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The Francois Frenkiel Award Lecture:
Fundamental aspects of Concentration Polarization
Todd Squires
Aditya Khair
Chemical Engineering
University of California, Santa Barbara
Copyright Aditya Khair and Todd Squires, 2009
Diffuse ions
Electrokinetic effects
Electric
fields
fluid flow,
particle motion
Copyright Aditya Khair and Todd Squires, 2009
Diffuse ionsElectro-osmotic
flow
Field-driven motion
qE
Electrokinetic effects
Electricfields
fluid flow,particle motion
Paul et al. ‘99
Copyright Aditya Khair and Todd Squires, 2009
Electrokinetic effects
Diffuse ionsElectro-osmotic
flow
Field-driven motion
qE
Electricfields
(Wikipedia)
Electrophoresis
Paul et al. ‘99
E
Happy 200th Birthday!
“Notice sur un nouvel effet de l'électricité galvanique”
F.F. Reuss 1809
fluid flow,
particle motion
Copyright Aditya Khair and Todd Squires, 2009
Diffuse ionsElectro-osmotic
flow
Field-driven motion Flow-driven fieldsStreaming potential
Electrokinetic effects
qE
Electricfields
(Wikipedia)
Electrophoresis
Paul et al. ‘99
Happy 200th Birthday!
“Notice sur un nouvel effet de l'électricité galvanique”
F.F. Reuss 1809
E
fluid flow,particle motion
Copyright Aditya Khair and Todd Squires, 2009
Broader impacts of electrokinetics researchConvective charge separation in low-conductivity fluids
Streaming potential
Copyright Aditya Khair and Todd Squires, 2009
Convective charge separation in low-conductivity fluids
QuickTime™ and a decompressor
Broader impacts of electrokinetics research
Source: youtube
Potentially highly transformative, in the exothermic sense
My Outreach to you: please remember to ground your gas containers.
decompressorare needed to see this picture.
Copyright Aditya Khair and Todd Squires, 2009
A modern renaissance in electrokinetics
Microfabrication:Fields and flows provide many new ways to
control micro & nano-scale transport
DNA translocation Nanofluidic Diode Energy Harvesting
Yossifon & Chang 2008
Overlimiting Currentsin Electrochemical Systems
Concentration Polarizationat Micro/Nano Interfaces
Leinweber et al 2006
Salt “de-mixing”
Kim et al. 2005
Karnik et al. 2007Keyser et al. 2006 van der Heyden et al. 2007
QuickTime™ and a decompressor
are needed to see this picture.
Copyright Aditya Khair and Todd Squires, 2009
How most of you view water.
Copyright Aditya Khair and Todd Squires, 2009
Water dissolves ions...
OHHNa
+
+
-
NaCletc.
+
-
Copyright Aditya Khair and Todd Squires, 2009
Ions ‘screen’ charged surfaces
screening length λλλλD varies from ~1 – 103 nm
PotentialDrop
Copyright Aditya Khair and Todd Squires, 2009
Electric fields: body force on fluid
net force within screening cloud
Copyright Aditya Khair and Todd Squires, 2009
Electro-osmosis: field drives flow
‘slip’ velocity (conveyor belt)
Copyright Aditya Khair and Todd Squires, 2009
“Standard Model” for electrokinetics
(2) Electrostatics
(1) Fluid flow: low-Re flow
Coupled,
Nonlinear
PDE’s
…what lurks behind all the cartoons I’ll show.
diffusion advection electrostatic
(3) Ion conservation
(2) ElectrostaticsCommon Approach:
Matched asymptotics
With “thin” double-layers
Copyright Aditya Khair and Todd Squires, 2009
Stroock et al 00
Microfabrication: natural surface charge inhomogeneities
Green, Gonzales, Ramos … 00
QuickTime™ and a
Karnik et al. 07
Leinweber & Tallarek 2005
Andy Pascall & TMS
Theory: J. Anderson, Ajdari, Stone, Long, Ghosal, Yariv, TMS, many many others
QuickTime™ and a decompressor
are needed to see this picture.
Kim et al. 2005
QuickTime™ and a decompressor
are needed to see this picture.
Leinweber & Tallarek 2005
Copyright Aditya Khair and Todd Squires, 2009
Surface charge discontinuity
Electro-osmotic flow:Similarity solution
outside double-layerYariv (2004)
Copyright Aditya Khair and Todd Squires, 2009
Surface charge discontinuity
Violates ion conservation!
Electro-osmotic flow:Similarity solution
outside double-layerYariv (2004)
Copyright Aditya Khair and Todd Squires, 2009
Surface charge discontinuity
Violates ion conservation!
Electro-osmotic flow:Similarity solution
outside double-layerYariv (2004)
Ion conservation:•Field pulled in from bulk
•How far downstream (L) before
standard “parallel field”?
Copyright Aditya Khair and Todd Squires, 2009
Surface charge discontinuity
Violates ion conservation!
Electro-osmotic flow:Similarity solution
outside double-layerYariv (2004)
No geometric length scale: “Healing length” emerges
•Can be very long!
Ion conservation:•Field pulled in from bulk
•How far downstream (L) before
standard “parallel field”?
Copyright Aditya Khair and Todd Squires, 2009
Universal flow problem: scale by healing length
• Ion conservation alters electrostatic boundary condition
Electric Field
x = σs/σB
Field
Lines
FlowStream Lines
Field & flow “heal” over length scale λλλλH=σ=σ=σ=σs/σσσσBKhair & Squires, JFM 08
Copyright Aditya Khair and Todd Squires, 2009
Question: what happens to the ions…
Electric
Field Lines
At first:
Seems ok
Copyright Aditya Khair and Todd Squires, 2009
Question: what happens to the ions…
Electric
Field Lines
At first:
Seems ok
Reverse field: Ion accumulation
Steady state solution?
