Open-Chip Droplet Splitting in Electrowetting

Electrowetting-on-dielectric (EWOD) has emerged as a powerful technique to perform on-chip droplet operations like transportation, dispensing, splitting, and mixing in sandwiched droplets. In contrast, open-chip droplet manipulation using electrowetting enables micro-total-analysis systems with multiple sensor integration and re-routing capabilities. Droplet splitting has been the bottleneck in developing open-chip platforms. Droplet splitting on an open-chip platform using electrowetting-on-dielectric is presented. An energy-based simulation model has been developed. It shows that splitting a sessile water droplet is impossible on an open-chip configuration because of the low pad contact angle requirement. Low contact angles cannot be achieved due to contact angle saturation in electrowetting. It is experimentally shown that splitting is possible if the droplet is engulfed in an oil shell (i.e., in compound droplets). The planar electrode configurations and regime of electrowetting numbers for which splitting can be achieved are identified. It is observed that larger gaps and higher electrowetting numbers favor symmetrical splitting because the electrostatic force driving the actuation is significantly higher than the retarding interfacial forces. Conversely, asymmetrical splitting has been obtained when the actuation force is barely sufficient. Further the splitting of surfactant-loaded single-phase sessile droplets is demonstrated and a preferential surface charging phenomenon is explained.

Automated summary

Electrowetting-on-dielectric (EWOD) has opened up new opportunities for droplet splitting on open-chip platforms, enabling the development of micro-total-analysis systems. Research shows that symmetrical splitting can be achieved with larger gaps and higher electrowetting numbers, while asymmetrical splitting occurs when the actuation force is barely sufficient.