Induced-charge electro-osmosis in dielectric annuli

This paper reports an analytical study on the induced-charge electro-osmosis (ICEO) within a leaky dielectric annulus subjected to an AC electric field. An interesting non-monotonic variation of the ICEO flow with the increasing AC frequency is revealed. This is different from the monotonic decrease of the ICEO flow around a cylinder submerged in an unbounded electrolyte solution upon increasing the AC frequency. Moreover, the ICEO flow is significantly reduced and may reverse direction due to the existence of the outer cylinder, depending on the charging responses of the annulus and the electrolyte solution, and the annulus geometry. In this analysis, we consider both the space charge layers (SCLs) and the electric double layers (EDLs) established within the solid and the liquid sides of the solid-liquid interfaces, respectively. The ICEO flow forms eight vortices within the annulus, which show a potential for mixing enhancement in micro/nanofluidics. As the AC phase increases, the ICEO flow changes periodically with a period half of the AC period. The outer cylinder presents a significant influence on the ICEO flow within the annulus since it affects the local electric fields and the induced zeta potentials of the cylinders. The present study may provide references for microchip fabrications with non-contact electrodes and biocell manipulations by electric fields.