Page 131 - Optofluidics Fundamentals, Devices, and Applications

Optofluidic Colloidal Photonic Crystals 111

environment, for example, with the refractive index or the birefrin-
gence. Especially, if one can infiltrate a fluid into the photonic crystal,
it will be easy to achieve good addressability and precise control of
the photonic properties. Also, the introduction of fluids is important
because it can transport readily an optical gain medium, such as a
fluorescent dye or quantum dots in microfluidic channels and enable
the fabrication of active optofluidic devices. This is the reason why
the combination of colloidal crystals and fluids has become such an
attractive and important subject in “optofluidics”—the rendezvous
between photonics and fluidics.
Colloidal photonic crystals are organized by the self-assembly of
building-block particles. However, the self-organization mechanism
involves intrinsic problems related to undesirable grain boundaries
and defects such as vacancies, faults, and cracks. In addition, control-
ling the crystal orientation and shaping the colloidal crystals in
regular forms bring additional difficulties in practical applications.
Therefore, in order to achieve optofluidic devices incorporating
colloidal-crystal units, it is imperative to solve all the aforementioned
problems.
At the microscale, fluids may exhibit quite different behavior
compared to that observed at the macroscale. The Reynolds number
(ρvD/μ) is very small in a microfluidic system, where ρ is the density
of the fluid, v the mean fluid velocity, D the characteristic diameter of
the capillary, and μ the fluid viscosity. This means that the flow will
always show laminar motion. In multicomponent systems, mixing is
restricted, because it is driven solely by molecular diffusion. How-
ever, if colloidal particles are packed in a microcapillary, the fluid
motion changes dramatically. In this case, the fluid must pass through
the interstices, whose sizes are much smaller than the particle
diameter. Therefore, a large pressure drop—resulting from the capil-
lary force and the large surface area which imposes no slip boundary
condition—is required to drive fluid flow. The surface treatment of
the colloidal particles represents an important point because it directly
affects the surface energy, and finally, the capillary force. Technically,
the aforementioned points are deeply related to the colloidal-crystal-
based optofluidic systems. In addition, in order to fabricate integrated
optofluidic devices with built-in colloidal crystals, we should be able
to tailor the colloidal crystals into desired shapes and locate them at
particular positions within the microfluidic system. Therefore, we
must understand both the nature of colloidal crystallization and the
behavior of fluids in small capillaries.

Evaporation-Induced Crystallization
The most basic and simple approach to the crystallization of colloidal
particles inside a capillary is evaporation-induced crystallization. If
we introduce a suspension of uniform colloidal particles at one end
of a capillary, the solvent medium will evaporate at the other
end. Therefore, the concentration of the colloidal suspension at the