Recent Progress and Continuing Puzzles in Electrostatics

Schein, L. B.

doi:10.1126/science.1142325

Cited by 103 publications

(84 citation statements)

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“…Models of such clustering typically focus on inelastic deformation and cohesion 1,4,6,8 . However, even in charge-neutral particle systems comprising grains of the same dielectric material, tribocharging can generate large amounts of net positive or negative charge on individual particles, resulting in long-range electrostatic forces [9][10][11] . The e ects of such forces on cluster formation are not well understood and have so far not been studied in situ.…”

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confidence: 99%

Direct observation of particle interactions and clustering in charged granular streams

et al. 2015

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Clustering of fine particles is of crucial importance in settings ranging from the early stages of planet formation [1][2][3] to the coagulation of industrial powders and airborne pollutants [4][5][6][7] . Models of such clustering typically focus on inelastic deformation and cohesion 1,4,6,8 . However, even in charge-neutral particle systems comprising grains of the same dielectric material, tribocharging can generate large amounts of net positive or negative charge on individual particles, resulting in long-range electrostatic forces [9][10][11] . The e ects of such forces on cluster formation are not well understood and have so far not been studied in situ. Here we report the first observations of individual collide-and-capture events between charged submillimetre particles, including Kepler-like orbits. Charged particles can become trapped in their mutual electrostatic energy well and aggregate via multiple bounces. This enables the initiation of clustering at relative velocities much larger than the upper limit for sticking after a head-on collision, a long-standing issue known from pre-planetary dust aggregation 1,12 . Moreover, Coulomb interactions together with dielectric polarization are found to stabilize characteristic molecule-like configurations, providing new insights for the modelling of clustering dynamics in a wide range of microscopic dielectric systems, such as charged polarizable ions, biomolecules and colloids [13][14][15][16] . One of the key difficulties in studying the interplay between repulsive contact forces, short-range cohesion and long-range electrostatic forces during cluster formation has been to obtain sufficiently detailed experimental data. Seeing how this process unfolds demands in situ observation of the collision trajectories among charged grains to extract quantitative information about their interactions. This requires the grains to be freed from gravity and tracked with high spatial and temporal resolution to capture individual collision events 17,18 . We overcome these obstacles with the set-up shown in Fig. 1a (refs 19,20). The granular material, in our experiments fused zirconium dioxide-silicate grains a few hundred micrometres in diameter, is contained in a vessel mounted inside a 3.0-m-tall cylindrical chamber. We evacuate this chamber to mitigate air drag. When a shutter covering a small orifice at the bottom of the vessel is opened, particles fall out freely, forming a dilute stream. Outside the chamber, a high-speed video camera falls alongside the grains, guided by low-friction rails. In the co-moving frame seen by the camera, the effect of gravity is eliminated, making it possible to track particle interactions in detail for about 0.2 s until the camera is decelerated by a foam pad. The same apparatus also allows determination of the net charge on individual grains: during free fall a horizontal electric field can be applied and the resulting horizontal acceleration observed by the camera gives the charge to mass ratio, q/m.Using particles with a narrow size d...

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Direct observation of particle interactions and clustering in charged granular streams

et al. 2015

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“…[3][4][5] Although the phenomenon of frictional charging of insulators had been described even in ancient times 6 and has been increasingly quantitatively studied over the last centuries, 7,8 no consensus on the charging mechanism has yet been reached. [9][10][11][12][13][14][15] As an indication of our ignorance on the subject, though the terms "electricity" and "electron", as is well known, were derived from the Greek word 022119-3 Nikolaus Knorr AIP Advances 1, 022119 (2011) temperature in an AFM glove box that was constantly flushed with nitrogen gas humidified by deionized water, to adjust for the desired RH. The rubbing direction in the scans was always from top to bottom.…”

Section: Introductionmentioning

confidence: 99%

Squeezing out hydrated protons: low-frictional-energy triboelectric insulator charging on a microscopic scale

Knorr¹

2011

AIP Advances

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Though triboelectric charging of insulators is common, neither its mechanism nor the nature of the charge is well known. Most research has focused on the integral amount of charge transferred between two materials upon contact, establishing, e.g., a triboelectric series. Here, the charge distribution of tracks on insulating polymer films rubbed by polymer-covered pointed swabs is investigated in high resolution by Kelvin probe force microscopy. Pronounced bipolar charging was observed for all nine rubbing combinations of three different polymers, with absolute surface potentials of up to several volts distributed in streaks along the rubbing direction and varying in polarity on μm-length scales perpendicular to the rubbing direction. Charge densities increased considerably for rubbing in higher relative humidity, for higher rubbing loads, and for more hydrophilic polymers. The ends of rubbed tracks had positively charged rims. Surface potential decay with time was strongly accelerated in increased humidity, particularly for polymers with high water permeability. Based on these observations, a mechanism is proposed of triboelectrification by extrusions of prevalently hydrated protons, stemming from adsorbed and dissociated water, along pressure gradients on the surface by the mechanical action of the swab. The validity of this mechanism is supported by explanations given recently in the literature for positive streaming currents of water at polymer surfaces and by reports of negative charging of insulators tapped by accelerated water droplets and of potential built up between the front and the back of a rubbing piece, observations already made in the 19th century. For more brittle polymers, strongly negatively charged microscopic abrasive particles were frequently observed on the rubbed tracks. The negative charge of those particles is presumably due in part to triboemission of electrons by polymer chain scission, forming radicals and negatively charged ions

