Page 46 - Optofluidics Fundamentals, Devices, and Applications

Basic Micr ofluidic and Soft Lithographic Techniques 27

phase can be varied independently by adjusting the pressure applied
to the gas stream, and the rate of flow of the liquid. The same device
can be used to generate liquid droplets in another immiscible liquid.

T-junction
Figure 2-9c and 2-9d illustrates the geometry of a T-junction [71,72].
Two channels merge at a right angle. The main channel carries the
continuous fluid (oil here) and the orthogonal channel supplies the
fluid that will be dispersed (water here). As the dispersed phase
penetrates into the main channel, shear forces generated by the con-
tinuous phase and the subsequent pressure gradient cause the tip of
the dispersed phase to elongate into the main channel until the neck
connecting the inlet channel with the droplet breaks. The discon-
nected liquid plug flows downstream in the main channel, while the
tip of the stream of the dispersed phase retracts to the end of the inlet
and the process repeats. The viscosity of the fluids, the interfacial ten-
sion, volumetric rates of flow of the two phases, and the geometry of
junction determine the size of the droplets or gas bubbles formed.

2-6-7 Optical Components
Because PDMS is soft and deformable, it is possible to form optical
components whose physical dimensions can be tuned mechanically
or thermally. These components can be prepared by molding PDMS
elastomers into the desired shapes. Tunable lenses and mirrors, dif-
fraction gratings, interferometric sensors, and distributed feedback
lasers have been fabricated out of PDMS [22,23,73–76]. Some of these
devices will be described in detail in later chapters.

2-7 Conclusions
We have illustrated the basic design and construction of some important
microfluidic components. Methods for the manipulation of fluids in
these microfluidic systems can be used to incorporate multiple functions
on the same chip, and to develop more complex optofluidic systems.
The fabrication of microfluidic components in PDMS is easier and
more flexible than in silicon or glass. The use of PDMS as a material
reduces the time, complexity, and cost of prototyping. Its influence on
costs of manufactured systems remains to be established, but poly-
mers are, in general, less expensive than ceramics as materials.
Some of the properties of PDMS may be disadvantageous in cer-
tain situations. For example, PDMS is incompatible with many organic
solvents; it has therefore been applied primarily to aqueous solutions.
When working with biological samples, nonspecific adsorption may
occur. The presence of nanoparticles of silica in commercial PDMS
causes undesired scattering of light. Methods to control the surface
chemistry of PDMS are being actively developed to overcome these