Directional flow induced by synchronized longitudinal and zeta-potential controlling AC-electrical fields

Abstract

Electroosmotic flow (EOF) in a microchannel can be controlled by electronic control of the surface charge using an electrode embedded in the wall of the channel. By setting a voltage to the electrode, the zeta-potential at the wall can be changed locally. Thus, the electrode acts as a "gate" for liquid flow, in analogy with a gate in a field-effect transistor. In this paper we will show three aspects of a Field Effect Flow Control (FEFC) structure. We demonstrate the induction of directional flow by the synchronized switching of the gate potential with the channel axial potential. The advantage of this procedure is that potential gas formation by electrolysis at the electrodes that provide the axial electric field is suppressed at sufficiently large switching frequencies, while the direction and magnitude of the EOF can be maintained. Furthermore we will give an analysis of the time constants involved in the charging of the insulator, and thus the switching of the zeta potential, in order to predict the maximum operating frequency. For this purpose an equivalent electrical circuit is presented and analyzed. It is shown that in order to accurately describe the charging dynamics and pH dependency the traditionally used three capacitor model should be expanded with an element describing the buffer capacitance of the silica wall surface.

Induced-charge electrokinetic phenomena

“…This reduces the magnitude of field-effect flow control, wherein an auxillary electrode is used to directly change the surface potential [92]. Varying both auxillary and bulk fields at the same frequency leads to a non-zero, time-averaged, AC flow-FET whose magnitude is well-described using surface adsorption of hydronium ions [93]. Recently, Suh and Kang [58] used a Langmuir isotherm for (non-specific) surface adsorption of ions in a model of ACEO flow over an electrode pair and demonstrated improved fitting of the original experimental data of Green et al [13].…”

On utilizing alternating current-flow field effect transistor for flexibly manipulating particles in microfluidics and nanofluidics

“…Electronic addresses: jyshao@mail.xjtu.edu.cn and rykhit@hit.edu.cn controlled by an external AC voltage source, so as to induce total ionic charge in phase with the AC background field oscillating at the same field frequency. This effect is, in essence, an AC generalization of the DC-flow field effect transistor, a phenomenon also termed as "ACflow field effect transistor" (AC-FFET), identical to the previous work of Van Der Wouden et al 35 Sun et al have recently developed a high-flux multifunctional ionic circuit platform, 36 on the basis of external ion concentration polarization due to electrical field applied across an ion-selective membrane, where a gate electrode located on top of the reservoir controls the direction and magnitude of the two imposed electric fields to result in flexible ion current rectification in the output microfluidic channel. So, the same mechanism appears to be work but with DC electrokinetics through a gating ion-selective nanoporous membrane that expands the polarized layer on the gate and induces ion concentration gradient.…”

Induced-charge electroosmotic trapping of particles

“…1 This effect is essentially an AC generalization of the "flow field effect transistor", 26,27 similar to the work of van der Wouden et al 28 When the AC voltage imposed on the left electrode is A 1 cosIJωt), the right electrode is grounded, and the potential distribution just outside the double layer of the middle electrode is ϕIJt), we can derive an analytical solution for the induced zeta potential ζ in the DC limit (0 Hz) to see whether the biased voltage has any effect on the induced surface charge 1. When the middle electrode is floating in potential…”