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“…7 Apesar da onipresença dos fenômenos eletrostáticos e das suas consequências, o conhecimento sobre este tópico ainda é bastante empírico. 8,9 Persistem lacunas importantes na correlação de ideias fundamentais da teoria atômico-molecular com as observações experimentais da eletrostática de materiais isolantes 10 e muitas questões importantes sobre esse assunto ainda não foram satisfatoriamente respondidas. A principal delas é a identidade das espécies portadoras de cargas em um material isolante, bem como a sua detecção e identificação.…”

Section: Eletrização De Materiaisunclassified

“…Creating a reproducible surface and obtaining experimental reproducibility among laboratories has been a challenge". 8 Portanto, não há consensos amplos sobre a teoria dos fenômenos eletrostáticos e também não há paradigmas experimentais reprodutíveis mesmo nos casos mais simples da eletrização por contato e triboeletrização.…”

Section: Eletrização De Materiaisunclassified

Eletrização de dielétricos: novas propostas para resolver velhos problemas

et al. 2010

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Recebido em 30/6/10; aceito em 30/8/10; publicado na web em 8/11/10 ELECTROSTATIC CHARGING OF DIELECTRICS: NEW APPROACHES TO SOLVE PERSISTING PROBLEMS. Electrostatic phenomena were discovered long ago but their interpretation according to well-established atomic-molecular theory is still lacking. As a result, electrostatic phenomena are often irreproducible and uncontrolled, causing serious practical problems. Highly reproducible recent experimental results on electrostatic charging from this and other laboratories are reviewed in this work, together with a description of the relevant but not so usual Kelvin probe and Faraday cup techniques. These results support a new model for electrostatic charging of dielectrics and insulated metals, based on the role of moist atmosphere as a charge reservoir.Keywords: water ion partition; electrostatic charging; Kelvin probe. INTRODUÇÃO Eletrização de materiaisO primeiro relato de um fenômeno de eletrização é devido a Tales de Mileto e os resultados experimentais obtidos até o século 19 foram consolidados no tratado de Maxwell sobre Eletromagnetismo. 1 Hoje, a eletrostática está presente em muitas tecnologias de produtos e processos importantes: as máquinas de eletrocópias, impressoras a laser, 2 eletretos 3 usados como transdutores acústicos, reciclagem de políme-ros isolantes, 4 fabricação de nanofibras e nanotubos poliméricos por fiação eletrostática, 5 pintura eletrostática e filtração eletrostática. 6 A eletrização atmosférica também é conhecida de praticamente todas as populações humanas devido à sua associação às tempestades. Poderosas descargas eletrostáticas são ainda observadas em erupções vulcânicas e em tempestades de areia, em regiões áridas. 7 Apesar da onipresença dos fenômenos eletrostáticos e das suas consequências, o conhecimento sobre este tópico ainda é bastante empírico. 8,9 Persistem lacunas importantes na correlação de ideias fundamentais da teoria atômico-molecular com as observações experimentais da eletrostática de materiais isolantes 10 e muitas questões importantes sobre esse assunto ainda não foram satisfatoriamente respondidas. A principal delas é a identidade das espécies portadoras de cargas em um material isolante, bem como a sua detecção e identificação. Schein, um ex-dirigente de pesquisas na Xerox afirmou em uma importante e recente revisão: "Most researchers believe that insulator charging is a surface phenomenon. Creating a reproducible surface and obtaining experimental reproducibility among laboratories has been a challenge". 8 Portanto, não há consensos amplos sobre a teoria dos fenômenos eletrostáticos e também não há paradigmas experimentais reprodutíveis mesmo nos casos mais simples da eletrização por contato e triboeletrização.O desconhecimento dos mecanismos de acúmulo e dissipação de cargas eletrostáticas não é somente restrito à área de materiais. As pesquisas em geofísica não produziram ainda um modelo efetivo para explicar a eletrização da atmosfera, 11 o que está certamente associado às frequentes perdas de vidas e propried...

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Recent Progress and Continuing Puzzles in Electrostatics

Cited by 103 publications

References 12 publications

Direct observation of particle interactions and clustering in charged granular streams

Direct observation of particle interactions and clustering in charged granular streams

Squeezing out hydrated protons: low-frictional-energy triboelectric insulator charging on a microscopic scale

Eletrização de dielétricos: novas propostas para resolver velhos problemas

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