Maximum efficiency of the electro-osmotic pump

Abstract

Electro-osmotic effect in a porous medium arises from the electrically charged double layer at the fluid-solid interface, whereby an externally applied electric field can give rise to fluid flow. The electro-osmotic pump (EOP) is potentially useful for a variety of engineering and biorelated applications, but its generally low efficiency is a negative factor in this regard. A study to determine the optimal efficiency of the EOP and the condition(s) under which it can be realized is therefore of scientific interest and practical importance. We present the results of a theoretical and experimental study on the maximum efficiency optimization of the electrokinetic effect in artificially fabricated porous media with controlled pore diameters. It is shown that whereas the EOP efficiency increases with decreasing channel diameter, from 4.5 to 2.5 μm for samples fabricated on oxidized silicon wafers as expected for the interfacial nature of the electro-osmotic effect, the opposite trend was observed for samples with much smaller channel diameters fabricated on anodized aluminum oxide films, with the pore surface coated with silica. These results are in agreement with the theoretical prediction, based on the competition between interfacial area and the no-slip flow boundary condition, that an optimal efficiency of ∼1% is attained at a microchannel diameter that is five times the Debye length, with a zeta potential of ∼100 mV.

Self-Consistent Approach to Global Charge Neutrality in Electrokinetics: A Surface Potential Trap Model

“…This time variation should be contrasted with the traditional picture that the time-dependent EO is a consequence of the inertial effect, which is small. In experiments, it is often observed that the flow rate can decay with time [73,74]. This is partly due to electrode screening by the ions, so the applied voltage seen by the sample actually decreases with time.…”

“…Since the final state represents a smaller fluid flux, it follows that the optimal operational mode of the EO effect should be periodically pulsed [73,74]. However, the timedomain optimization of the EO effect represents a new direction that will be pursued separately.…”

“…In that case, the average flow velocity (with no time dependence) is given by the expression ðεζ=ηÞE z − ða 2 =8ηÞð−dP=dzÞ [73,74], where the (averaged) pressure gradient is along the axial direction, and the ζ potential (¼ −29.8 mV) is defined by Eq. (15) by using as the boundary value −μða → ∞Þ ¼ −76.8 mV.…”

“…Such correlated variation also emphasizes the need for having both the ions and counterions be within the computational domain so that their interaction can be treated in a mathematically self-consistent manner. Fourth, the timevarying EO effect has been widely observed but rarely reported or addressed [73,74]. In particular, no theoretical study has been carried out to investigate whether there can be an intrinsic (noninertial) time dependence to the EO effect, even though from physical considerations the inevitable existence of diffusive counterion currents must give rise to a relaxation process.…”

Effect of interfacial Maxwell stress on time periodic electro‐osmotic flow in a thin liquid film with a flat interface

“…Flow actuation in microchannels due to an externally applied electric field has found remarkable applications in lab‐on‐a‐chip based microfluidics devices and systems . In many situations, such flows are effectively actuated by interactions between a wall‐adhering charged layer (also known as the electrical double layer) and an externally applied electrical field, resulting in so‐called EOFs, which have an enduring advantage over classical fluidic pumps in a sense that these do not require any moving components.…”