Hydroxyapatite and Other Biomedical Coatings by Electrophoretic Deposition - Biological and Biomedical Coatings: Processing and Characterization - page 91

pH Range

C B A B C

E particle repulsion > E external field

~ Constant
Saturated surface charge

OVERLY STABLE

+60

Changing
(Moderately saturated surface charge)

E particle repulsion < E external field

APPROPRIATELY STABLE

+30

Enhanced
sedimentation
region
(ESR)

Unsaturated surface charge UNSTABLE

0

-30

Changing
(Moderately saturated surface charge)

E particle repulsion < E external field

APPROPRIATELY STABLE

-60

Saturated surface charge

E particle repulsion > E external field

~ Constant

OVERLY STABLE

Basic

Acidic

pH

FIGURE 3.6
Effects of surface charge on pH-zeta potential relation during electrophoresis. (Reprinted from Taib, H.,
Synthesis and Electrophoretic Deposition of Tin Oxide (SnO 2 ). Ph.D. Thesis, University of New South Wales,
2009. With permission.)

the repulsive forces between particles cannot be overcome by the electric field ( E ), thereby
hindering deposition [19]. Accordingly, it has been commented that a suspension benefits
from being unstable near the electrode [22].

SuspensionStability
Suspension Stabilization

As mentioned, the effectiveness of electrophoretic deposition depends significantly on the
suspension stability. There are two types of suspension media used for electrophoretic
deposition [21]:

1. Aqueous (water-based)

2. Nonaqueous (organic)

Traditionally, suspensions and electrophoresis have been considered in terms of colloi-
dal particles, which can remain relatively stable in suspension since Brownian motion is
able to overcome gravitational settling of these particles of size ~1 μm or less [21]. Stable
suspensions present minimal tendency to flocculate, and upon eventual settling, pack
densely owing to the near-absence of irregular agglomerates [20]. Figure 3.7 illustrates the