Pii: S0304-3886(99)00005-4

0304-3886/99/$ see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S0304-3886(99)00005-4 80 T.J. Gung et al. / Journal of Electrostatics 46 (1999) 79-89 ments using the "onion-peeling" method of radial discretization of concentric circles centered on the axisymmetric axis of the point electrode at various heights above the ground plane. 3. Electrochemical Properties of Propylene Carbonate Propylene carbonate is very polar (e~ ,-~ 68) and is a good solvent for most plastic materials [5-8]. We use Delrin as the supporting structure for the electrodes as we have found that it does not dissolve in propylene carbonate. Propylene carbonate is also very reactive with many metals, in particular with stainless steel, generating particle layers on the positive electrode. We use aluminum for our electrodes as we have found that it reacts much less than most other common metals [8]. Our most successful measurements were with negative voltage polarity on the point electrode, as the particle layer on the large positive ground plane was negligible. However, when the point electrode was positive, the particle layer on the point electrode was irregularly shaped, destroying the axisymmetry, and blocked the light path near the point electrode. When we applied high voltage between electrodes immersed in propylene carbonate, particle motion could induce irregular liquid flow causing the transmitted light intensity to fluctuate and making the detected signal unstable. According to our investigation, keeping the applied dc voltage below 800 V for a 5 mm gap gives a stable optical signal. The increase in conductivity due to moisture can be reduced by drying propylene carbonate with circulation through 3A molecular sieves. 4. E x p e r i m e n t a l Setup The experimental setup is shown in Figure 1. The laser is moveable in the x-y plane, controlled by two linear stepper motors. The filtering polarizer is used to attenuate the light to prevent saturation of the photo-detector. The next polarizer's transmission axis is fixed at 0 ° to the vertical and the following quarter wave plate's slow axis is fixed at 45 ° . This arrangement converts the incident linearly polarized light to circularly polarized light. Inside the test cell is the axisymmetric point-plane electrodes within propylene carbonate. Electric field induced optical birefringence occurs when we apply high voltage between the point electrode and the ground plane. The applied electric field causes a small difference in the refractive indices for light polarized parallel and perpendicular to the applied electric field, proportional to the square of the electric field magnitude in the plane transverse to light propagation. The light exiting the Kerr cell is elliptically polarized. When it is passed through the analyzer and focused onto a light detector, the measured intensity gives information related to the electric field distribution between the electrodes. Because the Kerr effect change in refractive index is proportional to the square of the electric field in the plane perpendicular to the direction of light propagation, if the electric field has a dc component and an ac component at frequency f , the detected light intensity has components at dc, the fundamental frequency f and second harmonic frequency 2f. For our experiments, the gap distance between the point and ground plane electrodes is 5.25 mm, the point has about 1 mm radius of curvature and the ac frequency is chosen as f = 90Hz. T.J. Gung et al. / Journal of Electrostatics 46 (1999) 79-89 81