## 3 Results and Discussion
### Developing Microfluidic Pump and Mixer Driven by Bipolar Janus AC-FFET  
The two adjustable flow components u\({}_{x}\) and u\({}_{y}\) in the Janus AC-FFET are heavily expected to facilitate on-demand sample processing in multiple flow directions. The simultaneous active transport and stirring of analytes serves as the most typical application scenario for multiaxial fluid manipulation, which involves separate flow components both horizontally and laterally. Inspired by these observations, we attempted to develop an integrated microdevice with dual functionality in simultaneous electro-convective transport and the mixing of nanoscale analytes suspended freely in a buffer medium via the AC field-effect control of ICEO around metal-dielectric Janus micropillars, i.e., a microfluidic pump and mixer driven by the Janus AC-FFET. In the more advanced and highly integrated device design of the Janus-ACFFET employing two oppositely polarized gate terminals, instead of the single Janus pillar used in a previous study on controllable ICEO-flow behavior, an array of discrete Janus posts marked with i = 1, 2, 3,... n, with the metal phase of the adjacent pillar subjected to equal but opposite gate-voltage offset, namely, the bipolar Janus AC-FFET, is disposed consecutively along the centerline of the channel's bottom surface (Figure 1A). Here, n is specified as an even number. In this way, the odd-numbered Janus pillar has a gate voltage of V\({}_{\text{G-odd}}\) = \(\triangle\)V\({}_{\text{G}}\) + 0.5V\({}_{\text{D}}\) and a bias ratio of \(\beta_{\text{odd}}\) = \(\triangle\)V\({}_{\text{G}}\)/0.5V\({}_{\text{D}}\), while the even-numbered post has a gate voltage of V\({}_{\text{G-even}}\) = \(-\triangle\)V\({}_{\text{G}}\) + 0.5V\({}_{\text{D}}\) and a bias ratio of \(\beta_{\text{even}}\) = \(-\triangle\)V\({}_{\text{G}}\)/0.5V\({}_{\text{D}}\) = \(-\beta_{\text{odd}}\) when observing the sequence of central pillars forward toward the channel outlet. Figure 7: A simulation analysis of the driving-voltage effect on the resulting pump u\({}_{x}\) and mixing-flow behavior u\({}_{y}\), orthogonal to each other, originated by Janus AC-FFET, in which the metal phase of a Janus micropillar of given radius R = 5 \(\mu\)m is subjected to a constant gate-voltage-offset ratio \(\beta\) = 0.8 with V\({}_{\text{G}}\) = 0.9V\({}_{\text{D}}\) for a given field frequency of f = 200 Hz. (**A**) Numerical characterization of the ICEO pumping velocity, mixing velocity, and mixing/pumping-velocity-component ratio as a function of the AC-driven-voltage amplitude. (**B**) A quantitative comparison of the magnitude of induced zeta potential and tangential ICEO slipping between the dielectric and metal surfaces of the Janus micropillar as a function of the driving voltage. (**C**) Position-dependent IZP on the polarizable post surface as a function of the polar angle under varying AC-driven voltages. (**D**) Position-dependent azimuthal ICEO slip velocity on the polarizable post surface as a function of the polar angles for different AC-driven voltages. Both the pump-fluid motion \(u_{x}\) and the mixing-flow component \(u_{y}\) arise from ICEO slipping on the surface of the asymmetrically polarized Janus pillar. Therefore, different experimental parameters can influence the two electrokinetic flow components to different degrees. Since \(u_{x}\) is in charge of conveying the particle samples horizontally toward the channel exit, and \(u_{y}\) permits fluid mixing in the transversal direction, both ICEO flow components intersect orthogonally. On the one hand, a lower speed of electrokinetic pumping flow \(u_{x}\) leaves more time \(t=L_{c}/u_{x}\) for the incoming analyte to pass through the straight channel, so a larger value of \(u_{x}\) can produce a high throughput at the channel outlet, at the cost of lowering the mixing efficiency.