Page 147 - Optofluidics Fundamentals, Devices, and Applications

124 Cha pte r S i x

with layered structures. Silica particles with a diameter of 1 μm are
used for visualization through an optical microscope. Also, optical
microscopy images of blue, green, and red photonic balls and their
reflection spectra are shown in Fig. 6-5c and d, respectively, where the
positions of the reflection peaks are matched with the wavelength
estimated from Eq. (6-4).
Moreover, using a modified device containing a pair of
microcapillaries as the inner channel, it is possible to generate photonic
Janus balls, as schematically shown in Fig. 6-5e. When differently colored
suspensions are forced to flow through the paired inner capillaries,
coalesced droplets with two hemispherical domains are generated at
the end of the paired capillaries. Similar to the method described
previously, the Janus droplets are solidified by UV irradiation and
collected at the end of the outer capillary. The resulting balls show two
distinctive reflection colors from their own hemispherical domains, as
shown in Fig. 6-5f. Here, the two differently colored suspensions are
composed of silica particles of different sizes at the same volume
fraction. This enables a matching of the viscosities of both suspensions,
thus providing stable Janus drop-forming conditions. Especially, the
extremely high viscosity of the suspension compared to that of the
continuous phase (i.e., the aqueous solution) prevents twin recirculatory
flows in the droplets, which can induce severe mixing effects between
the two suspensions, even for a slightly misaligned system. In addition,
the diffusive mixing of colloids is also inhibited due to the high viscos-
ity and the repulsive interparticle interactions. Without considering the
interparticle potential at dilute suspension, the Stokes-Einstein law
2
(D = kT/3πμd) gives a very small diffusion coefficient of O (10 m /s)
−14
0
for 200-nm silica particles dispersed in a highly viscous photocurable
resin at room temperature. This value indicates that the diffusion length
of the particle is comparable to the particle size during 1 s, which is the
time interval between coalesced droplet generation and UV irradiation.
In the case of the repulsive interparticle potential in a concentrated
suspension, the diffusion coefficient is close to zero because the structure
is interlocked by the repulsive interparticle potential.
Photonic balls prepared by the optofluidic scheme are useful in
many applications. The rotation-independent reflection colors and the
wide reflection angles of the photonic balls enable their use as reflection
color pigments for microdisplays in the reflection mode. Especially,
Janus balls—which exhibit not only optical but also electrical
anisotropies—can be used in full color E-paper with rotating balls
(called Gyricon displays) [35]. In addition, photonic balls can be used
as decorative color pigments in the cosmetic or jewelry industry.

6-3-2 Optofluidic Encapsulation of
Crystalline Colloidal Arrays
Microfluidic devices composed of cylindrical glass capillaries have
many advantages in comparison with devices based on rectangular