Induced charge electroosmosis micropumps using arrays of Janus micropillars

Abstract

We report on a microfluidic AC-driven electrokinetic pump that uses Induced Charge Electro-Osmosis (ICEO) to generate on-chip pressures. ICEO flows occur when a bulk electric field polarizes a metal object to induce double layer formation, then drives electroosmotic flow. A microfabricated array of metaldielectric Janus micropillars breaks the symmetry of ICEO flow, so that an AC electric field applied across the array drives ICEO flow along the length of the pump. When pumping against an external load, a pressure gradient forms along the pump length. The design was analyzed theoretically with the reciprocal theorem. The analysis reveals a maximum pressure and flow rate that depend on the ICEO slip velocity and micropillar geometry. We then fabricate and test the pump, validating our design concept by demonstrating non-local pressure driven flow using local ICEO slip flows. We varied the voltage, frequency, and electrolyte composition, measuring pump pressures of 15-150 Pa. We use the pump to drive flows through a highresistance microfluidic channel. We conclude by discussing optimization routes suggested by our theoretical analysis to enhance the pump pressure. IntroductionSignificant research continues into the development of microfluidic devices for diverse applications including medical diagnostics, high-throughput chemistry and biology, and analyte monitoring and detection. 17-22 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. Specifically, our strategy uses oriented arrays of Janus metallo-dielectric micropillars to break the symmetry of the ICEO flow (Fig. 1a), so that AC electric fields applied across the pumping channel drive ICEO flows along the channel. In so doing, higher field strengths can be achieved with a given potential difference than in DC electrokinetic flow, where electric fields must be applied along the length of a pumping channel. Our proof of concept device (Fig. 1b) establishes pressures comparable to standard ACEO pumps, suggesting that further optimization and enhanced fabrication methods will enable higher pressures. We begin by describing electrokinetic flows (sec. 2.1) and electrokinetic pressure generation (sec. 2.2). We then describe the strategy for ICEO-based pressure generation using arrays of asymmetrically metallized micropillars (sec. 3.1). We analyze the theoretical performance of such arrays (sec. 3.2), using the Lorentz Reciprocal Theorem to derive expressions for the maximum pressure ΔP max and flow rate Q max to enable the rational analysis and design of such pumps. We then describe a method to microfabricate such arrays, and the experimental setup used to measure the pressure generate...

Moderately nonlinear diffuse-charge dynamics under an ac voltage

“…For reference, k B T /e ≈ 25 mV at T = 298 K. For instance, the majority of work in electrokinetics over the last century has considered fluid flow or particle motion driven by weak applied potentials, V < k B T /e [16], for which the equilibrium diffuse screening layer, or "Debye layer," around a charged surface or particle is only slightly perturbed. In contrast, modern applications such as AC, induced-charge, or secondkind, electro-osmotic pumps [17][18][19][20][21], and electrochemical supercapacitors [22,23] use time-dependent potentials on the order of a few volts, well above the thermal voltage. The relative scarcity of theoretical treatment for such larger voltages creates a need to study diffuse charge dynamics in this regime, where the Debye layer can be driven strongly out of equilibrium.…”

AC EOF in a rectangular microannulus

“…Velocity and friction were calculated and plotted. Paustian et al [31] investigated an induced-charge alternating current EOF. An array of Janus micropillars was used for breaking the symmetry of the flow; hence the flow was driven by applying AC electric fields.…”

Flexible particle flow‐focusing in microchannel driven by droplet‐directed induced‐charge electroosmosis

“…Induced-charge electrokinetics takes place when an applied background electric field E induces an ionic double layer over a polarizable surface immersed in electrolyte, then forces that induced double-layer (IDL) into induced-charge electroosmotic (ICEO) flow [30][31][32][33][34], and is a promising tool for particle sorting [35,36], liquid pumping [28,37,38] and mixing [39][40][41][42][43] etc. Nevertheless, how to use ICEO fluid motion to achieve particle manipulation with intended functions has rarely been exploited.…”