###### Abstract  
We propose herein a novel microfluidic approach for the simultaneous active pumping and mixing of analytes in a straight microchannel via the AC field-effect control of induced-charge electro-osmosis (ICEO) around metal-dielectric solid Janus cylinders of inherent inhomogeneous electrical polarizability immersed in an electrolyte solution. We coin the term \"Janus AC flow field-effect transistor (Janus AC-FFT)\" to describe this interesting physical phenomenon. The proposed technique utilizes a simple device geometry, in which one or a series of Janus microylinders are arranged in parallel along the centerline of the channel's bottom surface, embedding a pair of 3D sidewall driving electrodes. By combining symmetry breaking in both surface polarizability and the AC powering scheme, it is possible, on demand, to adjust the degree of asymmetry of the ICEO flow profile in two orthogonal directions, which includes the horizontal pump and transversal rotating motion. A comprehensive mathematical model was developed under the Debye-Huckel limit to elucidate the physical mechanism underlying the field-effect-reconfigurable diffuse-charge dynamics on both the dielectric and the metal-phase surfaces of the Janus micropillar. For innovation in applied science, an advanced microdevice design integrating an array of discrete Janus cylinders subjected to two oppositely polarized gate terminals is recommended for constructing an active microfluidic pump and mixer, even without external moving parts. Supported by a simulation analysis, our physical demonstration of Janus AC-FFT provides a brand-new approach to muti-directional electro-convective manipulation in modern microfluidic systems. microfluidic pump and mixer; AC field-effect flow control; induced-charge electro-osmosis; metal-dielectric Janus micropillar; inhomogeneous electrical polarizability; gate terminal +
Footnote †: journal: Applied Sciences  
either active or passive mixing devices, taking into consideration the driving mechanism based upon which they operate [4,5]. Because passive stirring expands the thickness of the two-phase contact interface, increases the time of interfacial mass exchange, and disturbs the laminar streamlines between parallel buffer solutions by inserting solid obstructions into the microchannel, it completely relies on the molecular-diffusion effect or chaotic advection [6]. In stark contrast, active micromixers exploit an external energy input to cause dynamic fluid motion and enhance the mixing performance, as in acoustic streaming [7], magnetic actuation [8], and electrokinetic (EK) forcing [9]. In particular, electro-hydrodynamics (EHDs) have been extensively investigated in microsystems to realize the flexible manipulation of leaky dielectric working fluid or nanoparticle samples monodispersed within it [10, 11, 12, 13, 14, 15, 16, 17, 18]. Within quasi-electrostatic limits, the common trait of EHDs manifests as an action of the local electric field on the space-charge density it induces to impart a net electrostatic body force and actuate the motion of the target medium via structural polarization [19]. This occurs in the so-called Ohmic model under an electroneutrality constraint [20], which is often accompanied by electrochemical polarization at charged solid/liquid interfaces [21, 22, 23, 24, 25]. In particular, the recent advances in microelectronic processing techniques have facilitated an easy way for compact metallic microelectrode arrays to be patterned and embedded into microfluidic devices, providing precious opportunities to exert ponderomotive Coulomb and dielectric forces directly on the working fluid--even during exposure to external AC forcing--on account of the field-induced features of the electrical charges involved in nonlinear electrokinetics [26].