Biomedical Engineering Reference
In-Depth Information
Cathode
Anode
Particle
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FIGURE 3.2
Schematic of cathodic electrophoretic deposition.
PrinciplesofElectrophoresis
Double Layer
Figure 3.3 illustrates the nature of a particle and the
surface potential
(
ψ
) associated with the
unsaturated chemical bonds that are present at the terminating grain surface [24-26]. A
double layer
forms when a charged particle in suspension is surrounded by ions of oppo-
site charge in concentrations higher than that of the bulk concentration of these ions in the
medium. The electrochemical double layer formed around the particle surface consists of an
inner
Stern layer
of counterions fixed on the particle surface, which produce the
Stern poten-
1
κ
tial
(
ψ
S
), and an outer
diffuse layer
. The
Debye length
is determined by the point at which
the effects of the electrical forces are countered by random thermal motion. The
zeta potential
(
ζ
; or
electrokinetic potential
), which is used widely to assess the magnitude of the electrical
charge at the double layer, is the electrical potential at the
slipping plane
, which is the inter-
face between the stationary adsorbed liquid layer and the mobile medium beyond it.
The stability of a suspension (its resistance to gravitational settling) is determined by the
Debye length, which is defined in Equation 3.1 [19,22,27
]
1
κ
kT
k
2
kT
εε
ε
o
o
=
=
(3.1)
∑
∑
2
2
e
n z
e
n z
i
i
i
i
where
ε
= permittivity of medium
ε
o
= permittivity of vacuum
1
κ
= Debye length
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