Page 150 - Optofluidics Fundamentals, Devices, and Applications

126 Cha pte r S i x

PDMS channels. First, cylindrical channels have a relatively low
hydrodynamic resistance (R ), which is given by the ratio of volu-
hyd
metric flow rate to pressure difference. For a cylindrical channel with
radius R and length L, the resistance can be expressed as 8ηL/πR 4 ,
cyl cyl
whereas a channel with a square cross section of equal sides h has a
4
resistance of 28.4 ηL/h . Considering the typical microfluidic channel
dimensions and the similar length scales of both geometries, the
cylindrical channel has a 10-times-smaller hydrodynamic resistance
than the square channel [36]. In addition, the glass capillary can
withstand various organic solvents whereas the PDMS channel can
be deformed by some of them. Especially, the axial symmetry of the
cylindrical channel is advantageous for generating emulsion droplets
because the outer flow can induce a drag force on the inner flow,
which is equal in all azimuthal directions. Therefore, the cylindrical
capillary device can be used to generate complex emulsion system,
such as multiphase emulsion droplets.
Double emulsion droplets, which are droplets in droplets, can be
fabricated by one- or two-step drop breakup in a microcapillary
device [37,38]. On account of the geometrical benefit, double emulsion
droplets can be used as capsules for confining the materials dispersed
in a core droplet. To ensure the long-term stability of the capsules, the
core and shell phases should be stabilized by adequate surfactant
molecules, and solidification of the shell phase is required to enforce
the structure. One of the most novel and simple strategies is
photopolymerization of the shell phase. Depending on the polymerized
shell properties, the capsule can either completely prevent the
penetration of small molecules through the membrane or permit the
transfer in a controlled manner.
Optofluidic devices enable the fabrication of microcapsules with
narrow size distributions by in situ photopolymerization of the shell
phase in double emulsion droplets, as shown in Fig. 6-6a. Especially,
if the core emulsion droplets contain PS particles with a high surface
charge density, the particles can spontaneously assemble into the
crystalline phase from the smooth inner wall of the shell [39]. In the
case of rigid and compact shells, which do not permit the penetration
of ionic species, the crystal phase in the capsule has long-term stability
in spite of its fragility in an ionic environment. In Fig. 6-6b and c,
still-shot images taken at the end of the middle capillary at the
moment of double-emulsion-droplet generation and in the downstream
are displayed, respectively. While the photocurable shell phase is
transparent, the core droplet is opaque due to scattering by concen-
trated PS particles. If the polymerization occurs downstream, the
solidified shell confines the PS particles without loss and the crystal
structure of the particles shows Bragg diffraction colors (see Fig. 6-6d).
On the other hand, an elaborate control of three flow rates enables
the preparation of capsules containing a specific number of small
core droplets, as shown in Fig. 6-6e and f. If the droplet-generation