Abstract
Electrowetting-on-dielectric (EWOD) has been widely exploited as an actuating force to manipulate liquids by surface tension and modulation of the contact angle on a microscale. To evaluate EWOD, an optical measurement of the droplet contact angle is conventional, but is constrained by the optical properties of the liquid, especially when two liquid phases (e.g., water in oil) are involved. We developed a non-optical method to study EWOD using a quartz-crystal microbalance (QCM). A QCM provides a promising technique for mass sensing, and has been developed for the study of liquid viscosity, density and contact angle. In this research, a QCM was employed to generate EWOD and concurrently to measure the variation of the contact angle. The contact angle of droplets of water in air and in oil was evaluated. The voltage-dependent oil film between a water droplet and the QCM surface was sensed. A modified QCM model considering a voltage-dependent oil film was derived for the analysis of the contact angle.
Highlights
Electrowetting-on-dielectric (EWOD) is a phenomenon to adjust reproducibly and reversibly the liquid-solid contact angle with appropriate electrical signals applied at the liquid–solid interface [1,2].For a reliable variation of the contact angle yet a simple configuration of a device, EWOD has been widely exploited as an actuating force to manipulate liquids for optoelectrical and biomedical applications [3,4,5,6,7,8]
To complement the inability of an optical goniometer under these conditions, we investigated a quartz-crystal microbalance (QCM) as an alternative method to determine the variation of the contact angle of a sessile droplet with EWOD
Our work provides the first report to demonstrate EWOD on a QCM chip for the has been shown previously, but the QCM worked primarily as a separate sensor to detect the dynamicformation measurement of contact angle; with wethe arefirst able to monitor the potential-dependent of a protein multilayer
Summary
Electrowetting-on-dielectric (EWOD) is a phenomenon to adjust reproducibly and reversibly the liquid-solid contact angle with appropriate electrical signals applied at the liquid–solid interface [1,2]. For a reliable variation of the contact angle yet a simple configuration of a device, EWOD has been widely exploited as an actuating force to manipulate liquids for optoelectrical and biomedical applications [3,4,5,6,7,8]. To characterize the actuating force of EWOD in various situations, an analysis of the voltage-dependent contact angle is essential. A more generic method to measure the variation of the contact angle with EWOD of a sessile droplet in an optically incompatible medium is important
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