1
Ionic and Electronic DC Conduction
• Electrolyte : a substance with ionic dc conductivity
– Charge carrier = ion(ionic current), not electron (electronic
current) in metals
• Two current carrying electrodes in an electrolyte are the
source and sink of electrons: from electrons of the metal
to ions or uncharged species of the electrolyte.
• The electrode is the site of charge carrier exchange
between electrons and ions.
• Migration velocity of
– Electrons in metal = ~0.3mm/s
– Ions in solution = ~10mm/s
• Living tissue = electrolytic
conductor : both intra- &
extracellular liquids contain
ions free to migrate
Ionic and Electronic DC Conduction
• Electric current flow
– No transport of substance
– Dc current flow without changing the conductor
• Ionic current
– Transport of substance
– Externally applied dc current changes the conductor
– First near the electrode, then spread to the bulk
– Electrolytic long duration dc conductivity is a difficult
concept in a closed system
– Transfer of electric charge across the solution
electrode interphase: electrochemical reaction at
each electrode(electrolysis).
– “We must keep the phenomena in the bulk of the
solution separate from the phenomena at the
electrodes.”
2
Ionization
• Electrochemical properties
– Determined by the inclination of
an atom to attain noble gas
configuration of the outer
electron shell
• Forces acting between atoms in a solid;
– Ionic bonding
– Covalent bonding
– Metallic bonding
– Van der Waals bonding
• Electronegativity
– Relative ability of an atom to
gain electrons and become a
negative ion
– Useful to predict the strength &
polarities of ionic bonds
between atoms, and thus
possible electrochemical
reactions.
Pauling’s scale of electronegativity
for some selected atoms
3
Basic Electrolytic Experiment
4
Pt & Carbon
– There must be energy barriers in the system
– Nonlinear system, not obeying Ohm’s law
– Bulk solution obeys Ohm’s law
– Energy barrier is not in the bulk but near the electrodes
– Without dc current, no electron transfer, no chemical reaction,
no faradaic current
At the cathode
–Na+ ions migrate and are discharged ? (hint : Na+ has a very
small electronegativity)
–Two processes with non-charged species transferred by
diffusion
• Reduction of dissolved neutral oxygen : at small current
• Decomposition of water molecules : at larger current
2H2O + 2e = H2 (gas) + 2OH- (base)
• Na+ need not be considered but is necessary for the conductivity of
solution, voltage drop in the solution is not too high
– Ag+ ions are reduced, AgCl layer is decomposed, pure Ag
appears
5
At the anode
– discharge of Cl- :
• chloride is highly electronegative, but less energy is necessary for
taking electrons from the chloride ions than from water
molecules
– Neutral Cl2 gas reacts with carbon, not with Pt
– Water decomposition
2H2O = O2 (gas) + 4H+ + 4e-(acid)
– Ag is oxidized and forms more AgCl
Redox process : the transfer of electrons oxidising or
reducing species at an electrode
“ the results indicate that if we are to apply large dc
currents to tissue, and we are to use noble metals as
electrode material directly on the tissue, the passage of
dc current is accompanied by the development of H2 gas
and a basic milieu at the cathode, and Cl2 gas and
perhaps oxygen and an acidic millieu at the anode”
Electrochemical Reactions
• An electrochemical reaction is a reaction involving the
transfer of charge as a part of a chemical reaction.
• Typical electrochemical reactions in corrosion are metal
dissolution and oxygen reduction
• In contrast a chemical reaction, such as the precipitation
of a metal hydroxide, does not involve a transfer of
charge
• Note that reduction and oxidation reactions have been
shown going in one direction only. While the reverse
reactions are perfectly possible, the reverse of an anodic
reaction is a cathodic reaction and vice versa.
6
Electrical Double Layer
Electrical Double Layer
There is a tendency for charged species to be attracted to
or repelled from the metal-solution interface.
This gives rise to a separation of charge, and the layer of
solution with different composition from the bulk solution
is known as the electrical double layer.
There are a number of theoretical descriptions of the
structure of this layer, including the Helmholtz model, the
Gouy-Chapman model and the Gouy-Chapman-Stern
model.
As a result of the variation of the charge separation with the
applied potential, the electrochemical double layer has
an apparent capacitance (known as the double layer
capacitance).
7
Electrical Double Layer
--perpendicular fields
• Charge transfer
– At the electrode-liquid interface the transformation from
electronic to ionic conduction occurs
– The electrode exchanges charges with the arriving ions or
ionizes neutral substances
– Oxidization of the electrode metal; the metal ion enters the
solution
• Electric double layer (EDL)
– In the solution, at the electrode surface an EDL is formed as soon
as the metal is wetted
– In all interphases, such as metal of an electrode & the electrolyte,
tissue or gel, or at a cell surface
– There will be a non-uniform distribution of charges -> an electric
potential across the interphase
– Particularly pronounced at the interphase between a solid and a
polar medium (e.g. water)
– When the polar medium is liquid and the ion mobility high, the
formation of an EDL will take place in the liquid phase.
8
General theory of Gouy-Chapman
9
10
Solid/Electrolyte Interface
Double layer
Surface
charge
Counter
ion
• Solid surface acquires charges in aqueous environment
• Counter-ions migrate to the solid to form double layer
• Electrically modeled as a capacitor
Potential —
Zeta potential:
Potential across
double layer:
σλ charge
ζ =
∝
ε
Cdbl
Cdbl = ε λ
Concentration —
ζ
Counter-ion
Co-ion
λ
y
distance
Double layer/Debye length
Electroosmosis and
Electrophoresis
11
Electro-Osmotic Flow (EOF)
High voltages
Bubble generation, pH
gradient
P. Mruetusatorn
Several KV is needed.
