Interdisciplinary Applied Mathematics

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Colloidal Aggregation

The colloidal particle aggregation on a charged surface under an electric field was first reported in (Richetti et al., 1984). Subsequent experiments

FIGURE 13.6. Colloid arrays obtained by electrodeposition on nonpatterned ITO substrates for 3.88 pm size polystyrene (PS) particles at a DC potential of 20 V. Times of electrodeposition were (a) 30 s, (b) 90 s, and (c) 210 s. (Courtesy of E. Kumacheva.)

on electrophoretic deposition have also confirmed the migration of colloidal particles toward each other over very large distances (even greater than five particle diameters) to form highly ordered two-dimensional structures. This long-range attraction between colloidal particles close to electrodes has been observed in both AC and DC electric fields (Bohmer, 1996; Trau et al., 1996; Trau et al., 1997). In addition, different two-dimensional colloidal phases (analogous to) gas, liquid, and solid (crystal structures) can be formed on the surface of the electrode by manipulating the current density, which alters the magnitude of lateral attractions between particles. In the following we review observations by (Bohmer, 1996) regarding the selfordering of colloidal particles on electrode surfaces:

1. The aggregation occurred after the particles were close to the surface or deposited.

2. The aggregation was reversed when the polarity of the applied field was reversed.
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20 V    on    ITO    electrodes    (Golding et    al.,    2004).    Evolution    of    the colloidal

system from a gaseous state in Figure (a) to a liquid phase in Figure (b) and finally to a crystalline phase in Figure (c) can be seen. The colloidal assembly follows a three-stage process. In the initial nucleation stage, the colloidal particles randomly deposit on the electrode surface (Figure a). Following deposition, the colloidal particles laterally migrate on the electrode surface, forming dyad- and triad-like structures, thereby increasing the surface area (Figure b). In the final stage, small particle clusters begin to merge to form two-dimensional islands (Figure c). It is important to understand that although the Brownian motion tends to redistribute and break up    the    particle    clusters    formed    after    aggregation,    it    is a    very    slow

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