Parallel-plate electric emulsification for repulsion-stabilized microdroplet arrays generation toward digital bioassays

Abstract

Digital bioassays enable absolute quantification of target analytes by partitioning samples into thousands of microcompartments and converting analog signals into discrete, countable events. This strategy improves sensitivity, precision, and resistance to amplification bias compared with conventional bulk assays. Among these technologies, channel-based droplet microfluidic systems have demonstrated exceptional scalability and statistical robustness. However, conventional droplet generation methods often require complex microfluidic architectures, external pumping systems, or surfactant-dependent stabilization, which limit device simplicity, portability, and integration flexibility. These limitations motivate the exploration of wall-less droplet manipulation platforms, such as digital microfluidics (DMF), for liquid compartmentation and digital bioassays. Nevertheless, existing electrowetting-on-dielectric (EWOD)-based DMF systems face inherent limitations in droplet size scaling and partitioning throughput, creating the need for alternative droplet generation strategies compatible with DMF platforms. This thesis investigates electric field-induced emulsification as a pump-free liquid partitioning strategy for digital bioassays and its application to digital polymerase chain reaction (dPCR). The study first examines electric emulsification originating from electric field-induced fingering instability at the three-phase contact line of a sessile droplet. The interplay between wetting dynamics, contact angle saturation, interfacial tension, and electric stresses is systematically characterized, revealing a transition from static deformation to fingering and ultimately droplet breakup. This mechanism is further extended to confined droplet geometries in a parallel-plate configuration, enabling controllable and continuous generation of microdroplet arrays. Distinct regimes, including wetting, fingering instability, labyrinthine instability, electric emulsification, and ordered droplet array states, are mapped as a function of electrical and material parameters. The generality of the electric emulsification strategy is validated across multiple aqueous phases and oil-surfactant systems, and a mechanistic framework is proposed to describe the observed morphological evolution under applied electric fields. Finally, the electric emulsification platform is integrated with dPCR for the detection of Shiga toxin-producing Escherichia coli (STEC) DNA. Device configuration, oil-surfactant formulation, and workflow parameters are systematically optimized to improve droplet generation and amplification reliability. The resulting platform demonstrates effective digital partitioning compatible with multiplex detection, providing a simplified and potentially scalable alternative to conventional droplet microfluidic systems and establishing a foundation for next-generation DMF-based digital bioassays.