Alternating current electroosmosis flow rate analysis based on electric-triple-layer model

ACEO (alternating current electroosmosis) flow rate is intimately associated with the structure and charge distribution of electric double layer (EDL). Flow field distribution and cross-over frequency at which velocity reaches its peak can be predicted by classic EDL theory and flow rate formula. But the maximum velocity obtained theoretically is several times bigger than the experimental value. Considering the influences of Stern layer and middle layer thickness on ACEO flow rate, the ETL (electric-triple-layer) model is presented and effective thicknesses and relative permittivity of each layer are described herein, and then the formula for the description of time average and position average flow rate, as well as the cross frequency, are deduced. In the tests, a four-phase travelling wave electric field with 90° phase difference is applied to interdigitated electrode array with equal width and space of 20 μm to observe the trajectory of fluorescence micro particles by charge-coupled device camera, from which the flow rate is calculated based on the micro-PIV method. Different thicknesses of three layers were used in simulation. The experimental results show good agreement with simulation data when the thickness of Stern layer is with the size of several solvent molecules (0.5 nm) and middle layer thickness as the height of electrode surface micro topography roughness (5 nm), which indicates the validity of the proposed ETL model and the modified velocity formula.