Open‐Chip Droplet Splitting in Electrowetting

Abstract

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.

Advances in Microscale Droplet Generation and Manipulation

“…Single-sided splitting of the droplet. Reprinted with permission from ref . Copyright 2022 Wiley-VCH GmbH.…”

“…In a study by Cho et al 49 for closed-chip splitting, a smaller gap between the bottom substrate and cover plate has been suggested to assist the splitting of droplets. In the recent work by Sagar et al, 119 an energy-based model has been developed to explain the challenge in open-chip droplet splitting. To demonstrate this, a pair of electrodes are fabricated with varying interelectrode gaps (Figure 11A).…”

Influence of the Ground Electrode on the Dynamics of Electrowetting

“…Electrowetting is used in several applications, such as micro-drop generation, mixing and splitting [ 1 , 2 ], high-speed droplet actuation [ 3 , 4 ], chip cooling [ 5 ], drug release and clinical diagnosis [ 6 , 7 ], e-paper and electronic display [ 8 , 9 ], energy harvesting [ 10 ], solar indoor lighting [ 11 ], optics and beam steering [ 12 , 13 ]. In most electrowetting studies, the primary focus has been to observe the drop deformation and contact-angle change when the applied voltage is varied.…”

Geometry-Gradient Magnetocontrollable Lubricant-Infused Microwall Array for Passive/Active Hybrid Bidirectional Droplet Transport

“…However, as a result of the high adhesive resistance and unidirectional transportability, the applications for passive transportation are still restricted by the uncontrollability, short distance, and low velocity. To overcome these shortcomings, a series of stimulus-responsive methods, such as electrowetting, , triboelectric wetting, pneumatic approach, mechanical vibration, light-responsive surfaces, , thermal-responsive surfaces, , and magnetic-responsive structures, , have been constructed to realize active droplet manipulation. For example, Xu et al investigated a previously underexplored phenomenon known as tribotoelectric wetting (TEW).…”