engineering applications infields as diverse as air pollution con-trol, printing and copying, painting,materials separation, and even sand-paper manufacture. Worldwide, theseapplications account for billions ofdollars in annual business. Moreover,after several millennia, when electro-statics was regarded as an interestingbut impractical scientific curiosity,all these various applications came to

fruition during the 20th century. Thecommon factor that they share is thecontrolled movement of small par-ticles, ranging in size from the scaleof millimeters to nanometers. Thesimple reason for this is that whencompared with various mechanicalforces and particularly that of gravity,the electrostatic Coulomb force be-comes dominant for very small par-ticles. This is usually quantifiedthrough the so-called charge-to-massratio (Q/M), which essentially providesan indication of the ratio of the electri-cal force for a given electric fieldstrength to the gravitational force.Since charge is dependent on surfacearea (radius squared) and mass is afunction of volume (radius cubed),the ratio varies inversely with size,showing that the smaller the particlethe more significant the electric forcebecomes. Moreover, since the force ofgravity is constant, the electrical forcecan be directly controlled in magni-tude and direction by simply varyingthe electric field strength and its ori-entation. An additional advantage isthat it is very energy efficient in thatthe electrical force acts only upon thecharged particle, and the surroundingmedium is unaffected.

What Is Electrostatics?All of this raises the question of whatis meant by the term electrostatics?Is it really static? Clearly it cannotbe, because no useful work would bedone otherwise, and it really wouldbe limited to a mathematical con-cept. Unfortunately, the existing IEEE

standard definition tends to encour-age this impression as it defines elec-trostatics as ‘‘the branch of sciencethat treats the electric phenomenaassociated with electric charges at restin the frame of reference” [1] (italicsadded). It seems that although thismay be acceptable to theoreticians, itclearly does not mirror the reality ofengineering applications. This defi-ciency was a bone of contention in the1960s, as recognition grew that it wasinadequate. However, modifying itproved to be difficult, as the IEEEstandards committee of the day wasunreceptive to change. This led toconsiderable frustration on the part ofsome proponents for a correction, inparticular, the late Prof A.D. Moore ofthe University of Michigan. His solu-tion was to form an independent orga-nization, the Electrostatics Society ofAmerica. This new society quicklyadopted a more relevant definition:‘‘the class of phenomena recognizedby the presence of electrical charges,either stationary or moving, and theinteraction of these charges, thisinteraction being solely by reason ofthe charges and their positions and not byreason of their motion” [2] (italicsadded). In practical application, thismeans that a process is governed byelectrostatics when the electric fieldeffects predominate over the mag-netic field effects. In electric circuitterms, this implies that the ratio ofvoltage to current is very high. Inother words, electrostatic devicescan simply be thought of as having avery high impedance.Digital Object Identifier 10.1109/MIAS.2010.937301

1077-2618/10/$26.00©2010 IEEE

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This insight allows us to refute acommon misconception that effectiveelectrostatic action must by necessityinvolve high voltages. This is clearlyincorrect, since what is involved in theCoulomb force is the product of chargeand electric field. The electric field isthe gradient of voltage, so it dependsboth on the geometry of the electrodesand the magnitude of the voltage. So,while high voltage may verywell be needed in some appli-cations, it is by no meansrequired for electrostatic forcesto be important. For example,certain electrostatic paintingprocesses usually require sepa-ration distances of tens of cen-timeters and voltages rangingfrom 50 to 100 kV. However,similar effects are achieved inlaser printers involving dis-tances of tens of micrometersby using only tens of volts.Another way of getting asense of the scale of voltage isto consider the breakdownstrength of air. For example,between parallel electrodes,the electric field normallyrequired for a spark to occur isconstant but can variously bedescribed as 3 MV/m, 30 kV/cm, or 3 V/lm.

Particle ChargingThe key to any practical appli-cation is a method to reliablycharge particles. Many of thehigh school triboelectric dem-onstrations referred to earlieroften failed because the rela-tive humidity on the day ofthe experiment was too high.This resulted in microscopiclayers of moisture coating thesurface of the materials, whichdid two things. First, sincecharge is a surface phenomena,the dissimilar materials wereeffectively shielded from eachother by the layers of mois-ture. Second, in cases wherecharge transfer did occur, thesurface conductivity due tothe moisture was high enoughto cause charge relaxation anddissipation of surface charges.So, the development of usefulapplications in the 20th cen-tury followed directly from theimprovements that occurred

in the methods of charging small par-ticles. There are three main techni-ques that are used: ionic, induction,and triboelectric (also more accuratelycalled contact) charging.

