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Optofluidic Trapping and Transport Using Planar Photonic Devices 77

3. Insensitivity to surface/solution conditions: Unlike electroos-
mosis, for example, this technique is largely independent
of surface/solution conditions and can be used for a much
broader class of bioanalytical operations.
In this chapter we begin by reviewing existing micro- and
nanoscale transport mechanisms and discuss the advantages of
optofluidic transport in the context of this state of the art. Following
this we present a review of a number of recently published optoflu-
idic transport architectures and introduce our own technique using
SU-8 waveguides and polydimethylsiloxane (PDMS) microfluidics.
A theoretical description of the transport is then provided and used
to back up the advantages purported above. The final section dis-
cusses the application of this technique to a specific application
area, namely, optofluidic chromatography.

5-1 Optically Driven Microfluidics

5-1-1 A Brief Review of Traditional Transport Mechanisms
in Microfluidic Devices
On length scales relevant to transport in micro- and nanofluidic
devices, fluid flow and species transport can be accomplished by a
number of elegant techniques, a few of which include pressure-driven
flow [1], electrokinetics [2–5], buoyancy [6], magnetohydrodynamics
[7], capillarity, electrowetting [8],and thermocapillarity [9] (see Stone
et al. [10] or textbooks by Nguyen and Wereley [11] or Li [5] for more
details). Of these techniques the former two are the most commonly
exploited, largely because of the relative ease with which they can be
implemented. Pressure-driven flow is likely the simplest to imple-
ment, requiring only a pressure or vacuum source to generate flow,
and is compatible with a broad range of fluid and surface conditions.
On-chip valving techniques such as those used in multilayer soft
lithography [1] enable precise and highly parallel flow control and
sample routing down to the scale of approximaely 1 μm. Since the
average velocity of a pressure-driven flow scales with the square of
the critical channel dimension, controlled manipulation of length
scales much smaller than this is exceptionally difficult. Another limi-
tation of pressure-driven transport is that it exhibits a parabolic veloc-
ity profile meaning that the flow is faster in the center of the channel
than at the edges near the walls. This causes an effect known as dis-
persion [12] (essentially the spreading out of a transported sample
because parts of it are moving faster than others), which is undesir-
able in many separation and some transport applications. Electroki-
netic transport, where flow is induced through the interaction of an
applied electric field and the charge in the electrical double layer near