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Liquid motion in benzene by negative and positive charge carriers
Volume 49A, number 5
PHYSICS LETTERS
7 October 1974
LIQUID MOTION IN BENZENE BY NEGATIVE AND POSITIVE CHARGE CARRIERS H. KRAUSE IV. Physikulische Institut der Universitdt C;iittingen, Germany Received 9 September 1974 In liquid benzene a macroscopic motion of the fluid is observed at very low densities of electrical current. The sign of charge carriers is determined by the direction of flow.
Because of the interaction of charge carriers with the surrounding molecules the electrical current in some dielectric liquids is accompanied by a macroscopic motion of the liquid [ 11. Therefore, in interpreting transient current measurements one has to correct for the motion of the liquid to obtain absolute values for the mobility. However, this electrohydrodynamic effect can be used to determine the sign of the charge carriers, because the liquid will move in the same direction as the charge carriers [2]. In liquid benzene the electrical current is mainly carried by negative ions, which are injected into the liquid from the electrodes [3]. But there is evidence of a small contribution from positive charge carriers, which is difficult to detect [4]. For the present measurements we used a teflon (ETFE) cuvette with glass windows. In fig. 1 the arrangement of the electrodes is shown: a plane gilded brass sheet (4X 25 mm) on the one side and a golden knife-edge electrode (0.1 X 5 X 25 mm) on the other in a distance of 10 mm. The electrode space was imaged by Schlieren optics to show the (very small)
skbn Fig. 1. Geometry of electrodes (schematic), a = golden knifeedge electrode, b = gilded brass electrode.
changes in the density of liquid caused by the current flow. Photographs were taken by special film material (Agfa ortho 25 professional) and the current was measured by a Keithley 610 C electrometer. The benzene used was of analytical grade purity (Merck AC). If the knife-edge electrode (a) is on negative polarity, the Schlieren image shows a distinct motion of the liquid benzene at voltages U > 0.5 kV. Although the current density j (through the plane electrode (b)) is verylow(j=5X10-11 Alcm2at U=0.5kV),aflow front builds up at the knife-edge and moves towards the plane electrode. With positive polarity of the knife-edge no flow is detectable up to U = 3 kV. However, above this value a flow front is observed again which also starts from the knife-edge in direction to the plane electrode. Then the current density (through the plane electrode) is 30% smaller at the same voltage and the contrast of the Schlieren image is low compared with the situation where the knife-edge has negative polarity. In both cases the flow front is not a straight line; ‘flow fingers are formed (right and left part of fig. 2) whose lengths gradually develop with increasing distance from the generating electrode. The mean velocity t+ of flow front has been estimated to uF & 1 cm/s at u= 3 kV (neg. polarity of knife-edge). This velocity UF should be comparable to the drift velocity uD of the charge carriers which can be calculated if the mobility P and the electric field are known. Takingp = 6X 10m4 cm2/Vs (at 2O’C) for negative charges in benzene from the work of Wollers [3] and a mean electric field (the field at half electrode distance) one obtains uD = 2.4 cm/s. Actually this is in the same order of magnitude as uF. Since we observe flow fronts with both polarities of the knife-edge electrode, both (positive or negative) 371
Volume 49A, number 5
PHYSICS LETTERS
b
a Fig. 2. Flow front in liquid benzene, electrode distance = 10 mm, voltage U = 5 kV, knife-edge neg. polarity, arrow shows direction of flow.
ions can be generated at the electrode surface. However, from the difference in the optical contrast of the flow fronts at lower fields only the front due to the motion of negative ions is detectable. As the generation rate depends on the electrode material [3], the situation may change if other electrode materials are used. However, the behavior of the liquid motion, which always starts at the electrode and not in the bulk obviously verifies that current in liquid benzene is carried by charges, injected from the electrode 13941. At voltages U > 4 kV and negative polarity of knife-edge we often observed a ‘turbulent breakdown channel’, shown in the middle of fig. 2. The flow fig-
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I October 1974
ures are reproducible in different experiments. This means that the regions at the surface, where the ‘flow fingers’ are generated stand fixed. Therefore charge injection is favored at certain positions at the surface and the channels obviously develop from the most effective injection regions. At current densities i > 1OPg A/cm2 (through the plane electrode) the liquid benzene is already in violently turbulent motion, consequently the measured current fluctuates. Apparently these turbulencies are responsible for the statistical current fluctuations observed by Kahlstatt [5] on liquid benzene at high fields.
The author would like to thank Professor Dr. H.-U. Harten and Dr. W. Schroten for valuable support.
References [l JE.J. Hopfinger and J.P. C&se, Phys. Fluids 14 (1971) 1671. [2] E. Gray and T.J. Lewis, Brit. J. Appl. Phys. 16 (1965) 1049. [3] F. Wollers, Ber. Bunsenges. Physik. Chem. 77 (1973) 1133. [4] G. Kleinheins, Ber. Bunsenges. Physik. Chem. 73 (1969) 1011. [5] P. Kahlstatt and F. Wollers, Phys. Lett. 38A (1972) 301.
