Page 101 - Optofluidics Fundamentals, Devices, and Applications

82 Cha pte r F i v e

5-2-2 Why Is Optofluidic Transport Interesting?
We list here a number of the different fundamental and practical
advantages of optofluidic transport over the traditional microfluidic
techniques. However, before we go to this list, let’s reexamine the
limitations of optical transport described in Sec. 5-1-3 and how this
method addresses them.

1. Solution to diffraction limitation: The high refractive index of
the waveguide serves to confine the optical mode to a much
smaller cross-sectional area than the free-space diffraction
limit. As such the cross-sectional area is lower and the inten-
sity of the light is greater for a given amount of optical power.
As was demonstrated by Ng et al. [48] the waveguide can be
designed such that the peak intensity occurs at the waveguide/
liquid interface.
2. Solution to light/species interaction length limitation: Since the
mode is confined by total internal reflection in the waveguide,
the interaction length can be extended indefinitely. In tele-
communications, for example, optical fibers carry signals
over kilometer scale distances. As such it should be relatively
easy to exploit this technology to create chip-based systems
that enable optical transport over the distances required for
microfluidic devices.
In addition to addressing these fundamental challenges with
optical manipulation in microfluidic devices, we can also list a few
additional advantages that optofluidic transport may have in com-
parison with some of the more traditional micro- and nanofluidic
transport mechanisms introduced earlier. Some of these advantages
are qualitative, whereas others are quantitative and rely on knowl-
edge of some of the transport theory that is expanded on in Sec. 5-4.
We summarize all these advantages here for continuity, but refer to
the relevant sections in the rest of the text where they are expanded
upon.

1. Favorable transport scaling laws: As the size of the photonic
device gets smaller, the optical energy/intensity increases
and with it the propulsive velocity. In Sec. 5-4-3, we will show
that the transport velocity is directly proportional to inten-
sity. As such as the cross-sectional area down to which the
light is confined is decreased (thereby increasing the optical
intensity) the transport velocity will increase. Pressure-driven
flow and electroosmosis have the opposite scaling (smaller
device sizes = slower transport).
2. Strong dependence of velocity on particle size and optical properties:
As will be further explained in Sec. 5-5, we show that the
optofluidic propulsive velocity has as much as a fifth power