Optimization of upstream particle concentration from flow using AC electro-osmosis and dielectrophoresis

There are many applications where upstream sample processing is required to concentrate dispersed particles in flow; this may be to increase the concentration (e.g., to enhance biosensor accuracy) or to decrease it (e.g., by removing contaminants from flow). The AC electrokinetic phenomenon, dielectrophoresis (DEP), has been used widely for particle trapping for flow, but the magnitude of the force drops reduces rapidly with distance from electrode edges, so that nm-scale particles such as viruses and bacteria are only trapped when near the electrode surface. This limits the usable flow rate in the device and can render the final device unusable for practical applications. Conversely, another electrokinetic phenomenon, AC electro-osmosis (ACEO), can be used to move particles to electrode surfaces but is unable to trap them from flow, limiting their ability for sample cleanup or trap-and-purge concentration. In this paper, we describe the optimization of ACEO electrodes aligned parallel to pressure-driven flow as a precursor/preconditioner to capture particles from a flow stream and concentrate them adjacent to the channel wall to enhance DEP capture. This is shown to be effective at flow rates of up to 0.84 ml min^-1. Furthermore, the analysis of the 3D flow structure in the ACEO device by both simulation and confocal microscopy suggests that while the system offers significant benefits, the flow structure in the volume near the channel lid is such that while substantial trapping can occur, particles in this part of the chamber cannot be trapped, independent of the chamber height.