###### Abstract
Figure 1: A schematic diagram of a microfluidic device using AC field-effect control on ICEO flow profiles consisting of both horizontal pump motion \(\mathrm{u_{x}}\) and transversal rotating motion \(\mathrm{u_{y}}\) around metal–dielectric Janus micropillars, which can actively deal with stratified liquid contents along the two orthogonal flow directions via pure nonlinear electro-osmosis. (**A**) The geometric configuration of the advanced 3D device design of an active microfluidic pump and mixer with dual functionality employing bipolar Janus AC-FFET, in which an array of discrete Janus pillars is arranged along the bottom surface of a straight microchannel embedding a pair of conducting DE plates on both sidewalls. The odd- and even-numbered Janus units in the array are each subjected to two oppositely polarized gate terminals, resulting in zigzag electrokinetic streamlines, which can effectively pump,  
rotate, and stretch the two-phase contact interface at the same time, even without an external moving part. (**B**) A 2D illustration of the geometric size of the discrete pillar arrangement and device channel, as well as the definition of Cartesian and polar coordinate systems. (**C-E**) A 2D schematic representation of the physical mechanism of AC field-effect-adjustable electrochemical polarization and ICEO streaming adjacent to a single metal-dielectric Janus microcylinder immersed in electrolyte solution, under the influence of a background electric field applied across the channel width. (**C**) When the target post is free from external wiring, i.e., \(\text{V}_{\text{G}}=0.5\text{V}_{\text{D}}\), two symmetric ICEO eddies appear selectively on its downstream conducting surface, leading to a forward ICEO pump motion along the horizontal direction, mainly due to the inherent uneven electrical polarizability of the Janus micropillar itself, with no net transversal rotating motion. (**D**,**E**) Once its metal phase is exposed to a positively (negatively) biased gate terminal, i.e., \(\text{V}_{\text{G}}>0.5\text{V}_{\text{D}}\) (\(\text{V}_{\text{G}}<0.5\text{V}_{\text{D}}\)), more anions (cations) than cations (anions) accumulate at the ideally polarizable metal surface, so the clockwise (anticlockwise) rotating vortex on the bottom (top) side becomes larger and dominates the anticlockwise (clockwise) counterpart on the top (bottom) side. This additional symmetry breaking in the transversal electrokinetic vortex flow pattern gives rise to a net rotating fluid motion \(\text{u}_{\text{y}}\) for sample mixing in the upward (downward) direction, respectively, and the original function of automatic fluid pumping in the forward direction \(\text{u}_{\text{x}}\) is maintained concurrently from an unchanged polarizability gradient. In this way, Janus AC-FFET proposed herein greatly facilitates simultaneous active pumping and mixing of stratified analyte streams with a simple device geometry. (**F**) A sketch of the 2D computation domain used in current numerical simulation. Therefore, we coin the term \"Janus AC flow-field-effect transistor (Janus AC-FFET)\" to highlight its unique feature of the controllable asymmetry of the ICEO flow profile in two orthogonal directions (Figure 1). It is worth mentioning that Janus AC-FFET is different from the traditional AC field-effect control of standard ICEO on uniform solid surfaces, since symmetry breaking in ICEO convection can only be realized along one specific orientation. Recently, [PERSON] et al. [52] demonstrated how various semiconductor diodes form a new category of self-propelling particles and pumps in microfluidic systems. The energy is supplied by a global external AC electric field. A DC voltage difference is then created between the electrodes of each diode due to the rectification of the global AC field. The uniform electric field between the electrodes gives rise to stable electro-osmotic flow, which can propel the floating diodes or pump the surrounding liquid.