Title: Hybrid electrodes effective for both electrowetting- and dielectrowetting-driven digital microfluidics
Authors: Hongyao Geng, Sung Kwon Cho
(Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania, USA)
Electrowetting on dielectric (EWOD) and dielectrowetting (DEW) are two major principles to drive droplets in digital microfluidics. EWOD is effective to manipulate (create, transport, split, and merge) conductive droplets being currently used for many biological, chemical, and optical applications. DEW can also manipulate droplets but more efficiently with dielectric (nonconductive) fluids. A digital microfluidic platform efficiently operable by both EWOD and DEW would offer higher versatility in handling a wide range of fluids, regardless of their conductivities. In this regard, this article presents a new hybrid electrode design enabling EWOD and DEW to drive various kinds of droplet fluids on a single platform. In addition, a slippery liquid-infused surface (SLIPS) is integrated with the hybrid electrodes. The SLIPS is well known to resist biofouling and repel sticky fluids, which endows the hybrid electrodes with much wider application spectra. As a result, the present SLIPS-integrated hybrid electrodes facilitate actuating various kinds of fluids which would not be driven by conventional EWOD and/or DEW electrodes. This paper presents the successful transportation of not only conductive fluids including water, protein solution, glycerol, and honey but also nonconductive fluids including dodecane, silicone oil, and light and heavy crude oil, all driven by the SLIPS-integrated hybrid electrodes. The performance comparisons among solid, interdigitating, and hybrid electrodes are made by testing both conductive and nonconductive droplets.
Fig.1 Configurations of (a) EWOD and (b) dielectrowetting (50 µm electrode width and 50 µm spacing). From bottom to top, each configuration is made of a glass substrate, electrode, dielectric layer, and SLIPS. EWOD and dielectrowetting effects on (c) DI water. As the voltage increases to 275 V AC, EWOD changes the contact angle by 50.5°, while the contact angle change is only 19.2° with dielectrowetting at 450 V AC. (d) Silicone oil (1000 cSt, dielectric constant:∼2.6). EWOD has no effect on nonconductive fluid while dielectrowetting can change the contact angle by 9.5°. All the voltages denote the input voltage to the electrodes (Ve). AC, alternating current; DI, deionized; EWOD, electrowetting on dielectric; SLIPS, slippery liquid-infused surface.
Fig.2 Spreading of ionic liquid (200 V) and 10 cSt silicone oil (450 V) at the same position. (a, b) showing when the middle area of the conductive droplet is at the EWOD zone, the droplet is elongated vertically to the electrode direction. (c, d) showing when dielectric fluid exists, it is stretched along the electrode direction. Scale bar = 1.5 mm. EWOD, electrowetting on dielectric.
Fig.3 Comparisons of transporting speeds. (a) 1000 cSt silicone oil (450 V). The speed is fastest on interdigital electrodes, while it closes to 0 on solid electrodes. (b) Images of silicone oil on different electrodes. (c) DI water (300 V). The speed is fastest on solid electrodes, and slowest on interdigital ones. (d) Images of DI water motions on solid and interdigital electrodes. (e) Transporting speed of silicone oil (10 cSt) at different voltages actuated by three types of electrodes. (f) Transporting speed of DI water. Scale bar = 1.5 mm. DI, deionized.
Droplet（《液滴》）是由吉林大学主办，与国际著名出版公司Wiley合作出版的英文国际性学术期刊，是国际上第一本全面报道液滴／气泡交叉领域科研成果的学术期刊。目前为季刊，主要发表液滴／气泡相关领域的原创性研究论文、综述及评论性文章，重点报道与液滴／气泡相关的结构、材料和系统设计、制备和调控等方面的基础研究及工程应用。现任主编为中国科学院院士任露泉教授、美国加利福尼亚大学洛杉矶分校C.J.Kim教授。执行主编由香港理工大学王钻开教授担任。
目前，Droplet（《液滴》）已通过全球最具影响力的开放存取期刊目录（Directory of Open Access Journals, DOAJ）评估，正式被DOAJ数据库收录。本刊旨在成为跨学科的高水平学术交流平台，展示液滴和气泡相关领域的前沿研究成果，推进国际科研传播与合作。
编辑部总编：张成春教授，副总编：王丹编审。
