Investigation on the thickness effect of a hydrophobic layer for operating voltage reduction in EWOD systems

This paper studies the optimum thickness of a hydrophobic layer for operating voltage reduction in electrowetting-on-dielectric (EWOD) systems by investigating the thickness effect on the surface wettability and hysteresis using an atomic force microscopy (AFM). To investigate the surface wettability, the thicknesses of Cytop and Teflon layers coated with different weight percent of Cytop and Teflon solutions are precisely measured by AFM. And the contact angles of D.I. water droplets sitting on the Cytop and Teflon layers with different thicknesses are measured by a microscope from the side. The results show that for Cytop films the contact angles of D.I. water droplets on the Cytop films thicker than 3 nm maintain around 110° on the other hand, the contact angles with the Cytop film thinner than 3 nm suddenly drop and decrease as the film becomes thinner. Teflon films show the similar trend as Cytop films, except the critical thickness (7 nm). The contact angle hysteresis on the different thicknesses of Cytop and Teflon films is separately investigated by the tilting base method. The results show that for the thin hydrophobic films, the contact angle hysteresis is over 10° however, it decreases as the film thickness increases. When the film is thicker than 12 nm, the values of the contact angle hysteresis are approximately halved and saturated. Based on the test results, the optimum hydrophobic film thickness (12 nm) is suggested. Finally, the thickness effect of the hydrophobic film on the operating voltage in EWOD actuation is tested. As expected the operating voltage in thin hydrophobic films is lower than in thick films. For the Cytop film, the saturation voltage in 12 nm is 20 V lower than 500 nm and 40 V lower than 1500 nm, respectively. Similarly, for the Teflon film, the saturation voltage in 12 nm is 20 V lower than 600 nm and 40 V lower than 1600 nm, respectively.