Page 277 - Book Hosokawa Nanoparticle Technology Handbook

FUNDAMENTALS CH. 4 CONTROL OF NANOSTRUCTURE OF MATERIALS

Nanostructure of Structure of
suspensions Coating and drying process Thin films Applications

Surface modification
Substrate treatment Memory media

Optical device of
Coating & Drying
nanoparticles

Rheology
Anti-reflection
viscosity

Nano-composite
Shear rate

Figure 4.6.1
Thin film structures desired for various applications.

determined by a surface potential on substrates is
self-assembly. For example, the pattern formations
due to the combination of both hydrophilic and order
hydrophobic characteristics and also due to positive
and negative electric potentials are classified as self-
assembly. On the other hand, the pattern formation Self-assembly Self-organization
under uniform surface potentials is due to self-organ-
ization. The difference of self-ordering between self-
assembly and self-organization on the same substrate
is due to the dependency on surface potentials. As a
result, the pattern of nanoparticles generated on the
latent pattern of surface potentials of a substrate is disorder
due to self-assembly. On the other hand, the pattern Nonequilibrium (super saturation)
formation due to self-organization is controlled by
particle–particle interactions and transport properties Figure 4.6.2
such as the diffusion of nanoparticles and the viscos- Schematic characteristics of self-assembly and
ity of suspensions. Furthermore, the structure forma- self-organization.
tion due to self-organization, which is a rate process
under non-equilibrium, depends on both the process
time and the relaxation time of nanoparticles struc- 2. Self-ordering modeling under drying process
tures. For instance, the complicated structure forma- The mechanism of self-ordering [8] in colloidal sys-
tion of nanoparticles in coating and drying process is tem can be explained by self-organization of
mainly determined by the balance between drying nanoparticles during drying. The colloidal system
time and particle diffusion time. The drying time is a during drying on a substrate is schematically shown
sort of quenching time to fix the structure of in Fig. 4.6.3. The nanoparticles in solvent reach the
nanoparticles. The fast quenching rate results in dis- substrate with the decreasing thickness of liquid film
order structures of nanoparticles. The transition from during drying. The ordering of nanoparticles takes
disorder to order is a function of the degree of non- place just before the dry up of solvent, because of the
equilibrium. The relationship is schematically shown capillary force, the attractive force, due to the change
in Fig. 4.6.2. The structure formation due to self- of the free surface shown in the middle of Fig. 4.6.3.
organization is not well-understood because of high The fcc ordering shown in the bottom of Fig. 4.6.3
non-equilibrium and strong non-linearity. can be observed [9–14]. Here, we explain the

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