Transient Effects in High Power Electroosmotic Pumps

Abstract

On generating high electroosmotic flows in microfluidic pumps under applied DC voltage, the flow rate and the drawn current drop with time. When generating high electroosmotic flows using microfluidic pumps under applied DC voltage, the flow rate and current draw decrease with time. The electroosmotic (EO) pump efficiency decreases with time due to flow rate deterioration. In order to study the transient effect in EO pumps, the mass transport of ions in the membrane is investigated. Ions mass transport are affected by the membrane surface charge, ion diffusion, ion migration and flow convection. Many studies investigate the mass transport in ion selective membranes, micro-channels, and nano-channels without focus on the transient effects at high electric fields. In most of these studies, the Poisson-Nernst-Plank and the Naiver stokes equations are used to model the ion transport in electrokinetic devices. Without applying simplifying assumptions, these system of equations can be only solved numerically. A theoretical model, based on diffusion and ion migration, is developed to predict the current drop and experiments are conducted to verify this model. EO flow can be neglected when there is no membrane installed between the pump electrodes (electrochemical cell). The current drop is predicted under no flow conditions using the advection diffusion equation and it solved analytically using the Ogata and Banks solution. In order to predict the current drop in the EO pump under flow conditions, the Helmholtz-Smoluchowski equation is used to calculate the EO flow velocity. This equation holds under thin electrical double layer assumption and small zeta potential. The current drop has been calculated theoretically and compared with the experimental data. The ion screening and depletion at the electrodes result in increasing the EO pump total resistance and decrease the total current. The calculated current drop time scale has been found to be in the order of 100 seconds under no flow conditions. As flow rate increases, the flow rate contributes to the mass transport of ions (convection current) and screens the ions faster, leading to a decrease in the current drop time scale. On the other hand, by increasing the fluid molar concentration, the current drop is much slower as more ions are available and need more time to be depleted. The current drop time scale decreases rapidly as higher DC voltages are applied, leading to low efficiency. The ion transport can be limited by applying a pulse voltage waveform instead of DC voltage, leading to more stable flow rate, current and hence constant EO efficiency. The pulse voltage waveform allows the ions to diffuse back from high to low concentration regions during the off-time, preventing ion depletion and current drop.