Real time contact angle measurement by quartz crystal microbalance for EWOD studies

We present real-time measurements of sessile contact angle variations by EWOD (electrowetting-on-dielectric) using QCM (quartz crystal microbalance). Different from the conventional contact angle goniometry, the QCM measurement abandons optical components involving in the system and hence has potential to prevent possible optical difficulties, such as blurred images caused by insufficient depth of focus of the video camera lenses and/or refractive index match of the droplet and the surrounding liquid. Here EWOD is conducted on a modified QCM by depositing proper dielectric and hydrophobic layers. The contact angle variation by EWOD is analyzed through both optical goniometry and QCM. The maximum contact angle difference between the methods was 6°. The volume limitation of the droplet and the resolution of the contact angle variation is studied and discussed.