Toshiyuki Sugimoto

Negative corona discharge at a tip of water cone deformed under dc field

Abstract The negative corona discharge phenomena occurring at a tip of water cone have been investigated focusing on the motion and the shape of the water surface using a high-speed video camera. The water cone is periodically formed from a water droplet located on a grounded electrode under dc field due to the electro-hydrodynamic instability. Negative corona discharge with trains of Trichel-like pulses occurred just after the formation of a cone jet. The magnitude and the pulse interval of the Trichel-like pulses gradually increase during the period that the conical surface returns to the initial ellipsoidal shape. The characteristics of the Trichel pulse occurring from the metal electrode simulating the shape of the water surface were also investigated. It is found that the increase in the magnitude and the interval of the trains of Trichel-like pulses with time at the tip of water drop are caused by the corona discharge from the tip of the water surface that deforms from a conical to an ellipsoidal shape.

DC corona discharge from water droplets on a hydrophobic surface

Corona discharge occurring in water droplets placed on the surface of a hydrophobic polymer sheet under DC field was investigated experimentally along with the behavior of droplets. Two kinds of hydrophobic insulating materials, PTFE and silicone rubber, were used. Two water droplets were placed at the border of the foil electrodes and the insulating sheet. The two droplets elongated in different ways and a water filament extended along the surface by the electrostatic force. Corona discharge occurred at the tip of water droplets before the filament formed. The discharge aspect and the behavior of water droplets strongly depends on the polarity of the applied voltage. This would affect the surface flashover voltage via water droplets.

Resonance phenomena of a single water droplet located on a hydrophobic sheet under AC electric field

Resonance vibration of a water droplet located on the surface of a hydrophobic sheet under an AC electric field was investigated experimentally. In order to confirm the effect of the surface property of hydrophobic material on the resonance phenomena of a water droplet, three kinds of hydrophobic materials-silicone rubber (SR), polytetrafluoroethylene (PTFE) and polydimethylsiloxane-grafted fluoropolymer (PGF) sheet-were used. A single water droplet with a volume from 3 to 30 /spl mu/l was placed between parallel electrodes which formed a uniform electric field in the tangential direction along the surface of the hydrophobic sheet. A water droplet under an AC field vibrates strongly only at a particular frequency range. The resonance frequency where the water droplet vibrates strongly decreases with increasing the volume of the water droplet. The resonance frequency on the SR sheet is lower than that on the PTFE and the PGF sheet. The resonance frequency and the magnitude of the deformation of the water droplet depend on the surface properties of the hydrophobic material.

Behavior of water droplets located on a hydrophobic insulating plate under DC field

The behavior of water droplets placed on the surface of a hydrophobic polymer plate under a DC field was investigated experimentally. In order to confirm the effect of hydrophobicity on the movement of the water droplet, three kinds of hydrophobic insulating materials-PTFE, silicone rubber (SR) and ethylene propylene rubber (EPDM)-were used. Water droplets were placed on the insulating plate in contact with the aluminum foil electrode or placed between the thin foil electrodes. A single droplet placed in contact with the electrode on the SR or EPDM plate was elongated and a water filament extended along the surface by the electrostatic force, while that on the PTFE plate left the electrode. The water droplet placed on the border of electrode at the PTFE plate was deformed and formed a semi-cone at a triple junction. The apparent contact angle varied with time for a constant applied voltage and some amounts of charge were emitted by micro discharge or by ejection of small droplets. The behavior of the droplets under DC field strongly depends on not only the hydrophobicity but also the surface property of an insulating plate.

Formation of the charged droplet cloud and corona discharge between the cloud and a grounded electrode

The formation of a charged-droplet cloud with high charge density and the corona discharge characteristics between the cloud and a grounded electrode have been investigated. To form a charged-droplet cloud an electrostatic spray system with an airless nozzle is adopted. The spray system consists of an airless nozzle, a ring electrode and a blower. Since an airless nozzle can ejected large amount of droplets at once, a high density cloud can be created. Droplets in the cloud are electrified by means of induction phenomenon with the ring electrode. A blower is used to promote the transportation of the charged droplets and to grow in the charge density of the cloud. As the air-flow is faster, the charge amount of the ejected droplets and the charge density of the cloud increase. The air flow also influences the corona discharge from the grounded electrode to the charged cloud, extremely. The discharge current with air flow becomes several times larger than that without air flow, because the air flow prevents the deposition of charged droplets on the grounded electrode and no small corona discharge from the deposited droplets occurs. It suggests that the air flow is quite effective to make the discharge between a charged-droplet cloud and a grounded electrode.

