Comment on “Theoretical Model of Electrode Polarization and AC Electroosmotic Fluid Flow in Planar Electrode Arrays”

Ramos, António; González-Weller, Dailos; Green, Nicolas G.; Morgan, Hywel; Castellanos, A.

doi:10.1006/jcis.2001.7957

“…Similar results hold for the relaxation time for the total half-cell charge, Eq. (36), which has the dimensionless form,…”

Section: B Time Scales For Linear Responsementioning

confidence: 99%

“…Recently, it has been rediscovered as the (inverse) frequency of "AC pumping" at patterned-surface micro-electrodes [1,6]. As in the past, its theoretical justification has sparked some controversy [35,36] related to the applicability of classical circuit models [37,38] in which τ c arises as the "RC time" of a bulk resistor in series with a double-layer capacitor (see below).…”

Section: Introductionmentioning

confidence: 99%

Diffuse-charge dynamics in electrochemical systems

Bazant

¹

,

Thornton

²

,

Ajdari

³

2004

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The response of a model microelectrochemical system to a time-dependent applied voltage is analyzed. The article begins with a fresh historical review including electrochemistry, colloidal science, and microfluidics. The model problem consists of a symmetric binary electrolyte between parallel-plate blocking electrodes, which suddenly apply a voltage. Compact Stern layers on the electrodes are also taken into account. The Nernst-Planck-Poisson equations are first linearized and solved by Laplace transforms for small voltages, and numerical solutions are obtained for large voltages. The "weakly nonlinear" limit of thin double layers is then analyzed by matched asymptotic expansions in the small parameter epsilon= lambdaD/L, where lambdaD is the screening length and L the electrode separation. At leading order, the system initially behaves like an RC circuit with a response time of lambdaDL/D (not lambdaD2/D), where D is the ionic diffusivity, but nonlinearity violates this common picture and introduces multiple time scales. The charging process slows down, and neutral-salt adsorption by the diffuse part of the double layer couples to bulk diffusion at the time scale, L2/D. In the "strongly nonlinear" regime (controlled by a dimensionless parameter resembling the Dukhin number), this effect produces bulk concentration gradients, and, at very large voltages, transient space charge. The article concludes with an overview of more general situations involving surface conduction, multicomponent electrolytes, and Faradaic processes.

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“…The same time scale, τ c , also arises as the inverse frequency of AC electroosmosis (Ramos et al 1999) or AC pumping (Ajdari 2000) at a micro-electrode array of characteristic length a, where again an RC-circuit analogy has been invoked to explain the charging process. This simple physical picture has been criticized by Scott, Kaler & Paul (2001), but Ramos et al (2001) and Gonzalez et al (2000) have convincingly defended its validity, as in the earlier Russian papers on polarizable colloids.…”

Section: Iceo Around a Conducting Cylinder In A Sinusoidal Ac Fieldmentioning

confidence: 99%

Induced-charge electro-osmosis

Squires

¹

,

Bazant

²

2004

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We describe the general phenomenon of 'induced-charge electro-osmosis' (ICEO) -the nonlinear electro-osmotic slip that occurs when an applied field acts on the ionic charge it induces around a polarizable surface. Motivated by a simple physical picture, we calculate ICEO flows around conducting cylinders in steady (DC), oscillatory (AC), and suddenly-applied electric fields. This picture, and these systems, represent perhaps the clearest example of nonlinear electrokinetic phenomena. We complement and verify this physically-motivated approach using a matched asymptotic expansion to the electrokinetic equations in the thin double-layer and low potential limits. ICEO slip velocities vary like u s ∝ E 2 0 L, where E 0 is the field strength and L is a geometric length scale, and are set up on a time scale τ c = λ D L/D, where λ D is the screening length and D is the ionic diffusion constant. We propose and analyze ICEO microfluidic pumps and mixers that operate without moving parts under low applied potentials. Similar flows around metallic colloids with fixed total charge have been described in the Russian literature (largely unnoticed in the West). ICEO flows around conductors with fixed potential, on the other hand, have no colloidal analog and offer further possibilities for microfluidic applications.

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“…The same time scale, τ c , also arises as the inverse frequency of AC electro-osmosis (Ramos et al (1999)) or AC pumping (Ajdari (2000)) at a micro-electrode array of characteristic length a, where again an RC-circuit analogy has been invoked to explain the charging process. This simple physical picture has been criticized by Scott et al (2001), but Ramos et al (2001) and Gonzalez et al (2000) have convincingly defended its validity, as in the earlier Russian papers on polarizable colloids.…”

Section: Time Scales For Iceo Flowsmentioning

confidence: 99%

Induced-Charge Electro-Osmosis

SpringerReference

0

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We describe the general phenomenon of 'induced-charge electro-osmosis' (ICEO) -the nonlinear electro-osmotic slip that occurs when an applied field acts on the ionic charge it induces around a polarizable surface. Motivated by a simple physical picture, we calculate ICEO flows around conducting cylinders in steady (DC), oscillatory (AC), and suddenly-applied electric fields. This picture, and these systems, represent perhaps the clearest example of nonlinear electrokinetic phenomena. We complement and verify this physically-motivated approach using a matched asymptotic expansion to the electrokinetic equations in the thin double-layer and low potential limits. ICEO slip velocities vary like u s ∝ E 2 0 L, where E 0 is the field strength and L is a geometric length scale, and are set up on a time scale τ c = λ D L/D, where λ D is the screening length and D is the ionic diffusion constant. We propose and analyze ICEO microfluidic pumps and mixers that operate without moving parts under low applied potentials. Similar flows around metallic colloids with fixed total charge have been described in the Russian literature (largely unnoticed in the West). ICEO flows around conductors with fixed potential, on the other hand, have no colloidal analog and offer further possibilities for microfluidic applications.

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Comment on “Theoretical Model of Electrode Polarization and AC Electroosmotic Fluid Flow in Planar Electrode Arrays”

Cited by 5 publications

References 9 publications

Diffuse-charge dynamics in electrochemical systems

Diffuse-charge dynamics in electrochemical systems

Induced-charge electro-osmosis

Induced-Charge Electro-Osmosis

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