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The works of Michael Faraday related to aerosol science
J. Aerosol Sci., Vol. 26. Suppl l, pp. $241-$242, 1995
Elsevier Science Ltd Printed in Great Britain 0021-8502/95 $9.50 + 0.00
Pergamon
THE WORKS OF MICHAEL FARADAY RELATED TO AEROSOL SCIENCE J.W. GENTRY Department of Chemical Engineering, University of Maryland, College Park, Maryland, 20754 USA
INTRODUCTION The principal content of this paper are those studies of Michael Faraday which are related to aerosol science. Those papers which were described in depth in the recent essay ( Gentry (1995)) are briefly sketched here while the significant study on tribocharging by Faraday which had not been previously discussed, is described in detail in this mini-paper. COLLOIDS AND AEROSOLS Faraday's principal contribution to aerosol science was presented in a paper which was first presented as the Baker Lecture to the Royal Society in 1857. The material covered a wide range of topics suggesting several methods for generating colloidal particles and aerosols and procedures for producing a stable colloid. He was the first to systematically use the exploding wire procedure for producing gold aerosols and his procedure for producing gold colloids with the assistance of phosphorous dissolved in CS2 was later adapted by Zsigmondy in his Nobel prize work and by Mie in the study of light scattering of colloids. He demonstrated that the color of a colloidal dispersion differed according to whether the light was scattered or transmitted and that as colloidal particles coagulated and increased in size the color of transmitted light shifted from ruby red to blue. Faraday discovered and utilized the FaradayTyndall effect to examine colloids and aerosols indirectly from their scattered light. The topics discussed in the paper are presented in the first two columns of Table I. In a prescient observation , Faraday describes the use of a observation tube at an angle to the transmitted light. It was this idea which was exploited a century later in the development of the OWL. Faraday's study of electrical discharge in gases is unquestionably the deepest study between those of Watson and PI0cker. He demonstrated that the probability of discharge depended on the gas composition as well as the electrical potential gradient. Also, he showed that the presence of dust enhances the probability of discharge because of discharge from the
Figure 1: Schematic of Faraday Contributions
$241
$242
J.W. GENTRV
electrode to the dust particles as well as the effect of the "electrical wind". He developed a clever way of visualizing the shape of the Taylor cone, using conducting particles in turpentine. TRIBOCHARGING In additions to the papers mentioned above and discussed in detail in Gentry (1995) there is the paper " On the Electricity Evolved by the Friction of Water and Steam Against other Bodies" which is germane to the aerosol related contributions of Faraday. Essentially this paper describes some of the earliest work in the area of tribocharging an area which has been studied recently by Scarlet, Yammamoto, H. Masuda, and Dunn among other contemporary Aerosol scientists. Tribocharging describes the change in electrical charge due to the friction of impaction by liquid or solid particles on a surface. This study was one of two which the obituary for Faraday printed by the Royal Society in Edinburgh stated as far removed from the other offerings in the series, This may explain its relative neglect for it seems not to have played a significant role in the development of Tribology in contrast to his studies on regelation which played a major role in attempts to find the explanation as to why ice melts when under pressure. An excellent overview of this latter controversy including short papers by Forbes, Tyndall, Faraday, and James Thomson are available in the Complete Works of James Thomson. The tribocharging study arose from observations by Armstrong, Pattinson, and Schafhaeutl on the production of electricity in air by the issue of pressurized steam into air. The Faraday paper describes a number of qualitative experiments which allowed Faraday to conclude that "...particles of water rubbed against other bodies by a current of steam evolve electricity. For this purpose, however, it is not merely water but pure water which must be used " This statement covers two of the major results in this paper. The electrical charge is conducted by entrained particles and not by the steam. Moreover the charge is result of contact friction between the particles of water and a solid impaction surface. The second result that the effect does not occur when tap water is used. This phenomenon is due to the fact that because tap water contains dissolved salts it is conducting and as consequence the frictional charge is dissipated. Faraday carried out experiments with solutions which are poorly conducting using ammonia dissolved in water to test his hypothesis. The droplets with poor conductivity result in electrification confirming his explanation. Next Faraday considered the case where the water used to form the steam contained trace amounts of an organic non-conductor. He conjectured that: " When olive oil or oil of turpentine is present these globules are ... virtually converted into globules of these bodies, and it is no longer water, but the new fluids which are rubbing the rubbed bodies". He showed that saline or acid solutions would electrify the target while alkali drops and olive oil would not show tribocharging whereas alkali and oil of turpentine would. His explanation is that olive oil is soluble in alkali surfaces whereas oil of turpentine is not. These experiments demonstrate that the tribocharging phenomena is associated with the surface character of the drop and not its bulk properties. The fourth point which Faraday examined was the effect of the composition of the contact surface on the charge behavior. These were examined in experiments where the steam was emitted through a tube of variable composition and impacted on a cone or wire of a second composition placed in the path of the fluid stream. Consequently the polarity of the charge induced on the cone depended both on its composition and the composition of the emission tube. Faraday found that the polarity could be adjusted at will. Tubes or cones were constructed of 18 different materials. The tube or impaction wire were connected to a gold leaf electrometer which provided an indication of the quantity of electricity produced. Typical of these results were that a neutral stream of particles were generated using ivory as the emission tube. Ivory proved to be conspicuous in that impaction cones from ivory showed very little electrical effect. Faraday, M., (1845), Experimental Research in Electricity, Vol. 2, p 106-126 Gentry, James W., (1995), "The Aerosol Contributionsof Michael Faraday", J. Aerosol Sci.,26