Ionic charging normally involvesthe use of localized ionization of airthrough a stable discharge known ascorona discharge. This occurs in ahighly nonuniform electric field that

exists when a sufficiently high volt-age is applied between electrodes thatdiffer markedly in geometry, forexample, a point to a plane. Depend-ing upon the polarity of the voltage, aunipolar cloud of ions is produced,which can be used to bombard aparticle, thus giving it a net charge.This is a very reliable and controlledmethod for applying charge and has

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the advantage that the particles maybe either conductors or insulators.Induction charging involves exposinga conductive particle that is in con-tact with a grounded surface to anelectric field and then breaking con-tact with the ground, trapping chargeon the surface. Clearly, this techniqueis restricted to materials that havefinite conductivity. Triboelectric charg-ing occurs whenever dissimilar mate-rials are brought into contact. Chargeis exchanged between the materialsin proportion to the difference intheir effective work functions and,provided at least one of the materi-als is a good insulator, opposite signcharge will be trapped on the twosurfaces after separation.

To illustrate the developments thathave occurred in the last century, it isinstructive to look at three importantexamples: electrostatic precipitation(ESP), electrostatic painting and coat-ing, and electrophotography [3], [4].

Electrostatic PrecipitationESP represents the oldest successfulprocess, and the year 1907 may beconsidered to be the real beginning ofthe industrial application of electro-statics. This was the year that F.G.Cottrell installed the first ESP at achemical plant south of San Fran-cisco. The concept was to charge par-ticles in an air stream using a coronadischarge and then to collect them ona grounded plate through Coulombicattraction. Although simple in prin-ciple, it proved very difficult to trans-late into practice, as an ESP involves acomplex interaction of electrical,mechanical, and chemical factors. Infact, it is instructive to look at thisdevelopment in a historical sequenceto see how the science evolved inengineering application. It was ac-tually first demonstrated by Hohl-field in Germany in 1824 when heused a corona discharge to clear thefog inside a glass jar. However, thisdid not progress further because hisinterest was not to precipitate the fogbut rather to see if he could explainwhy rain often falls following a light-ning strike (he couldn’t). The firstperson to recognize the potential ofusing the process for air cleaning wasO.J. Lodge, who patented the processand installed a commercial precipita-tor at a smelter in North Wales in1885. Unfortunately, although the

High-VoltageTransformer Mechanical

Rectifier

Insulating Bushing

Bushing Heater Coil

PubescentDischargeElectrode

GasOutlet

Gas Inlet1

Illustration from Cottrell’s first electrostatic precipitation patent, U.S. Patent No.895,729 (1908).

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demonstration unit worked well in thelaboratory, it failed to work as well inthe field. In hindsight, we recognizethat the two main reasons for this werebecause of the inadequate power supplyand the fact that he was attempting tocollect lead oxide fumes, which eventoday are difficult to precipitate becauseof their fineness and high resistivity.

The concept then stagnated untilthe turn of the 20th century when Cot-trell realized that the recently devel-oped mechanical high-voltage rectifier,when used in conjunction with hispubescent discharge electrode, wouldprovide sufficient corona current tosuccessfully charge large quantities ofdense fumes. Furthermore, he discov-ered that negative corona was more sta-ble than the positive, and to maintainhigh voltage in practical cases, it wasnecessary to heat the high-voltagebushing. These four points were thekey components of the patent issued tohim in 1908 (Figure 1). Another factor

that was helpful inthis case was thatCottrell had someluck on his side.Unlike the lead fumefaced by Lodge, thesulphuric acid misttargeted by Cottrellis particularly ame-nable to collectionby ESP.

