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.