Steric effects on ac electroosmosis in dilute electrolytes
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Steric effects on AC electroosmosis in dilute electrolytes. Brian D. Storey 1 , Lee R. Edwards 1 , Mustafa Sabri Kilic 2 , Martin Z. Bazant 2 1 Olin College of Engineering 2 MIT. Storey, Edwards, Kilic, Bazant PRE 2008 Bazant, Kilic, Storey, Ajdari , arXiv , 2007.

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Steric effects on ac electroosmosis in dilute electrolytes l.jpg

Steric effects on AC electroosmosis in dilute electrolytes

Brian D. Storey1, Lee R. Edwards1, Mustafa Sabri Kilic2, Martin Z. Bazant2

1Olin College of Engineering

2MIT

Storey, Edwards, Kilic, Bazant PRE 2008

Bazant, Kilic, Storey, Ajdari, arXiv, 2007


Motivation iceo microfluidics l.jpg
Motivation – ICEO microfluidics

  • For engineers, ICEO operates at low voltage (portable microfluidics).

  • For electrokinetic theorists, ICEO operates at high voltage ~100 kT/e.

  • Current simulations of ICEO microfluidic devices rely on classical electrokinetic theory.

  • What’s complicated about ICEO? Large voltage, dynamics, long range flow patterns, complex geometry.

  • Can we develop a simple theory that can be implemented as boundary conditions in commercial finite element codes for predicting macro-scale flows?

Bazant & Squires PRL & JFM2004


Slide3 l.jpg

AC Electroosmosis

Ramos, Morgan, Green, Castellenos 1998


Aceo pump l.jpg

Ajdari, PRE 2000

ACEO Pump

Exp. data from Urbanski et al, MIT


Slide5 l.jpg

Classical electrokinetics

Chemical potential of dilute point ions:

Near a wall, steady state, 1D:

Applied voltage =.025 V

Applied voltage =0.75 V

Would need ions to be 0.01 angstrom


Bikerman 1942 l.jpg
Bikerman (1942)

@ equilibrium

C, dimensionless,

ν, volume fraction in bulk

Kilic, Bazant, Ajdari – PRE 2007


Model applied to aceo l.jpg
Model applied to ACEO

Linearized, DH

Non-linear, GCS

Bikerman model



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Theory and experiment

Ion is 4 nm to best fit data.

Exp. from Studer, Pepin, Chen, Ajdari Analyst 2004


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Better steric models

Carnahan Starling (2 nm)

“volume effects can be underestimated significantly” using Bikerman model.

(Biesheuvel & van Soestbergen, JCIS 2007).

We can reduce to ~1 nm if we assume ε changes in double layer


Slide11 l.jpg

Concentration effects-ACEO experiments

Urbanski et al. 2007

Studer et al, 2004


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Concentration effects - simple model

Electroosmotic mobility

Valid for any continuum model

Simplest model of a viscoelectric effect

(Bazant, Kilic, Storey, Ajdari 2007 arXiv)


Viscoelectric model applied to aceo l.jpg
Viscoelectric model applied to ACEO

Increasing concentration

Need an ion size of ~4 nm to fit forward flow data

Lose the flow reversal when we combine models


What s missing l.jpg
What’s missing?

  • Correlations effects.

  • Faradaic reactions (recall talk of Ramos et al on TWEO).

  • …..

What’s needed?

  • Experiments designed to study these effects.

  • Multi-scale modeling


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Conclusions

  • ICEO applications has opened new avenues for study in theoretical EK.

  • Crowding of ions, increased viscosity, and decreased permittivity are not new ideas (Bikerman, 1970).

  • Accounting for steric effects can effect qualitative and quantitative predictions in ACEO.

  • Still more work is needed for a truly useful theory based on first principles.


Induced charge electroosmosis iceo l.jpg
Induced charge electroosmosis (ICEO)

Bazant & Squires PRL & JFM2004


Slide18 l.jpg

Theory & experiment

Exp. from Studer, Pepin, Chen, Ajdari Analyst 2004


Ac electroosmosis l.jpg
AC Electroosmosis

*simulations account for EK through BC.

Ramos, Morgan, Green, Castellenos 1998


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