Following thissuccess, the processshowed that it wasvery advantageousin collection effec-tiveness, energy ef-ficiency, and cost.As a result, ESP became the preferredmethod for cleaning large-scale indus-trial particulate emissions such as flyash from the exhaust of coal-firedpower plants. In the succeeding deca-des, corporate competition resulted inmajor advances in collection efficiency

(>99% by mass),capacity, range ofapplications, andreliability. By theearly 1960s, ESPhad evolved intoamature technology.In the latter years ofthe 20th century,rapidly changingenvironmental leg-islation continuallyincreased the de-mands on precipi-tator performance.For example, thecollection efficien-cies of >99.9% be-

came the norm along with very tightrestrictions on the number of escapingfine particles below ten micrometersin size. As a result, major improve-ments continued with such develop-ments in the areas of gas conditioning(to modify particle resistivity), corona

ESP BECAMETHE PREFERREDMETHOD FORCLEANING

LARGE-SCALEINDUSTRIALPARTICULATEEMISSIONS.

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wire geometry, power supplies, pre-charging, rapping optimization, andintermittent and pulse energization.Today, many challenges still exist. Cur-rently, efforts are being made to adaptthe process to use things such as non-thermal discharge plasmas, electronbeam reactors, and advanced oxidationtechniques to stimulate gas-phasechemical reactions for removal of gase-ous pollutants along with particles.

Electrostatic Paintingand CoatingThe success and reliability of ESPswere noted in other fields. In particu-lar, in the 1930s, several researchersrecognized the potential advantage ofusing electrostaticforces to improvethe deposition effi-ciency in liquid spraypainting. This was anew process quicklycoming into use, butit suffered from thefact that in manycases only 30% ofthe paint reachedthe target. Severalpatents were issuedbased upon ideasthat were essentiallymodifications of theESP. Commercialversions soon followed. By 1940s, auto-mated electrostatic paint lines wereintroduced, where transfer efficienciesof more than 70% were commonlyachieved, a remarkable achievement atthe time. This was soon followed byfurther improvements in the chargingand delivery systems using electrifiedatomizers in the form of a blade, rotat-ing bell or disk, and air or hydraulicspray guns. All of these systems haveone thing in common. Unlike the caseof ESP where the electrostatic forces arethe basis of operation, they are notessential to the process but rather act togreatly enhance the coating uniformityand improve the transfer efficiency. Inother words, should the electrical pow-er fail, some deposition still takes placedue to mechanical forces.

However, by the early 1960s, acompletely different approach wasdeveloped in which electrostaticsplayed an essential role. The conceptwas to entirely remove the solventfrom the paint and spray it in theform of a finely dispersed, electrically

insulating, thermoplastic powder.This powder was electrostaticallycharged using either a triboelectric orcorona technique. It was then electri-cally attracted to a grounded metaltarget, where the powder adhered tothe surface by electrostatic imageforces. Then, the powder was fused tothe surface by heating it in an oven.This process developed rapidly,stimulated by the adoption of increas-ingly stricter environmental lawsstarting in the 1970s that limited thevolatile organic compound (VOC)emissions from paint solvents. As aresult, improvements in both applica-tion equipment and powder formula-tions allowed more decorative and

thinner layers ofpaint to be applied.Although the trans-fer efficiencies arecomparable to thatfound with electro-static liquid paint-ing (now typicallyin the range of>80%), in modernsystems, the recy-cling of oversprayedpaint powder allowspowder utilizationof >95% to be com-monly achieved.

Electrophotographyor XerographyThe use of electrostatic forces toenhance particle depositions such asin ESPs and liquid painting weresignificant improvements in technol-ogy. The development of powdercoating and the elimination of sol-vent in painting can be considered amajor technological advance. How-ever, the demonstration of the firstelectrophotographic image by Ches-ter Carlson in 1938 can truly beregarded as revolutionary [5]. Thereis no question that it represented themost significant development in thehistory of applied electrostatics, as itallowed for the first time to make truedry copies of documents. It is notonly the most commercially success-ful one but also involves the mostcomplex and integrated combinationof electrostatic processes, includingphotoconductivity, corona charging,triboelectric charging, Coulombic at-traction, image force adhesion, and ionicneutralization. To fully understand the

impact of this technology, it should berealized that it was only in 1950 thatthe first relatively primitive copier wasdeveloped by the Haloid Company(later to become Xerox). In the earliestform, it was very cumbersome andinvolved three separate units. One ofthese housed the camera that capturedthe image on a photoconductor. Thesecond used the manual cascade devel-opment to form the image using finesolid ink particles called toner andtransfer it to paper. The third unit wasa fuser that heated the copy to bondthe toner to the paper. To make onephotocopy using this equipment in-volved three minutes of concentratedeffort. In 1959, the first automated,plain black and white photocopier, theXerox 914, came on the market. In1973, the first printer using a flash-lamp exposure system was commer-cialized. This was followed in 1975with the first commercial form of thelaser printer. Today, the modern elec-trostatic copying and laser printerindustries provide full color reproduc-tion capabilities and account for manybillions of dollars in worldwide com-merce every year. It has changed theway business and home offices operateand has created the whole new indus-try of desktop publishing.

