Page 21 - Optofluidics Fundamentals, Devices, and Applications

2 Cha pte r O n e

1-2 What Is Optofluidics? A Historical Perspective
Remarkably, the definition of optofluidics has evolved significantly
over the few years that it has been in existence. The term “optofluidics”
first appeared in the name of a University Research Center funded
by the Defense Advanced Research Projects Agency (DARPA) in
2003. The charter of the center was to “develop adaptive optical cir-
cuits by integrating optical and fluidic devices.” This optics-centric
definition points to an interesting aspect of this field’s origin—optics
researchers were trying to incorporate microfluidic technologies
into their research to create novel optical devices. It was recognized
from the start that microfluidic technologies can enable changeable
and reconfigurable optical devices (see Chaps. 2, 3, and 4 for some
examples).
It quickly became apparent that microfluidics can bring other
advantages to bear. In Ref. [1], several other aspects of fluidics were
identified as key advantages for optofluidics: “the ability to change
the optical property of the fluid medium within a device by simply
replacing one fluid with another; the optically smooth interface
between two immiscible fluids; and the ability of flowing streams of
miscible fluids to create gradients in optical properties by diffusion.”
The focus of optofluidics on the creation of novel optical devices
remained.
A review paper in 2007 [2] marked the shift to a more symmetric
definition in which the advantages of optofluidic technologies were
discussed as beneficial to both the optics and the microfluidics fields.
In the present context, an appropriate description of optofluidics
would be to broadly define it as the combination of optics and micro-
fluidics in the same platform to leverage specific advantages of these
two disciplines.

1-3 Fluidic Advantages
There are numerous advantages associated with fluid media that
optofluidic researchers have utilized. In this section, we shall look at
some of these features.

1-3-1 Immiscible Fluid-Fluid Interfaces Are Smooth
It has long been recognized that the optical smoothness of fluid inter-
faces can be a useful and cost-effective way to create optical surfaces.
Due to surface tension, an immiscible fluid-fluid interface is uniform
and smooth. Liquid telescope mirrors that are created by spinning
large dishes of mercury work on this principle [4]. On a much
smaller scale, most optofluidic lens projects, likewise, make use of
this principle (Chap. 9). It is worth noting that the meniscus between
two immiscible fluids of equal density in a column is perfectly