Progress of electrowetting applications in micro-nano energy conversion and utilization systems

Abstract

Dynamic wetting is a process that one fluid replaces the other fluid (usually gas) on solid surfaces, which is very common and widely involved in our daily activities and many industrial applications, such as printing, dyeing, coating, drag reduction, and many others. Due to the complexity of the dynamic wetting, most of studies have focused on understanding the dynamic wetting behaviors of simple fluids on homogeneous flat surfaces without any external fields in early ages. The gaps between the research with ideal wetting conditions and real applications facilitate the studies of dynamic wetting with complex conditions, such as complex fluids (non-Newtonian fluids, colloidal solutions, nanofluids etc.), real substrates (hydrophobic surfaces, hydrophilic surfaces, micro-nano hybrid surfaces or others), and complex external fields (electric fields, thermal fields or magnetic fields). Among these studies, the dynamic wetting with external electric fields, also known as electrowetting, is a hot topic. Electrowetting is a modification method of the surface wettability using an applied electric field. Electrowetting has been widely used in many microfluidic systems, such as electronic microlenses, electronic-paper, electrowetting display and others. This paper provides a review on the recent developments of electrowetting applications in the emerging micro-nano energy conversion and utilization systems, ranging from the physical and theoretical research, fundamental applications to industrial applications. The fundamental physical concepts and principles involved in electrowetting phenomena are reviewed in the first part. The mainstream physical mechanisms related to electrowetting are summarized and compared to illustrate their successes and limitations. The challenges in the topic of electrowetting phenomena are summarized, which lie in four aspects: (1) the contact angle saturation; (2) the contact angle hysteresis; (3) the electric puncture & electrolysis; (4) electrowetting reversibility on super-hydrophobic surfaces. This paper also reviews and summarizes the new achievements and contributions on material, structure and complex external conditions related to electrowetting applications. The gaps between electrowetting fundamental research and industrial applications lie in several facts, including the new electrowetting materials with low triggered electric field strength, the novel structures which can easily manipulate the liquid transport using electrowetting. Among the electrowetting structures, the “sandwich” style, the open-style, the twins-style, as well as the pole-array-style are summarized to illustrate their advantages and disadvantages. The liquid manipulation, the complex inner flows and the droplet vibration with external electric fields using electrowetting provide potential applications in the energy conversion or thermal management devices. There have been some pioneering studies on micro-nano energy conversion and utilization systems using electrowetting. The latest development on the research of the applications in the micro-nano energy conversion and utilization systems is reviewed, including micro-channel systems, electronic cooling, phase change and solar photovoltaic system applications. The outlooks on the topic of electrowetting are provided in end of this review, including: (1) new materials and EWOD (electrowetting-on-dielectric) structures; (2) electrowetting of complex fluids containing functional particles; (3) electrowetting coupled with thermal field or magnetic field; (4) enhancing boiling and condensation heat transfer using electrowetting; (5) smart thermal management methods using electrowetting.