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

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
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
AC Electroosmosis

Ramos, Morgan, Green, Castellenos 1998

aceo pump
Ajdari, PRE 2000ACEO Pump

Exp. data from Urbanski et al, MIT

slide5
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
Bikerman (1942)

@ equilibrium

C, dimensionless,

ν, volume fraction in bulk

Kilic, Bazant, Ajdari – PRE 2007

model applied to aceo
Model applied to ACEO

Linearized, DH

Non-linear, GCS

Bikerman model

theory and experiment
Theory and experiment

Ion is 4 nm to best fit data.

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

better steric models
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
Concentration effects-ACEO experiments

Urbanski et al. 2007

Studer et al, 2004

concentration effects simple model
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
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
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
conclusions
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
Induced charge electroosmosis (ICEO)

Bazant & Squires PRL & JFM2004

slide18
Theory & experiment

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

ac electroosmosis
AC Electroosmosis

*simulations account for EK through BC.

Ramos, Morgan, Green, Castellenos 1998

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