3.5. Induced-charge electrophoresis (ICEP)
The ICEP concept was first introduced by Bazant and Squires (Bazant & Squires, 2004). It was suggested that asymmetries on polarizable objects such as polarizable Janus particles or tear shape objects immersed in an electrolyte result in non-uniform charge distribution on the surface. Therefore, ICEO on one part of the surface is dominant and drives the particle in the electrolyte under either AC or DC electric fields (Bazant & Squires, 2004). Squires and Bazant (Todd M Squires & Bazant, 2006) further studied the effective asymmetries on the conducting particle motion, including inhomogeneous surface properties on the particle, nearly and highly asymmetry objects, and symmetric body in an electric-field gradient.
The inhomogeneity in surface properties of particles was considered in some studies. Gangwal et al. (Gangwal, Cayre, Bazant, & Velev, 2008) observed the motion of a surface-coated Janus particle perpendicular to the axis of the applied AC electric field. Kilic and Bazant (Kilic & Bazant, 2011) then numerically validated such motions. Boymelgreen and Miloh (Boymelgreen & Miloh, 2012) derived a theoretical presentation for the motion of inhomogeneous Janus particles. Daghighi et al. (Daghighi, Sinn, Kopelman, & Li, 2013) experimentally characterized the motion of Janus particles under DC electric field and observed that particles, unlike under the AC field, are aligned with the axis of the DC electric field.
The particle motion under an electric field was further studied for different types of particle geometries, including non-spherical particles (Yariv, 2005), colloidal rods (Saintillan, Darve, & Shaqfeh, 2006), striped micro rods (Rose, Meier, Dougherty, & Santiago, 2007), cylindrical particles (H. Zhao & Bau, 2007), and slender particles (Yariv, 2008). Characterizing the translational and rotational motions of these particles has shown that the rods or slender objects tend to align properly with the electric field (Saintillan, Darve, et al., 2006; Yariv, 2008).
The ICEP phenomenon was also studied within microfluidics, where the attraction and repulsion of particles from the channel walls were characterized in different conditions (Saintillan, 2008; Z. Wu & Li, 2009). Wu et al. (Z. Wu, Gao, & Li, 2009) numerically simulated the motion of an ideally polarizable particle in a microchannel and demonstrated that the velocity of these particles was equal to the electrophoretic velocity of a non-conducting particle. However, the polarizable particles were repelled from the microchannel wall by the generated MVs around them, transporting the particles toward the centerline. The ICEP motion of conducting particles has been used for developing microsensors (Gangwal et al., 2008) microvalves and microactuators, (Daghighi & Li, 2011; Sugioka, 2010), micropumps (Nobari et al., 2016), micromixers (Daghighi & Li, 2013), and stabilization of sedimenting rods (Saintillan, Shaqfeh, & Darve, 2006). In most of these studies, spherical conducting particles were employed within microfluidics, though further investigation is needed to characterize how other particle shapes contribute to enhancing the performance of ICEK-based microfluidic systems.