Page 138 - Optofluidics Fundamentals, Devices, and Applications

Optofluidic Colloidal Photonic Crystals 117

Figure 6-3b shows optical and electron microscopy images of
sectioned colloidal crystals composed of silica particles of different
sizes.

Electrically Addressable Crystallization
Electrowetting is the basic technique for actuating fluids in microflu-
idic chips. The wettability of a fluid on a substrate varies with the
applied electric field. In the absence of an electric field, the fluid
becomes stationary inside the hydrophobic microfluidic channels,
whereas under a sufficiently large field, the surface tension changes
and the fluid wets the surface. Eventually, the fluid is forced to flow
inside the microfluidic channel by capillary forces.
Shiu et al. applied the electrowetting method to manipulate the
flow of a colloidal suspension [21,22]. By using microfluidic chips
with electrode patterns, they were able to induce colloidal crystalliza-
tion in a desired region. In addition, the sequential crystallization of
colloids of different sizes enabled the simple preparation of hybrid
colloidal crystals inside microfluidic channels. Unlike the aforemen-
tioned centrifugal crystallization, in this case the length of the colloi-
dal crystal blocks can be intentionally controlled by modulating the
electrowetting. Figure 6-3c shows sectioned colloidal-crystal patterns
composed of three different sizes of colloidal particles. In addition,
combined with a multilayer pneumatic valve system, colloidal-crys-
tal patterns with multiple reflection colors could be digitally fabri-
cated in certain fluidic cells. Figure 6-3d shows the designs of the
electrodes, the flow channels, and the control channels in a pixellated
microfluidic chip. Using this complex microfluidic chip, it is possible
to fabricate patterned colloidal-crystal arrays with freely controllable
addressability. Figure 6-3e shows an optical image of the pixellated
microfluidic chip with colloidal crystals. However, in this case, the
complexity of the fabrication process still represents a significant
problem. Therefore, the remaining issue should be “how to prepare
colloidal crystals at a specific place within microfluidic chips in a
simple way?” A possible means to achieve this is the use of
photoresponsive colloidal particles [23]. Through proper surface
modification, the colloidal particles may exhibit controllable surface
properties, which can be modulated by photon irradiation. Therefore,
it will be possible to crystallize colloids at desired areas by remotely
changing the electrostatic interactions without the need for complex
electrodes or valves integrated with the microfluidic chips.

6-2-2 Applications of Integrated Colloidal Photonic Crystals

Chemical and Biological Sensors
Microfluidic systems with built-in colloidal crystals can be applied
to develop label-free chemical and biological sensors. As discussed
earlier, colloidal crystals have peculiar reflectance properties that