Electrohydrodynamic deformation of water surface was investigated in a pin-to-plate gas discharge system that consisted of a pin electrode made of metal and an ion-conductive water electrode. In the condition of a lower applied voltage than the corona threshold, because an extremely small electrostatic attractive force, a Coulomb force in the order of 10 μN, was induced, water lifted up in the order of several tens of micrometres at the centre. Over the threshold voltage corona discharge took place and a relatively large repulsive force, in the order of 100 μN, was induced due to the ionic wind. It depressed water in the order of several hundred micrometres at the centre. Deformation of the water level coincided with the pressure distribution on the metal plate electrode, if the surface tension of water and the Coulomb force was included in the estimation to derive the pressure distribution from the measured deformation of the water level. If the applied voltage was lower than the corona threshold, total force to the water electrode coincided with that to the pin electrode. However, it was larger than that to the pin electrode at the corona discharge because the reaction force due to the ionic wind was applied not only to the pin electrode but also to other parts of the electrode at the corona discharge.
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