Page 154 - Optofluidics Fundamentals, Devices, and Applications

Optofluidic Colloidal Photonic Crystals 129

derivatives (such as inverse opal) lead to interesting applications in
microfluidic continuous reactors by incorporating catalysts at the
surfaces of the colloidal crystals [42]. Both mixers and reactors are
important units in optofluidic systems to extend their applications to
lab-on-a-chip or micro-TAS based on optics. Therefore, in the near
future, we expect colloidal crystals to act as multifunctional units in
integrated optofluidic systems.

6-5 Summary
Colloidal crystals have a photonic stop band that results from the
periodic modulation of the refractive index at the half-wavelength
scale of interacting light. This stop band can be controlled by
infiltration of fluids into the crystal interstices. Therefore, the
integration of colloidal crystals into microfluidic systems is important
in order to exploit this property. Integration can be simply achieved
by evaporating a colloidal suspension in microchannels or capillaries
with one open end. Here, crystallization leads to a close-packed fcc
structure, which has a volume fraction of colloids of 0.7404. However,
evaporation-induced crystallization has many disadvantages. First of
all, the crystallization process is too slow because evaporation occurs
only at the small opening. In addition, soft spheres can induce cracks
or gaps between the walls and the crystals because they can form
non-close-packed crystals before the complete evaporation of the sol-
vent. Moreover, the generation of open gaps between the colloidal
crystals and the channel walls represents a severe problem in
optofluidic applications. Because of the low hydrodynamic resistance
at the gap, the majority of the fluid flows through the gap instead of
through the crystal interstices. To solve these problems, centrifugal-
force-induced crystallization was developed. Colloidal particles
located in the rotating centrifugal chip move radially outwards. These
particles are arranged into close-packed crystals much faster than
with the evaporation-based method. Here, the crystallization time is
determined by the particle size, the rotation speed, and the density
contrast between the particles and the solvent. The colloidal crystals
prepared in the centrifugal chips can be directly used as optofluidic
devices, and hybrid colloidal crystals of different sizes and materials
can also be prepared. However, in order to increase the flexibility of
optofluidic systems containing colloidal crystals, crystallization
should be located at a desired area. To achieve this, electrowetting is
applied, whereby the microfluidic channel is combined with the
electrode pattern. Electrowetting enables us to move the colloidal
suspension into the desired position, and thus pixellate the colloidal
crystals in electrically addressable microfluidic chips.
Colloidal crystals integrated in microfluidic devices can also be
used as refractive index sensors because the reflection spectra of the