The role of Faradaic reactions in microchannel flows

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The role of Faradaic reactions in microchannel
flows

• David A. Boy
• Brian D. Storey
• Franklin W. Olin College of Engineering
• Needham, MA
• Sponsor NSF CTS, Research in Undergraduate
  Institutions.

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Motivation ACEO ICEO
Electric Field
Positive Ions
Flow
Negative Ions

-----------------------------------
Negative Electrode
Positive Electrode

• Advantages over DC
• Low voltage, portable (1 10 volts)
• Good flow rates (mm/s)

Green et al PRE 2000, 2002 Ajdari PRE 2000 Brown
PRE 2000 Bazant Squires JFM 2004 Olesen et al
PRE 2005
Soni, Squires, Meinhart, BC00004 Swaminathan ,
Hu FC00003 Yossifon, Frankel, Miloh, GC00007
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Experimental observations(reactions have been
proposed as possible mechanism for each of these)

• Reversal of net pumping in ACEO is observed at
  high frequency.
• Most flow stops at 10 mM in ACEO ICEO
• Typically, only qualitative flow is predicted.

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Our goals

• Understand the general coupling between reactions
  and flow.
• Account for non-linear effects
• Surface conduction
• Mass transfer concentrations at electrodes are
  not the same as the bulk.
• Body forces outside of EDL.

Olesen et al PRE 2005
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A simpler system to study body forces
current
reactions at electrodes
Binary, symmetric electrolyte
reactions at electrodes
R. F. Probstein. 1994. Physicochemical
Hydrodynamics. Wiley.
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Bulk equations (symmetric, binary, dilute
electrolyte)
Voltage scaled thermal voltage (25 mV) ? 0.1
to 0.0001 Pe 100 to 1,000,000
Small device Large device
Dilute High Concentration
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Boundary conditions
boundary conditions at electrodes - fixed
voltage difference - No slip - reactions
periodic boundary conditions in x
Butler-Volmer reaction kinetics
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1D Solutions ?0.01
K. T. Chu and M. Z. Bazant. 2005. SIAM J. Appl.
Math. 65, 1485-1505.
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1D Voltage-Current Behavior (fixed geometry
fluid properties)
unstable
Dilute
K. T. Chu and M. Z. Bazant. 2005. SIAM J. Appl.
Math. 65, 1485-1505. Rubinstein Zaltzman PRE
(2000, 2003, 2005 )
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Fixed Debeye length 0.1
unstable
Stable
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Streamlines for ?.02, k2.5, V9.5
0 1 2 3
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Unsteady flow at high voltages
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Voltage-Current behavior
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ACEO Pumping Geometry

• When reactions occur
• Flow occurs for all voltages
• Flow occurs in AC and DC case
• Flow is not symmetric even when electrodes are

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ACEO Symmetric Electrodes (DC, ?0.01, Pe1000,
V10)
Potential
Charge Density
Streamlines
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ACEO Typical Streamlines (DC, ?0.01, Pe1000)
V1
V5
Neg.
Neg.
Pos.
Pos.
V20
V10
Neg.
Neg.
Pos.
Pos.
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Reverse the sign on the electrodes (DC, ?0.01,
Pe1000, V5)
Pos.
Neg.
Neg.
Pos.
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Frequency response (AC, ?0.05 Pe1000)
Olesen et al. PRE 2005.
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Future work

• Complete the parameter study of ACEO geometry.
  Can body forces destabilize the flow?
• Compare ACEO flow computed with our full
  simulation to simpler models (i.e. Olesen et al.
  PRE 2005).
• Use realistic reactions and electrolyte
  parameters as opposed to model binary, symmetric
  electrolyte.
• Incorporate non-dilute effects. All applications
  well exceed kT/e 25 mV.

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Conclusions

• Body force in extended charge region can induce
  instability in parallel electrode geometry.
• Instability occurs in parameter range found in
  microfluidic applications.
• Thus far, we have not flow instability due to
  body forces in ACEO applications. Apparently,
  steady flow overwhelms the instability. (Note
  our study is currently incomplete).

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