Induced Charge Electro-osmosis over Controllably Contaminated Electrodes

Abstract

Recent studies in nonlinear electrokinetics reveal the standard theory to generally overpredict measured velocities, sometimes dramatically. Contamination of the driving surface provides a natural mechanism for electrokinetic suppression. We measure induced charge electro-osmosis over gold electrodes "contaminated" with silica layers of controlled thickness for nearly a thousand distinct conditions, in a system that enables direct comparisons between theoretical predictions and experimental measurements. Both the magnitude and frequency dependence of the measured slip velocity are captured quantitatively over the entire range of experiments by accounting for the physical capacitance and surface chemistry of the dielectric layer. More generally, the quantitative characterization enabled by our apparatus will prove invaluable for the rational design and prediction of electrokinetic systems.

Induced-charge electro-osmosis around metal and Janus spheres in water: Patterns of flow and breaking symmetries

“…However, the velocity reduction factor cp  caused by concentration polarization has been predicted to be only about 5 [17], thus the effect is unlikely to be the sole factor responsible for the smallness of velocities. Among other reasons are surface contamination [23] and roughness [24], interactions with walls [13], confinement-enhanced viscous friction [25] and the assumption of small voltages Metal-dielectric Janus spheres produce dipolar flows and pumping effects, with contributions ~cos , ~sin to the radial and azimuthal components of ICEO velocities, respectively.…”

Induced charge electroosmosis micropumps using arrays of Janus micropillars

“…2d). 22,35,36 Although ACEO and ICEO share their central physical mechanism -an electric field that induces, then forces, an EDL -in ACEO, the electric field and induced double-layer are both driven on the same structure, whereas the driving field in ICEO is established by external electrodes, and the induced double-layer forms over electrically floating objects. The ICEO velocity obeys eqn (1); under an AC applied field, however, only the induced potential (ζ i~E a, where a is the length of polarized surface) contributes to the time-averaged flow 〈u ICEO 〉~〈Eζ i 〉 to give…”

“…[37][38][39] Previous experiments have investigated ICEO flows near individual polarizable objects, 19,22,[35][36][37] measured the induced-charge electrophoretic velocity of metallodielectric colloids, 40 and used conductive micropillars for ICEO-driven mixing. 41,42 More recently, experimental application of ICEO has focused on local flow control within microchannels, for example by using electroplated gold 43 47 and surface conduction.…”

“…Furthermore, eqn (2) for the ICEO slip velocity assumes a fully-charged induced EDL, an assumption that is violated at sufficiently high frequencies. 17,22 54 An electric field applied along a microchannel (or capillary) with cross-sectional area A establishes an electroosmotic slip u HS , driving a volumetric flow rate Q EO max ,…”

“…We adapt the classic strategy for electrokinetic pressure generation 16 to be compatible with Induced-Charge Electro-Osmotic (ICEO) flows. [17][18][19][20][21][22] ICEO flows arise when an applied electric field polarizes a metal surface, inducing a non-uniform electric double layer, then drives that induced double layer into electroosmotic flow. Like conventional methods for eletrokinetic pressure generation, the ICEO strategy described here exploits the ease of driving flows electrokinetically through small pores, so that large pressures naturally arise to establish mass-conserving backflows.…”