Electrical actuation-induced droplet transport on smooth and superhydrophobic surfaces

Abstract

E El le ec ct tr ri ic ca al l a ac ct tu ua at ti io on n--i in nd du uc ce ed d d dr ro op pl le et t t tr ra an ns sp po or rt t o on n s sm mo oo ot th h a an nd AbstractElectrical control of liquid droplet motion and wettability has wide-ranging applications in the field of MEMS, lab-on-a-chip devices and surface engineering, in view of the resulting enhanced flow control opportunities, low power consumption and the absence of mechanical moving parts. This article summarizes recent progress towards understanding of the fundamentals underlying electrical actuation of droplets on smooth and superhydrophobic surfaces. Electrical actuation of liquid droplets with widely differing electrical properties on smooth surfaces is first discussed. Electromechanical considerations are employed to study the actuation force on a generic liquid droplet across the entire spectrum of electrical actuation regimes. The challenges in understanding the fluid flow and dissipation mechanisms associated with a discrete moving droplet are discussed. The role of electrical voltages, interfacial energies and surface morphology in determining droplet states (nonwetting Cassie state and wetting Wenzel state) and triggering state transitions on superhydrophobic surfaces is then mapped out. Critical phenomena associated with droplet transitions on superhydrophobic surfaces (energy barrier for the Cassie-Wenzel transition, lack of spontaneous reversibility of the Cassie-Wenzel transition, robustness of the Cassie state, and the role of the roughness elements) are analyzed. The article also highlights key avenues for future research in the fields of electrical actuation-based microfluidics and superhydrophobic surfaces. INTRODUCTIONElectrical actuation of liquid droplet motion on smooth and superhydrophobic surfaces has been studied as a tool for controlling microfluidic operations which determine the performance of lab-on-a-chip devices, optical systems, fluidic displays and other MEMS-based fluidic devices. Key advantages of electrical actuation of liquids (in the form of discrete droplets) over mechanical actuation include the absence of moving mechanical parts, opportunities for enhanced and reconfigurable flow control, compatibility with liquids of widely different electrical properties, compatibility with existing fabrication techniques, and ease of integration with a chip-based microfluidic platform. Electrical actuation techniques are also very energy-efficient, typically requiring only microwatts of power per microfluidic operation. Owing to these advantages, electrical actuation-based control has attracted significantly more research interest than competing non-mechanical liquid control technologies such as thermocapillarity [1] (reduction of surface tension by temperature, leading to fluid motion), optoelectrowetting [2] (light-controlled surface tension induced pumping) and vapor bubble-based pumping [3]. Electrical actuation of liquid droplets on smooth surfaces and its potential applications have been reviewed by Mugele and Ba...

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