PHYSICS LETTERS
7 October 1974
LIQUID MOTION IN BENZENE BY NEGATIVE AND POSITIVE CHARGE CARRIERS H. KRAUSE IV. Physikulische Institut der Universitdt C;iittingen, Germany Received 9 September 1974 In liquid benzene a macroscopic motion of the fluid is observed at very low densities of electrical current. The sign of charge carriers is determined by the direction of flow.
Because of the interaction of charge carriers with the surrounding molecules the electrical current in some dielectric liquids is accompanied by a macroscopic motion of the liquid [ 11. Therefore, in interpreting transient current measurements one has to correct for the motion of the liquid to obtain absolute values for the mobility. However, this electrohydrodynamic effect can be used to determine the sign of the charge carriers, because the liquid will move in the same direction as the charge carriers [2]. In liquid benzene the electrical current is mainly carried by negative ions, which are injected into the liquid from the electrodes [3]. But there is evidence of a small contribution from positive charge carriers, which is difficult to detect [4]. For the present measurements we used a teflon (ETFE) cuvette with glass windows. In fig. 1 the arrangement of the electrodes is shown: a plane gilded brass sheet (4X 25 mm) on the one side and a golden knife-edge electrode (0.1 X 5 X 25 mm) on the other in a distance of 10 mm. The electrode space was imaged by Schlieren optics to show the (very small)
skbn Fig. 1. Geometry of electrodes (schematic), a = golden knifeedge electrode, b = gilded brass electrode.
changes in the density of liquid caused by the current flow. Photographs were taken by special film material (Agfa ortho 25 professional) and the current was measured by a Keithley 610 C electrometer. The benzene used was of analytical grade purity (Merck AC). If the knife-edge electrode (a) is on negative polarity, the Schlieren image shows a distinct motion of the liquid benzene at voltages U > 0.5 kV. Although the current density j (through the plane electrode (b)) is verylow(j=5X10-11 Alcm2at U=0.5kV),aflow front builds up at the knife-edge and moves towards the plane electrode. With positive polarity of the knife-edge no flow is detectable up to U = 3 kV. However, above this value a flow front is observed again which also starts from the knife-edge in direction to the plane electrode. Then the current density (through the plane electrode) is 30% smaller at the same voltage and the contrast of the Schlieren image is low compared with the situation where the knife-edge has negative polarity. In both cases the flow front is not a straight line; ‘flow fingers are formed (right and left part of fig. 2) whose lengths gradually develop with increasing distance from the generating electrode. The mean velocity t+ of flow front has been estimated to uF & 1 cm/s at u= 3 kV (neg. polarity of knife-edge). This velocity UF should be comparable to the drift velocity uD of the charge carriers which can be calculated if the mobility P and the electric field are known. Takingp = 6X 10m4 cm2/Vs (at 2O’C) for negative charges in benzene from the work of Wollers [3] and a mean electric field (the field at half electrode distance) one obtains uD = 2.4 cm/s. Actually this is in the same order of magnitude as uF. Since we observe flow fronts with both polarities of the knife-edge electrode, both (positive or negative) 371
Volume 49A, number 5
PHYSICS LETTERS
b
a Fig. 2. Flow front in liquid benzene, electrode distance = 10 mm, voltage U = 5 kV, knife-edge neg. polarity, arrow shows direction of flow.
ions can be generated at the electrode surface. However, from the difference in the optical contrast of the flow fronts at lower fields only the front due to the motion of negative ions is detectable. As the generation rate depends on the electrode material [3], the situation may change if other electrode materials are used. However, the behavior of the liquid motion, which always starts at the electrode and not in the bulk obviously verifies that current in liquid benzene is carried by charges, injected from the electrode 13941. At voltages U > 4 kV and negative polarity of knife-edge we often observed a ‘turbulent breakdown channel’, shown in the middle of fig. 2. The flow fig-
372
I October 1974
ures are reproducible in different experiments. This means that the regions at the surface, where the ‘flow fingers’ are generated stand fixed. Therefore charge injection is favored at certain positions at the surface and the channels obviously develop from the most effective injection regions. At current densities i > 1OPg A/cm2 (through the plane electrode) the liquid benzene is already in violently turbulent motion, consequently the measured current fluctuates. Apparently these turbulencies are responsible for the statistical current fluctuations observed by Kahlstatt [5] on liquid benzene at high fields.
The author would like to thank Professor Dr. H.-U. Harten and Dr. W. Schroten for valuable support.
References [l JE.J. Hopfinger and J.P. C&se, Phys. Fluids 14 (1971) 1671. [2] E. Gray and T.J. Lewis, Brit. J. Appl. Phys. 16 (1965) 1049. [3] F. Wollers, Ber. Bunsenges. Physik. Chem. 77 (1973) 1133. [4] G. Kleinheins, Ber. Bunsenges. Physik. Chem. 73 (1969) 1011. [5] P. Kahlstatt and F. Wollers, Phys. Lett. 38A (1972) 301.