The role of Faradaic reactions in microchannel flows
description
Transcript of The role of Faradaic reactions in microchannel flows
The role of Faradaic reactions in microchannel
flows
David A. BoyBrian D. Storey
Franklin W. Olin College of EngineeringNeedham, MA
Sponsor: NSF CTS, Research in Undergraduate Institutions.
Motivation: ACEO & ICEO
Advantages over DC• Low voltage, portable (~1 – 10 volts)• Good flow rates (~mm/s)
Green et al PRE 2000, 2002Ajdari PRE 2000Brown PRE 2000Bazant & Squires JFM 2004Olesen et al PRE 2005
Positive ElectrodeNegative Electrode
Soni, Squires, Meinhart, BC00004Swaminathan , Hu FC00003Yossifon, Frankel, Miloh, GC00007
++++++++++++++++++++++++-----------------------------------
Electric Field
Negative IonsPositive Ions Flow
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.
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
A simpler system to study body forces
reactions at electrodes
reactions at electrodes
Binary, symmetricelectrolyte
R. F. Probstein. 1994. Physicochemical Hydrodynamics. Wiley.
current
ratesreaction essDimensionl :K voltageapplied essDimensionl :V
number Reynolds:numberPeclet :
length Debye essdimensionl:
field electric:
density charge:tyconductivi electrical:
potential electric:
RePe
EE
Bulk equations (symmetric, binary, dilute electrolyte):
Voltage scaled thermal voltage (25 mV)λ = 0.1 to 0.0001Pe = 100 to 1,000,000 Small device Large device Dilute High Concentration
E2
2 1
EEE
Pet
1v
EPet1v
EvvvvERe
pt
21
0 v
0vVS n
RE n
boundary conditions at electrodes: - fixed voltage difference - No slip - reactions
RE n expexp CCR
DKH
periodic boundary conditions in x
yfyf ,2,0
Butler-Volmerreaction kinetics:
layer Stern across drop voltage:
:1y
Boundary conditions
K. T. Chu and M. Z. Bazant. 2005. SIAM J. Appl. Math. 65, 1485-1505.
1D Solutions λ=0.01
K. T. Chu and M. Z. Bazant. 2005. SIAM J. Appl. Math. 65, 1485-1505.Rubinstein & Zaltzman PRE (2000, 2003, 2005 )
1D Voltage-Current Behavior (fixed geometry & fluid properties)
Dilute
unstable
Fixed Debeye length 0.1
Stable
unstable
Streamlines for λ=.02, k=2.5, V=9.5
x
y
0 1 2 3
Unsteady flow at high voltages
Voltage-Current behavior
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
AC
Time averagedflow
ElectrodeElectrode
ACEO: Symmetric Electrodes (DC, λ=0.01, Pe=1000, V=10)
Potential
ChargeDensity
Streamlines
ACEO: Typical Streamlines (DC, λ=0.01, Pe=1000)
V=1 V=5
V=10 V=20Pos.Neg.
Neg.
Neg.
Neg.Pos.
Pos.
Pos.
Reverse the sign on the electrodes (DC, λ=0.01, Pe=1000, V=5)
Pos.
Pos.
Neg.
Neg.
Frequency response (AC, λ=0.05 Pe=1000)
Olesen et al. PRE 2005.
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.
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).