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.