A Contact Angle Measurement Method for the Droplets in EWOD-based Chips

Electrowetting-on-dielectric (EWOD) is a droplets handling method in microfluidic chips. Compared to other driving methods, EWOD is considered as a most promising way for droplet-based lab on a chip (LOC). In the basic research of the electrowetting principle and droplets driving experiments, accurate and reliable measurements of the droplet solid-liquid contact angles are significant. This paper presents an automated contact angle measurement method mainly including image segmentation and curve fitting steps. In the image segmentation step, marker-controlled watershed transform, a morphological image segmentation technology, is used to extract the droplet edge to acquire a single-pixel width, continuous profile which can be directly applied to the later step. The circle functions, which are similar with the real droplet shape, are selected to fit the droplet profile data. This method is successfully used for the droplets in the EWOD chips with the advantages of high accuracy, automated and little dependent on the image quality. Moreover, this method can also be used to measure sessile or pendent droplets contact angles in other microfluidic devices.