Optimum thickness of hydrophobic layer for operating voltage reduction in EWOD systems

This paper presents the optimum thickness of a hydrophobic layer for operating voltage reduction in electrowetting-on-dielectric (EWOD) systems by investigating the effects of thickness on the surface wettability and hysteresis using atomic force microscopy (AFM). To investigate the surface wettability, the thicknesses of Cytop and Teflon layers coated with different weight percentages of Cytop and Teflon solutions are precisely measured by AFM. The contact angles of deionized water droplets on the Cytop and Teflon layers of different thicknesses are measured using an optical microscope from the side. The results of the contact angle measurements of deionized water droplets on Cytop layers indicate that layers thicker than 3 nm maintain an angle of approximately 110°; the contact angles of the drops on Cytop layers thinner than 3 nm abruptly drop and decrease as the layer becomes thinner. The Teflon layers show a similar trend to the Cytop layers, except that the critical thickness is larger (7 nm). The contact angle hysteresis for different thicknesses of Cytop and Teflon layers is investigated by the tilting plate method. The results reveal that for thin hydrophobic layers, the contact angle hysteresis is over 10° but decreases as the layer thickness increases. When the layers are thicker than 12 nm, the contact angle hysteresis is reduced by approximately 4° for Cytop and 7° for Teflon and becomes saturated. To investigate the effect of aging on the surface wettability and hysteresis, the film stability of Cytop and Teflon layers is separately tested. The initial contact angles are reduced about 1.2% for Cytop and 1.6% for Teflon within 1 day and then maintained up to 10 days, while the initial contact angle hysteresis is increased about 117% for Cytop and 39% for Teflon within a day and then maintained up to 10 days, regardless of the thickness of the Cytop and Teflon layers. Based on the results of the surface wettability and hysteresis, an optimum hydrophobic layer thickness of 12 nm is established. Finally, the thickness effects of the hydrophobic layer on the operating voltage in EWOD actuation are tested. As expected, the operating voltage for thin hydrophobic layers is lower than that for thick layers. For the Cytop layer, the saturation voltages for 12 nm, 500 nm, and 1500 nm are 80 V, 100 V, and 120 V, respectively. Similarly, for the Teflon layer, the saturation voltages for 12 nm, 600 nm, and 1600 nm are 90 V, 110 V, and 130 V, respectively. The relative differences in the saturation voltages for both materials with respect to thickness are approximately the same.