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Electrophoretically Deposited Polymers for Organic Electronics 365

the particle size. Under such a condition when the electric field is
applied, the particles move relative to the liquid phase. In the second
step (deposition), the particles are collected at one of the electrodes
and form a coherent deposit on it. The deposit takes the shape
13
imposed by the electrode. Zhitomirsky has proposed a mechanism
to explain the deposition of material on the electrode. The mechanism
of deposition can be divided into three different categories depend-
ing upon the type of the material to be deposited and the nature of
the suspension used for the deposition These include charge neutral-
ization, zeta-potential lowering or electrochemical coagulation, and
particle accumulation.
Particle Charge Neutralization
According to this concept, the particles undergo charge neutraliza-
14
tion as they reach the electrode surface and become static. This
mechanism is important for single particles and monolayer deposits.
It explains deposition of powders that get charged upon salt addition
15
to the suspension, e.g., the deposition of aluminum. But the limita-
tion of this concept is that it explains the initial stage deposition from
very dilute suspensions, but it is invalid under certain conditions
such as (1) when EPD is performed for a longer time (thick deposits),
(2) when particle-electrode processes are prevented (e.g., a semiper-
meable membrane induces deposition between the electrodes), and
(3) when reactions occur at the electrode which alter the local pH.

Electrochemical Coagulation of Particles
This mechanism is based on the coagulation of the particles due to
the reduction of repulsive forces between the particles near the elec-
trode surface. The increase in electrolyte concentration around the
particles near the depositing electrode lowers the zeta potential and
1
thus induces flocculation. But this mechanism is possible only when
the deposition is in the aqueous phase where electrode reactions gen-
−
erate OH ions and is invalid when there is no increase of electrolyte
concentration near the electrode. For such cases, Sarkar and Nicholson 16
gave an explanation by considering a positively charged oxide particle/
lyosphere system moving toward the cathode in an EPD cell. Accord-
ing to this concept, the applied electric field and the fluid dynamics
distort the double-layer envelope around the particle in such a manner
that it will become wider behind and narrow in front. Moreover, the
counter-ions on the wider end start reacting with the cations drifting
toward the cathode and result in the thinning of the double layer.
Under such conditions, the next particle with the thin, leading-edge
double layer can now approach close enough for London van der Waals
attractive forces to dominate and induce coagulation/deposition.
The schematic of this mechanism is shown in Fig. 10.2.
This mechanism is, however, invalid when there is excessive con-
centration of cations near the cathode. Under such conditions, there is