Electrical actuation of electrically conducting and insulating droplets using ac and dc voltages

Abstract

Electrical actuation of liquid droplets at the microscale offers promising applications in the fields of microfluidics and lab-on-chip devices. Much prior research has targeted the electrical actuation of electrically conducting liquid droplets using DC voltages (classical electrowetting). Electrical actuation of conducting droplets using AC voltages and the actuation of insulating droplets (using DC or AC voltages) has remained relatively unexplored. This paper utilizes an energy-minimization-based analytical framework to study the electrical actuation of a liquid droplet (electrically conducting or insulating) under AC actuation. It is shown that the electromechanical regimes of classical electrowetting, electrowetting under AC actuation, and insulating droplet actuation can be extracted from the generic electromechanical actuation framework, depending on the electrical properties of the droplet, the underlying dielectric layer and the frequency of the actuation voltage. The paper also presents experiments which quantify the influence of the AC frequency and the electrical properties of the droplet on its velocity under electrical actuation. Velocities of droplets moving between two parallel plates under AC actuation are experimentally measured; these velocities are then related to the actuation force on the droplet which is predicted by the electromechanical model developed in this work. It is seen that the droplet velocities are strongly dependent on the frequency of the AC actuation voltage; the cut-off AC frequency, above which the droplet fails to actuate, is experimentally determined and related to the electrical conductivity of the liquid. This paper then analyzes and directly compares the various electromechanical regimes for actuation of droplets in microfluidic applications.

Electrical actuation-induced droplet transport on smooth and superhydrophobic surfaces

“…A typical microfluidic device for the electrical actuation of droplets is represented schematically in Figure 1 [12]. It consists of two flat plates separated by a known spacing.…”

“…Kumari et al [12] studied the three different regimes in which a liquid droplet can be electrically actuated: classical electrowetting (EW), AC electrowetting and actuation of dielectric liquids. The electrical properties of the droplet and the underlying dielectric layer along with the frequency of the AC actuation voltage determine the actuation regime.…”

“…Water droplets are made electrically conducting by adding a small amount of salt (e.g., potassium chloride). It should be noted that EW systems have been extensively modeled using Lippman's equation [4] which quantifies the reduction in the solid-liquid interfacial energy upon the application of an EW voltage as: [12].…”

“…The regime of AC electrowetting [12,[27][28][29][30][31][32][33][34] corresponds to the actuation of an electrically conducting droplet using an AC voltage. Use of AC voltages for electrical actuation is advantageous since the possibility of chemical reactions in the droplet is minimized [29].…”

“…The focus of this paper is on the use of electromechanical models to study droplet actuation; in particular, the scaling analysis developed by Kumari et al [12] is utilized to delineate three different regimes of electromechanical actuation. The fluid mechanics and dissipative mechanisms influencing droplet motion are also summarized; it is seen that significant research is needed to understand the complex fluid mechanics and nonconservative dissipative mechanisms in such systems.…”