3.1.1. ACEO micropumps
ACEO phenomena employed non-linear electrokinetics by using a locally
asymmetric polarizable electrode to induce fluid flow in microchannels
(Ajdari, 2000). Brown et al. (Brown, Smith, & Rennie, 2000) utilized a
pair of planar electrodes with different edge sizes to induce fluid flow
in channels. This work was further optimized by Studer et al. (V Studer,
Pepin, Chen, & Ajdari, 2002) to improve the flow control within
microchannels (Fig. 2A ). This design configuration showed that
breaking the electrode symmetry created an asymmetry charge distribution
along the channel’s axis, which led to a frequency-dependent pumping.
The asymmetry pair of a planar electrode array was also used for
micro-pumping thanks to its ease of fabrication, low-voltage operation,
and capability in local flow control (Jie Wu, 2008). Theoretical
modeling of the frequency-dependent pumping was further developed
(Ramos, Gonzalez, Castellanos, Green, & Morgan, 2003) and used to
simulate different aspects of the ACEO pumping (Debesset, Hayden,
Dalton, Eijkel, & Manz, 2004; Vincent Studer, Pépin, Chen, & Ajdari,
2004). Furthermore, the effect of several parameters and pumping
configurations were investigated, such as studying the effect of channel
height, electrochemical reactions, and non-linear surface capacitance of
the Debye layer (Olesen, Bruus, & Ajdari, 2006); controlling the
pumping direction by switching the voltage using an inclined electrode
array (Hilber, Weiss, Saeed, Holly, & Jakoby, 2009; Loucaides, Ramos,
& Georghiou, 2007), pumping of two different electrolytes
simultaneously through microchannels (Morgan, Green, Ramos, &
García-Sánchez, 2007), bubble-free pumping (Kuo & Liu, 2008; Tawfik &
Diez, 2017) as well as ACEO pumping using biased AC/DC signals (Islam &
Reyna, 2012; Lian & Wu, 2009; Piñón et al., 2017; Jie Wu, 2008; Yang
Ng, Ramos, Cheong Lam, & Rodriguez, 2012), pulse voltage waveforms
(Tawfik & Diez, 2017), square pole-slit electrode arrays (Yoshida,
Sato, Eom, Kim, & Yokota, 2017), and arrays of asymmetric ring
electrode pairs in the cylindrical microchannels (Gao & Li, 2018).
Also, the comparison between fluid velocity on arrays of identical
electrodes with AC voltage and a traveling-wave potential demonstrated
that traveling-wave potential resulted in a higher fluid velocity (Ramos
et al., 2005; Yang, Jiang, Ramos, & García-Sánchez, 2009).
Besides using a planar array of electrodes, Urbanski et al. (Urbanski,
Thorsen, Levitan, & Bazant, 2006) introduced the first nonplanar ACEO
pump by using three-dimensional (3D) stepped electrodes. They
demonstrated the capability of higher flow rate pumping compared to
planar ACEO pumps (Fig. 2B ). Senousy and Harnett (Senousy &
Harnett, 2010) introduced a less expensive and an easy fabrication
method for constructing 3D stepped electrodes. Rouabah et al. (Rouabah
et al., 2011) further studied the performance of a new set of 3D
high-aspect-ratio electrodes for ACEO pumping while Huang et al. (C.-C.
Huang, Bazant, & Thorsen, 2010) employed these 3D electrode micropumps
for liquid manipulation in portable biomedical microfluidic devices.