Bi-directional extension of the travel range of electrostatic actuators by open loop periodically switched oscillatory control

Pull-in instabilities occur in the dynamics of several classes of electromechanical systems which are of interest in connection with a number of actuator technologies. We describe a gap-closing type electrostatic actuator for which pull-in instabilities occur, and we propose and analyze a novel structure and an open-loop oscillatory control design to stabilize and extend the operation range of a parallel plate actuator bi-directionally; for both gap-closing and gap-enlarging. Our approach involves averaging theory and the design of an averaged potential function around whose critical points the dynamics of the controlled system are organized. An oscillatory stabilizing control law is designed by parametrically shaping the averaged potential. Practical features of the control law are discussed, and performance of the controlled system is shown to be robust with respect to disturbances.

Corona Charging and Current Measurement Using Phi-Type Corona Electrodes

In order to develop a noncontact surface resistivity measurement technique, the corona charging of a test material with simultaneous measurement of an induction current caused by a traveling surface charge was investigated using phi-type electrodes. The phi-type electrode consists of a high-voltage needle electrode that penetrates a circular hole of a grounded planar electrode. The phi-type electrode is positioned above an electrically isolated test material. The purpose of the electrode design is to supply static charge to the test surface, to produce a ground potential close to the charged test material, and then to measure the induction current or the surface potential caused by the propagated surface charge. Test materials with surface resistivities from 106 to 1012 Ω/□ were prepared by coating conductive polymer layers onto polyvinyl chloride disks. Two setups were prepared for higher surface resistivity (Model H) and lower surface resistivity (Model L). For Model H, a surface voltmeter was used to measure the slow propagation of surface charge. The rise time of the surface potential increased linearly with the surface resistivity from 3 × 109 to 1 × 1012 Ω/□. The Model L had two induction probes to measure the fast propagation of surface charge. The rate of the total induction charges was a function of the surface resistivity from 3 × 106 to 3 × 109 Ω/□. Experimental results obtained from both the H and L models agreed with the predicted results. The phi-type electrode was verified as effective for noncontact surface charge measurements.

Vibration of water droplet located on a hydrophobic sheet under the tangential AC field

The behavior of a water droplet placed on the surface of the hydrophobic polymer sheet under AC field was investigated experimentally. In order to confirm the effect of hydrophobicity on the movement of the water droplet, two kinds of hydrophobic insulating materials, PTFE and silicone rubber (SR) were used. A single water droplet was placed on the insulating sheet where AC electric field was applied. The frequency of AC voltage was varied from 10 to 100 Hz. The change of the droplet shape during vibration was investigated in detail. The droplet was deformed and synchronized with the AC field. The amplitude of vibration strongly depends on the frequency and the volume of water droplet. It was revealed that water droplet vibrates strongly at a definite frequency and the resonance frequency of a droplet with a volume of 10 /spl mu/l and 30 /spl mu/l is 30 Hz and 22 Hz, respectively.

Non Contacting Measurement of Surface Resistivity Using Phi Type Electrodes

Non contacting measurement of surface resistivity has been investigated by simultaneous applications of corona charging to a test sample and measuring surface potential around the charged spot. The corona charging electrodes are consisted of a grounded plane electrode with a circular hole and a needle electrode penetrating the plane electrode at the center of the hole, defined as phi type electrodes. The test sample to be measured is placed underneath of the phi type electrode and is charged partially around the needle electrode where the corona ions emitted from. An electrostatic field meter is located on the grounded plane electrode to measure the surface potential around the needle electrode. Because the surface potential distribution around the charged area is determined by its surface resistivity, the electrostatic properties of the sample, i.e. dissipative or insulative, can be detected without any contact electrode onto the sample. The experimental setup, measured and predicted surface potentials are presented. Distribution of surface potential at the test surface partly covered with antistatic spray is also presented for a possible application of non contacting measurement of the surface resistivity.

Noncontact Measurement of Surface Resistivity Using a Phi-Type Electrode

In this paper, a noncontact surface-resistivity measurement has been developed for the evaluation of electrostatic properties in plastic materials coated with an antistatic spray. The system includes a corona-charging electrode and an electrostatic field meter. The corona-charging electrode consists of a grounded planar electrode with a circular hole and a needle electrode that penetrates the planar electrode at the center of the hole and is thus referred to as a phi-type electrode. The test sample is placed underneath the phi-type electrode and charged by corona charging. An electrostatic field meter located on the grounded planar electrode measures the surface potential outside of the charging area. Because the surface potential is determined by the surface resistivity of the test sample, the electrostatic properties, i.e., dissipative or insulative, can be detected from the surface potential without contacting the test sample. The system for charging the test samples and measuring the surface potential is presented. The measured surface potentials are in approximate agreement with the calculated potentials. Measurement of the surface-potential distribution can also identify where a plastic sample is coated with an antistatic spray.