1. Introduction
Lab-on-chip (LOC) microfluidic devices have been developed for various biomedical and biochemical applications (Azarmanesh et al., 2019; Sackmann, Fulton, & Beebe, 2014; Shamsi, Mohammadi, Manshadi, & Sanati-Nezhad, 2019; Jing Wu, He, Chen, & Lin, 2016). Components, such as micromixers, micropumps, and microvalves are integrated into LOC devices to automate their function (Bhagat et al., 2010). While passive platforms operate without any external energy source, active systems operate mainly based on one of thermophoretic (Vigolo, Rusconi, Stone, & Piazza, 2010), acoustophoretic (Barani et al., 2016; Lenshof, Magnusson, & Laurell, 2012), magnetophoretic (S. Kim et al., 2013; M. K. Manshadi et al., 2018), electrokinetic (Mohammadi, Madadi, Casals-Terré, & Sellarès, 2015; C. Zhao & Yang, 2013) or centrifugal forces (Kinahan et al., 2016; Mohammadi, Kinahan, & Ducrée, 2016; Tang, Wang, Kong, & Ho, 2016). Electrokinetic techniques have become one of the most popular active methods for chemical analysis and biomedical diagnostics due to their inherent characteristics, including their high controllability, flat velocity profile in microchannels, independency form channel size, and function without need to moving parts (Cunlu Zhao & Yang, 2012).
Electrokinetics refers to fluid or particle motion in liquid electrolytes under an external electric field (Bazant & Squires, 2010). In classical electrokinetics, the main assumption is a linear response to the applied electric potential within microchannels that have constant surface charge (Todd M Squires & Bazant, 2004). Therefore, classical electrokinetic phenomena are also called linear electrokinetics. However, the possible drawbacks of linear electrokinetics are: i) weak flow rate, ii) zero time-averaged liquid flow under alternating current (AC) electric fields, and iii) the need to a high electric potential along the microchannel (with the length order of centimeter) to generate a strong electric field strength (Bazant & Squires, 2004, 2010). These drawbacks have been improved by introducing non-linear electrokinetics (Ajdari, 2000; Ramos, Morgan, Green, & Castellanos, 1999). Such non-linearity was first examined by Ramos et al. (Ramos et al., 1999) around locally asymmetric polarizable electrodes where they observed AC electroosmotic (ACEO) fluid flow over a pair of adjacent metal microelectrodes fabricated on planar glass substrates (Ramos et al., 1999). Ajdari (Ajdari, 2000) further employed locally asymmetric arrays of electrodes to demonstrate low-voltage ACEO micro-pumping. Non-linear AC electrokinetics was further used for particle manipulation (Mirzajani et al., 2016), DNA concentration (Bown & Meinhart, 2006), and micromixing (Sasaki, Kitamori, & Kim, 2010).
Bazant and Squires (Bazant & Squires, 2004) first generalized ACEO fluid flow to dielectric and conducting structures by introducing the term of induced-charge electroosmosis (ICEO) in a weak DC or AC electric field. The ICEO describes the interactions between the applied electric field and the induced ionic charge near the polarizable surface (electrode or object) (Todd M Squires & Bazant, 2004). The induced-charge electrokinetic (ICEK) theory and applications, as well as theoretical and experimental advances on ICEK, were further presented (Bazant, Kilic, Storey, & Ajdari, 2009; Bazant & Squires, 2010). Then, Bazant (Bazant, 2011), in his book chapter, introduced the basic physical concepts of ICEK. Zhao and Yang’s review article (Cunlu Zhao & Yang, 2012) then focused on the breakthrough in electrokinetics and its physical mechanisms in micro and nanofluidics. Ramos et al. (Ramos, García-Sánchez, & Morgan, 2016) studied AC electrokinetic characteristics of polarizable microparticles with a focus on the theory and experimental observations. The field of non-linear ICEK has expanded rapidly over the past few years to various microfluidic and LOC applications, such as induced-charge electrophoresis (ICEP) (Todd M Squires & Bazant, 2006), ICEO micropump (Paustian, Pascall, Wilson, & Squires, 2014), ICEO microvalve (C. Wang, Song, Pan, & Li, 2016), ICEK micromixer (M. K. D. Manshadi, Khojasteh, Mansoorifar, & Kamali, 2016; M. K. D. Manshadi, Nikookar, Saadat, & Kamali, 2019), and recently in two-phase flow research studies (Bazant, 2015; W. Liu et al., 2017; Ren, Liu, Liu, et al., 2018).
This work reviews theoretical, numerical and experimental studies on the physics and applications of ICEK within microfluidics. The characteristics and performance of ICEK-based microfluidic components in active micromixers, micropumps, and microvalves are then reviewed, Furthermore, the opportunities and challenges of ICEK-based microfluidic devices are highlighted.