Page 23 - Optofluidics Fundamentals, Devices, and Applications

4 Cha pte r O n e

example, depend on the switching of laser dye medium as a way to
accomplish wavelength tuning.
Easy fluid transport is also useful for “renewing” optofluidic
devices—an advantage that solid devices do not possess. Specifically,
as and when the fluid media in an optofluidic device deteriorates, we
can easily infuse the device with fresh fluid replacements. This advan-
tage is very useful for optofluidic lasers as the lasing media in such
devices need to be replaced when the dyes are bleached.
Finally, easy fluid transport enables the intriguing possibility of
on-chip chemical analysis and synthesis by providing an easy means
for inputs and transport. See Chap. 16 for a discussion on this
topic.

1-3-4 Fluid Can Be an Excellent Buoyancy Mediator
The density of fluid media ranges widely—mercury has a density of
3
13.6 g/cm while pentane has a density of 0.63 g/cm . By mixing two
3
miscible fluids, we can create fluid with arbitrary intermediate density
values.
The buoyancy of fluid facilitates manipulation of small objects
that are suspended in a suitable fluid medium. Optical tweezer
technology (Chaps. 5 and 15) provides an excellent illustration of
this advantage. Optical tweezing force is relatively weak in com-
parison with gravitational pull. It is only by neutralizing the
impact of gravitational pull by suspending objects in fluid that we
can manipulate these objects by optical tweezing. The assembly of
colloidal photonics crystal (see Chap. 6) is another good example
of an application where neutralizing gravitational pull by using
fluid is important.

1-4 Optical Advantages
Optics brings a complementary (and sometimes, overlapping) set of
advantages to optofluidics. In this section, we shall look at some of
these features.

1-4-1 Numerous High-Sensitivity Optical
Sensing Techniques Exist
The range of light-matter interaction mechanisms is remarkably broad;
to name a few of these mechanisms—fluorescence, phosphorescence,
Raman scattering, polarization, elastic scattering, refraction, second
harmonic generation, and stimulated emission. These mechanisms
form the basis of optical sensing methods that are broadly used for
chemical and biological sensing, because of their fast response and
high specificity and sensitivity that are ideal for sensing applications.
For example, fluorescence and Raman scattering are commonly
used tools to probe the dynamics of biological processes.