E~ 104 V/cm
12
Modulation of Zeta Potential
Passive interface at channel walls
Anode
Cathode
+
-
Reactions
to conduct
electricity
Gate--Field effect, Capacitive charging,
Induced ions can be externally modulated (+/-)
Fluid Surface Velocity:
u II = −
ε
⋅ Δζ ⋅ E II
μ
double layer charge density x EII
Mixer
13
Electrophoresis
Applications
14
Capillary Electrophoresis
Principle of CE
15
Microfabricated CE
16
DC Electrokinetics (EK)
Fluids
Particles
17
Four electrokinetic effects due to the electric charge of the
double layer at the solid-liquid interphase
– Electrophoresis : the migration of charged particles through a
liquid
– Electro-osmosis : bulk liquid flow through a pore caused by a
migrating ionic sheath
– Sedimental potential : potential difference generated by
falling charged particles
– Streaming potential : potential difference when a liquid is
pressed through a pore
– Body or podermotive forces :
• Field-induced polarization will make uncharged particles move in
inhomogeneous or moving electric field,
• The resultant forces increase with the volume of the particle
– These forces are the basis of phenomena & techniques such
as “electrorotation”, “levitation”,”dielectrophoresis”, “pearl
chain formation”, “travelling wave dielectrophoresis”
Issues with DC Electrokinetics
+
• High voltages
– Bubble generation, blocking channels,
open reservoirs, losing pressure
– pH gradients
– Electric insulation
– Impractical for large network
• Poor mixing
• Velocity scales linearly with voltage
• etc
1 sec
3 sec
Power
supply
DC EO
Acid
E
Base
Electrochemical
reaction
18
Active Solid/Electrolyte Interface
V
+
_
+ Co-ions ~
+ Reactions
_
_
Charges induced by external voltages can
be positive or negative.
_
_
Counter-ions
~Capacitance
+
+
DC/AC path
V
Electrode
Rlead
V
by Reactions
AC path only
DC/AC path
Rrct
Fluid bulk, Rsolu
Cdbl: Double layer capacitance
Charge transfer paths:
Ccell: system
dielectric coupling
AC Voltage Case
– At high frequency (~1MHz) : back and forth
migration process in the bulk electrolyte will take
palce, no accumulation or reaction will take
place at the electrodes
– At low frequency (~0.1Hz) : result will depend on
the dimension of the cell and the degree of
reversibility of the reactions. Id the gas has time
to bubble away, the process is certainly
irreversible.
19
AC Electrokinetics
• Dielectrophoresis (DEP)
– Particle manipulation, positive & negative
DEP
• AC electro-osmosis (ACEO)
– Fluid force exerted by electrode/sidewall
charging
• AC electrothermal (ACET) effect
– Fluid force from thermal gradient in the
bulk
20
Vac ⋅ exp( jωt )
u II = −
ε
⋅ Δζ ⋅ E II
μ
E II =
∂
Δζ
∂x
Δζ : voltage drop in the double layer
AC EO Principle (con’t)
—Surface Vortices and Stagnation
Four vortices on an isolated electrode pair.
FEA simulation of
electric field distribution
Electrode
0
Flow
Flow
x
Electrode
a
1
xstagn = ±
(L + a )2 + a 2
2
a : half of electrode gap
L : electrode width
L+a
Nulls of tangential field
Stagnation point
E-field
+ + + + + + + + + + + + ++
+
- - - - - - - - - - - - - -
Capacitive Charging
Particle trapping at
fluid velocity minimum
21
ACEO Features
• Electric fields induce charge density on metal
surfaces
• Induced charges cause electric double layer
formation
• Electric field becomes tangential Bulk resistance
• Electric field sets the double layer in motion
Double layer
capacitance
V cos ω t
− V cos ω t
Induced-Charge Electro-Osmosis
TODD M. SQUIRES
MARTIN Z. BAZANT
22
Induced-charge electro-osmosis:
Fundamental picture
The evolution of the electric field around a solid, ideally polarizable conducting
cylinder immersed in a liquid electrolyte, following the imposition of a DC field at t =
0 (a), where the field lines intersect normal to the conducting surface.
Over a charging time, a dipolar charge cloud forms in response to currents from the
bulk, reaching steady state (b) when the bulk field profile is that of an insulator.
Steady ICEO around an uncharged
conducting cylinder
The steady-state induced-charge electro-osmotic flow around (a) a conducting
cylinder with zero net charge and (b) a positively charged conducting cylinder.
The ICEO slip velocity depends on the product of the steady field and the
induced zeta potential. The charged cylinder (b) simply involves the superposition of the standard electro-osmotic flow.
23
Simple microfluidic devices
exploiting ICEO
24
Induced-Charge Electrophoresis
of Metallodielectric Particles
Asymmetrical Particles
25
Nature Materials 6, 235 - 240 (2007)
Remotely powered self-propelling particles and
micropumps based on miniature diodes
Lab Chip, 2008, 8, 117 - 124,
Remotely powered distributed microfluidic pumps
and mixers based on miniature diodes
Orlin D. Velev’s group, NDSU
•http://www.che.ncsu.edu/velevgroup/nmat1843_movies.htm
26
27