Adjustment and Measurement of Contact Angle with Electrowetting on a Quartz-Crystal Microbalance

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.

Assessment and Interpretation of Surface Wettability Based on Sessile Droplet Contact Angle Measurement: Challenges and Opportunities

“…Previous measurements revealed that the parameter σ value ranges from 2 to 2.15 . The droplet CA (θ) can also be determined from the variation in resonant frequency when the liquid droplet volume ( V L ) and radius ( r d ) are known …”