3.4. ICEK for two-phase liquid flow systems
Two-phase flow microfluidic devices have been widely used for chemical
filtration, pharmacology and drug delivery (Azizian et al., 2019;
Bazazi, Sanati-Nezhad, & Hejazi, 2018). When a leaky dielectric droplet
immersed in another immiscible leaky dielectric liquid, four symmetric
MVs are induced inside and outside the droplet (Lin, Skjetne, &
Carlson, 2012). This phenomenon was first observed by Taylor (Taylor,
1966) in his experimental work. Small deformation of the droplet was
predicted theoretically by calculating the exerted electrical stress on
droplet-medium interface (Taylor, 1966). This deformation is generated
due to the distribution of the interfacial polarization charge on the
interface (Lin et al., 2012). The induced charge on the interface of the
liquid and the generated MVs around the droplet have a resemblance to
the generated MVs around polarizable solid cylinder immersed in the
electrolyte under an external electric field (Bazant, 2015). Squires and
Bazant (Todd M Squires & Bazant, 2004) utilized this analogy to
determine radial and azimuthal fluid velocities of the ICEO flow around
a conducting cylinder. Jung et al. (Jung, Oh, & Kang, 2008) calculated
the amount of induced charge in a leaky dielectric water droplet
oscillated between two fixed electrodes. Flittner and Přiby (Flittner &
Přibyl, 2017) proposed a mathematical model for such oscillatory
behavior of water droplets. Wuzhang et al. (Wuzhang, Song, Sun, Pan, &
Li, 2015) investigated oil droplet motion in different ionic surfactant
solutions and observed that an increase in surfactant concentration led
to a higher droplet velocity due to the enhancement in surface charge
density. It was also found that two MVs are generated around the oil
droplet as a consequence of the redistribution of the mobile surface
charges on the droplet surface (Daghighi, Gao, & Li, 2011). Li et al.
(Li & Li, 2016a) studied this phenomenon more precisely and proposed an
analytical presentation for the local zeta potential distribution on the
oil droplet. Furthermore, they experimentally observed the accumulation
of passivated aluminum nanoparticles on one side of the droplet, which
confirmed the charge redistribution on the droplet (Li & Li, 2016b).
Mori and Young (Mori & Young, 2018) further improved the Taylor model
(Taylor, 1966) and simulated the droplet deformation using
electro-diffusion theory, as an essential tool for describing
electrokinetic phenomena by considering the charge diffusion model.
Besides the above studies for describing ICEK occurrence in two-phase
flows, this phenomenon has also been employed in various microfluidic
applications, such as electric separation of droplets (Guo et al.,
2010), droplet coalescence (Xiaodong Chen, Song, Li, & Hu, 2015; Y. Jia
et al., 2018), emulsion micro-pumping (Bhaumik, Roy, Chakraborty, &
DasGupta, 2014), controlling microdroplet generation (Azizian et al.,
2019; Kamali & Manshadi, 2016), regulating the micro reactions (Y. Jia
et al., 2017), droplet separation (Li & Li, 2017; K. Zhao & Li, 2018),
controlling multiphase flow systems (W. Liu et al., 2017), particle
flow-focusing (Ren, Liu, Liu, et al., 2018), droplet motion in water-air
and water-oil interfaces (C. Wang, Li, Song, Pan, & Li, 2018; C. Wang,
Song, Pan, & Li, 2018a, 2018b), and microvalves (Li & Li, 2018).