Elsevier Science Ltd Printed in Great Britain 0021-8502/95 $9.50 + 0.00
Pergamon
THE WORKS OF MICHAEL FARADAY RELATED TO AEROSOL SCIENCE J.W. GENTRY Department of Chemical Engineering, University of Maryland, College Park, Maryland, 20754 USA
INTRODUCTION The principal content of this paper are those studies of Michael Faraday which are related to aerosol science. Those papers which were described in depth in the recent essay ( Gentry (1995)) are briefly sketched here while the significant study on tribocharging by Faraday which had not been previously discussed, is described in detail in this mini-paper. COLLOIDS AND AEROSOLS Faraday's principal contribution to aerosol science was presented in a paper which was first presented as the Baker Lecture to the Royal Society in 1857. The material covered a wide range of topics suggesting several methods for generating colloidal particles and aerosols and procedures for producing a stable colloid. He was the first to systematically use the exploding wire procedure for producing gold aerosols and his procedure for producing gold colloids with the assistance of phosphorous dissolved in CS2 was later adapted by Zsigmondy in his Nobel prize work and by Mie in the study of light scattering of colloids. He demonstrated that the color of a colloidal dispersion differed according to whether the light was scattered or transmitted and that as colloidal particles coagulated and increased in size the color of transmitted light shifted from ruby red to blue. Faraday discovered and utilized the FaradayTyndall effect to examine colloids and aerosols indirectly from their scattered light. The topics discussed in the paper are presented in the first two columns of Table I. In a prescient observation , Faraday describes the use of a observation tube at an angle to the transmitted light. It was this idea which was exploited a century later in the development of the OWL. Faraday's study of electrical discharge in gases is unquestionably the deepest study between those of Watson and PI0cker. He demonstrated that the probability of discharge depended on the gas composition as well as the electrical potential gradient. Also, he showed that the presence of dust enhances the probability of discharge because of discharge from the
Figure 1: Schematic of Faraday Contributions
$241
$242
J.W. GENTRV
electrode to the dust particles as well as the effect of the "electrical wind". He developed a clever way of visualizing the shape of the Taylor cone, using conducting particles in turpentine. TRIBOCHARGING In additions to the papers mentioned above and discussed in detail in Gentry (1995) there is the paper " On the Electricity Evolved by the Friction of Water and Steam Against other Bodies" which is germane to the aerosol related contributions of Faraday. Essentially this paper describes some of the earliest work in the area of tribocharging an area which has been studied recently by Scarlet, Yammamoto, H. Masuda, and Dunn among other contemporary Aerosol scientists. Tribocharging describes the change in electrical charge due to the friction of impaction by liquid or solid particles on a surface. This study was one of two which the obituary for Faraday printed by the Royal Society in Edinburgh stated as far removed from the other offerings in the series, This may explain its relative neglect for it seems not to have played a significant role in the development of Tribology in contrast to his studies on regelation which played a major role in attempts to find the explanation as to why ice melts when under pressure. An excellent overview of this latter controversy including short papers by Forbes, Tyndall, Faraday, and James Thomson are available in the Complete Works of James Thomson. The tribocharging study arose from observations by Armstrong, Pattinson, and Schafhaeutl on the production of electricity in air by the issue of pressurized steam into air. The Faraday paper describes a number of qualitative experiments which allowed Faraday to conclude that "...particles of water rubbed against other bodies by a current of steam evolve electricity. For this purpose, however, it is not merely water but pure water which must be used " This statement covers two of the major results in this paper. The electrical charge is conducted by entrained particles and not by the steam. Moreover the charge is result of contact friction between the particles of water and a solid impaction surface. The second result that the effect does not occur when tap water is used. This phenomenon is due to the fact that because tap water contains dissolved salts it is conducting and as consequence the frictional charge is dissipated. Faraday carried out experiments with solutions which are poorly conducting using ammonia dissolved in water to test his hypothesis. The droplets with poor conductivity result in electrification confirming his explanation. Next Faraday considered the case where the water used to form the steam contained trace amounts of an organic non-conductor. He conjectured that: " When olive oil or oil of turpentine is present these globules are ... virtually converted into globules of these bodies, and it is no longer water, but the new fluids which are rubbing the rubbed bodies". He showed that saline or acid solutions would electrify the target while alkali drops and olive oil would not show tribocharging whereas alkali and oil of turpentine would. His explanation is that olive oil is soluble in alkali surfaces whereas oil of turpentine is not. These experiments demonstrate that the tribocharging phenomena is associated with the surface character of the drop and not its bulk properties. The fourth point which Faraday examined was the effect of the composition of the contact surface on the charge behavior. These were examined in experiments where the steam was emitted through a tube of variable composition and impacted on a cone or wire of a second composition placed in the path of the fluid stream. Consequently the polarity of the charge induced on the cone depended both on its composition and the composition of the emission tube. Faraday found that the polarity could be adjusted at will. Tubes or cones were constructed of 18 different materials. The tube or impaction wire were connected to a gold leaf electrometer which provided an indication of the quantity of electricity produced. Typical of these results were that a neutral stream of particles were generated using ivory as the emission tube. Ivory proved to be conspicuous in that impaction cones from ivory showed very little electrical effect. Faraday, M., (1845), Experimental Research in Electricity, Vol. 2, p 106-126 Gentry, James W., (1995), "The Aerosol Contributionsof Michael Faraday", J. Aerosol Sci.,26