Page 153 - Optofluidics Fundamentals, Devices, and Applications

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frequency at the end of the inner capillary is twice that at the middle
capillary, two droplets will be confined into a single-shell drop.
Likewise, four or more droplets can also be confined into the shell
drop by controlling the relative generation frequency through the
flow rates.
Microcapsules containing colloidal crystals in the liquid state can
be used as tunable color pigments because the lattice constant of the
crystal in the liquid state can be modulated by an external stimulus,
such as a magnetic field, if the colloids contain magnetic nanoparticles
[33,40]. In addition, high-throughput encapsulation without material
loss will become a useful technique in many areas including drugs,
cosmetics, and microreactions.

6-4 Conclusions and Outlook
Colloidal photonic crystals integrated in microfluidic devices are useful
for many optofluidic applications, such as chemical and biological
sensors or lasing cavities. This is because of their unique bandgap
properties. However, there are many challenging issues that have to be
solved to realize an optofluidic platform with colloidal crystals. First of
all, the integration of colloidal crystals into optofluidic devices at a
desired position is still difficult, although many integration techniques,
such as evaporation-induced crystallization, centrifugal-force-induced
crystallization, and electrically addressable crystallization, have been
developed. In addition, the thickness of the crystal should be above the
penetration depth of Bragg diffraction to achieve a high performance.
This thickness should be on the order of 100 μm due to the small index
contrast between the colloidal particles and the infiltrated fluid. Above
all, the colloidal crystals should fill the channels completely, without
leaving any gaps between the channel wall and the crystal. Because the
hydrodynamic resistance in the gap is much smaller than that in the
colloidal crystal, most of the fluid will pass through the gap without
replacing the preoccupied fluid in the interstices if a gap exists. This
means that effective tuning of the bandgap is impossible. Also, the
crystal should have a large stiffness to prevent drift of the colloids by
flow. Especially, a high flow rate of a fluid with a high affinity for
colloids (for example, silica particles and ethanol) will easily wash
away the particles. Therefore, an interconnection between the colloidal
particles is required, which is achievable by annealing or neck formation
through an etching process.
Once the colloidal crystals have been properly incorporated into
the optofluidic devices, they can be used not only as bandgap
materials but also as mixers or reactors for microfluidic units. A struc-
tured flow path for the colloidal crystals (at the submicron scale) can
effectively induce mixing of the neighboring streams [41]. In addition,
the high surface-area-to-volume ratios of colloidal crystals and their