Optimization of physical properties of liquid and substrate to reduce response time of an electrowetting liquid lens

Abstract

Electrowetting is a phenomenon that controls the surface tension of droplets by electricity and changes the contact angle of the droplets on the surface of the substrate. There are many applications of electrowetting, such as tunable liquid lenses, electronic paper, and 3D displays. Among them, electrowetting multi-view lenticular 3D displays have many advantages, such as a 2D/3D switching function, an increased viewing zone, and super multi-view capability using time multiplexing. Of course, these functions are only possible if the operating speed of electrowetting is fast enough. To realize a super multi-view display, the chamber size of the lenticular lens must be increased, but the response speed is slowed down. In addition to the lenticular structure, fast response time is essential for many large-scale applications that require real-time processing. The response time of electrowetting applications is very important, but the relationship between response time and the physical parameters for electrowetting operations has not been deeply investigated. Therefore, the effects of physical properties, such as viscosity, interfacial tension, and substrate roughness, on the response speed in AC electrowetting were investigated and the optimal conditions for fast electrowetting were found. When the contact angle of the droplets on the surface of the solid substrate changes, the three-phase contact line moves. Contact line velocity is related to driving force and contact line friction. The driving force of electrowetting is proportional to the interfacial surface tension of two immiscible liquids, and the contact line friction is related to the viscosity of droplets and the substrate roughness. Liquids that have very low viscosity have high contact line velocity but under-damped response. Hence, some liquids with a certain viscosity may exhibit the fastest response time. On the other hand, the substrate roughness in areas that the conducting liquid touches typically slows down the wetting speed of droplets. However, the contact line friction can be reduced by roughness. Liquid droplets exhibit the Cassie-Baxter wetting state on substrates that have roughness with a high aspect ratio. In the Cassie-Baxter wetting state, a shear-free interface enhances the mobility of droplets. An experiment was conducted on a 0.4 mm-thick aluminum plate with a 1 um-thin Parylene C film deposited with a 50 nm Teflon coating. The experimental results showed that the shortest response time was obtained for $5 mPa \cdot s$ conducting liquid (water-glycerol mixture) with 0 wt % sodium dodecyl sulfate (SDS) on a default aluminum plate (RMS roughness 270 nm). Through this experiment, it was possible to control the spreading response pattern of electrowetting from an under-damped response to an over-damped response by adjusting the conditions of the viscosity of the conducting liquid, the surface tension between two immiscible liquids, and the substrate roughness. Also, a critical damping response was implemented using a hardware method by applying the optimum condition without voltage shape variation technology. Finally, two types of electrowetting liquid lenses (circular and lenticular) were fabricated by applying optimal conditions and their performances were evaluated. It was confirmed that the response times of the circular and lenticular electrowetting lenses were reduced by 50 % and 70 % reduced respectively.