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.