Of course, many sequential im-provements occurred in the latterdecades of the 20th century, but spacedoes not permit a detailed discussionof this interesting story. However, forthose unfamiliar with the details ofthe process, it is instructive to brieflymention each step. The key compo-nent is the photoconductor, a mate-rial that has the unique property ofbeing an insulator when in darknessbut becomes conductive in light. Byuniformly precharging the photocon-ductor in the dark, a latent imageconsisting of charged areas can thenbe established by exposing it to alight image. This image is developedby coating it with an oppositelycharged toner. The toner is then trans-ferred onto paper using electrostaticattraction and then heated to fuse it tothe paper. Finally, any remaining toneris carefully cleaned from the photocon-ductor to allow the cycle to be repeated.It is of interest to note that five of thesesix steps rely on the presence or removalof electrostatic charges. The develop-ment of practical and reliable equip-ment suitable for operation in office or

THE KEY TO ANYPRACTICAL

APPLICATION ISAMETHOD TO

RELIABLYCHARGEPARTICLES.

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home involved animmense amount ofengineering ingenu-ity, leading to theautomation and in-tegration of these sixseparate steps. Com-monly, it involves arotating cylindricaldrum containing thephotoconductorpassing through thesegments , whereeach of these sixsteps is carried out.

It is somewhatironic to note that the reliability ofgood image reproduction that weexperience everyday in photocopyingand laser printing depends cruciallyupon the consistency of the tribo-charging of the toner. Clearly, we havecome a long way from the variability

found in the highschool experimentsmentioned earlier.

The FutureAlthough this is anarticle detailing his-tory, it does providesome insight intowhat we can expectin the future by re-calling some of themain characteristicsof electrostatic forces.First is their un-equaled ability to

control the trajectories of particles inthe size range from millimeters tonanometers. Second is the dependenceof electric fields upon the inverse squarelaw that results in rapidly increasingforces as the physical dimensions de-crease. Third, they are energy efficient

because of the very small currents thatare involved. Of the many possibleapplications that are actively understudy, three particular areas stand out:microelectrical mechanical systems(MEMS), biotechnology, and ultrafineparticles and nanotechnology. Clearly,the smaller the scale of the device, themore dynamic the field of electro-statics becomes.

References[1] The Authoritative Dictionary of IEEE Standards

Terms, 7th ed., IEEE Standard 100, 2000.[2] Available: http://www.electrostatics.org/

electrostatics.html[3] G. S. P. Castle, ‘‘The evolving field of elec-

trostatics,” in Proc. Electrostatics 1991 (Insti-tute of Physics Conference Series No. 118).Bristol: IOP, 1991, pp. 1–12.

[4] G. S. P. Castle, ‘‘Industrial applications ofelectrostatics; The past, present and future,”J. Electrostat., vol. 51–52, pp. 1–7, 2001.

[5] D. Owen, ‘‘Making copies,” SmithsonianMag., pp. 90–97, Aug. 2004. IAS

f rom the edi to r ’ s desk(continued from p. 2)

?-p

erformances of indus-trial electrostatic sepa-rators. In theircontribution to thisspecial issue, Senouci

et al. propose the association of She-whart and cumulative sum charts todetect not only abrupt alterations butalso small shifts in the responses ofvery complex, multifactorial electro-static processes.

Unfortunately, physical phenom-ena related to static electric chargesare not only beneficial in many fieldsof application but also a source of haz-ard: electrostatic discharges can ig-nite explosions and fire or damagemicroelectronic devices. Several groupsof researchers in are involved in pre-dicting and minimizing the damagecaused by electrostatic discharge invarious industry applications. Sodaand Sekine’s article concentrates on theelectrostatic discharge between the

ELECTROSTATICDEVICES CANSIMPLY BE

THOUGHT OF ASHAVING A VERY

HIGHIMPEDANCE.