Copyright Aditya Khair and Todd Squires, 2009
Simplest
model
system
Inhomogeneous surface transport:Ion conservation Khair & Squires, PoF (08)
Electro-osmosis over a periodically-varying charged surface
•Low-ζ:ζ:ζ:ζ: can be solved exactly•Steady-state concentration and fields•Oscillatory concentration and fields•Suddenly-applied field -- evolution of “concentration polarization”
•High-ζ ζ ζ ζ also amenable to analysis•Avoids field curvature effects •Convective transport due to EOF
Copyright Aditya Khair and Todd Squires, 2009
Concentration Polarization
E
Loss of (+) ions:
Need inward field
Copyright Aditya Khair and Todd Squires, 2009
Concentration Polarization
E
Loss of (+) ions:
Need inward field
Gain of (+) ions:
Need outward field
Copyright Aditya Khair and Todd Squires, 2009
Concentration Polarization
E
Loss of (+) ions:
Need inward field
Gain of (+) ions:
Need outward field
Gain of (-) ions:
Need inward field
Copyright Aditya Khair and Todd Squires, 2009
Concentration Polarization
E
Loss of (-) ions:
Need outward field
Loss of (+) ions:
Need inward field
Gain of (+) ions:
Need outward field
Gain of (-) ions:
Need inward field
This is a cartoon. All details emerge quantitatively from analysis
Copyright Aditya Khair and Todd Squires, 2009
Concentration polarizationsuddenly-applied field
Inhomogeneous double-layer: salt source/sink disribution
Outside DL: Salt transport via diffusion
Natural time scale: ττττD∼λ∼λ∼λ∼λ2/D
t = 10-3 ττττDt = 10-2 ττττD
E
t = ττττDt =10 ττττD
λλλλ
E
Copyright Aditya Khair and Todd Squires, 2009
Convection and diffusion of salt
PeE = 10-3
+_
E
Electro-osmoticPeclet number:
+_
Copyright Aditya Khair and Todd Squires, 2009
Convection and diffusion of salt
PeE = 10-3 PeE = 1
+_
E
Electro-osmoticPeclet number:
PeE = 10 PeE = 100
Flow asymmetry gives concentration asymmetry, form salt plumes
+_
Copyright Aditya Khair and Todd Squires, 2009
Convection-diffusion: salt plumes
E
Electro-osmoticPeclet number:
Leinweber & Tallarek 2005
Salt “de-mixing”
CP + Convection
EQuickTime™ and a
decompressorare needed to see this picture.
Salt “de-mixing”
Leinweber et al 2006
Yossifon & Chang 2008
Copyright Aditya Khair and Todd Squires, 2009
Surface charge discontinuities2D diffusion from line source/sink: ill-posed.
Regularization mechanisms:•Convective transport from EOF
•Convection-diffusion boundary layer, ‘upwind’ stabilizes•Downwind: salt gradually reduced
•Plate ends, providing a line source of salt
Time scale to reach steady state: •diffusion time L2/D•convection time L/UEOF
Copyright Aditya Khair and Todd Squires, 2009
Where we’ve since gone
Electrophoresis with slip & stericsUniversal EK mobility
Of highly-charged bodies
Surface conductiondetermines mobility
Steric ions
Roughness effects
Gating in ‘tunable’ nanochannels
Remarkable Collapse!
Partial Slip Khair & Squires, PoF 09, JFM 09
Voltage
Cu
rre
nt
QuickTime™ and a decompressor
are needed to see this picture.QuickTime™ and a
decompressorare needed to see this picture.
Squires, preprint
Copyright Aditya Khair and Todd Squires, 2009
Inhomogeneous surface transport
• Surface charge gradients:– Ion conservation necessitates bulk field perturbations
– At step changes: “healing length” ~ σS /σB can be long
– Field into/out of DL creates sources/sinks of salt– Field into/out of DL creates sources/sinks of salt
– Concentration Polarization• Established over ‘macro’ time and length scales
• Nontrivially influenced by convection with EOF
– Step Change:• ill defined without regularization
– Trailing edge - diffusive dipole
– Convection with EOF - boundary layer
Copyright Aditya Khair and Todd Squires, 2009
AcknowledgmentsThe Frenkiel Award Committee & DFD Community
Aditya Khair, Andy Pascall, Rob Messinger
Carl Meinhart
•Khair & Squires “Fundamental aspects of concentration polarization arising from non-uniform
surface transport”, Phys. Fluids 20, 087102 (2008)
•Khair & Squires, J. Fluid Mech. 615, 323-334 (2008)
References
UCSB nanofab
Copyright Aditya Khair and Todd Squires, 2009
Concentration polarizationElectrokinetic formation of salt concentration gradients
+E
Permiselective membrane
-Salt Source
+
-
+
-
+
-
+
-
+
-
E
Permiselective membrane
High Salt
QuickTime™ and a decompressor
are needed to see this picture.
Tallarek et al 2005
E +- Salt Sink+-
+- +- +-+- E Low salt
CP in porous granules
Kim, Han et al 2007
CP across nanochannels
See e.g. Rubinstein
•Induced-charge
electrokinetics
(e.g. Chu & Bazant)
Copyright Aditya Khair and Todd Squires, 2009