Page 97 - Optofluidics Fundamentals, Devices, and Applications

78 Cha pte r F i v e

a channel wall (electroosmosis) or a flowing particle (electrophoresis),
exhibits a more favorable scaling ratio. Outside the limit where two
electrical double layers overlap, the speed of electrokinetic transport
is more or less independent of channel height. Within the double layer
overlapped regime the velocity scales approximately with 1–1/κd [2],
where 1/κ is the double layer thickness (which varies in thickness
between 10 nm and 1 μm depending on the ionic strength of the solu-
tion) and d is the channel half-height. As such when κd is on the order of
1 (as it is in the case of many nanofluidic systems) the flow can be nearly
entirely impeded. In practice electrokinetic techniques are compatible
only with a limited class of fluids (low-ionic strength aqueous solu-
tions), exhibit extreme sensitivity to surface conditions, and generally
cannot be used with semiconductor substrates such as silicon. Current
flow through the channel results in significant Joule heating [13], which
can lead to problems ranging from nonuniform viscosity fields to cata-
strophic boiling particularly in polymeric substrates.

5-1-2 Optical Manipulation in Microfluidic Devices
Free-space optical manipulation techniques in microfluidic systems
have recently generated a significant amount of interest. Such tech-
niques range from traditional optical tweezing (see a recent review by
Grier [14], and some other papers of interest [14–18]) rotational manip-
ulation of components based on form birefringence [19] to more recent
electro-optic approach such as that by Chiou et al. [20]. As an example
of a direct device integration, Wang et al. [17] developed an optical-
force-based cell-sorting technique whereby radiation pressure was
used to direct rare cells into a separate stream following a green flores-
cent protein (GFP) detection event. Unlike the traditional transport
techniques described above, the main advantage of these optical
approaches lies in their ability to handle individual particles directly, as
opposed to indirect manipulation of the surrounding flow field.
Broadly speaking, although very complex manipulations have
been demonstrated, the majority of optical tweezer-based implemen-
tations tend to be “binary.” This means that they rely on the ability
either to trap or not to trap a particle based on whether the conditions
for trapping stability are met [21–23]. Recently, however, a number of
works have extended these ideas to exploit the dependence of this
trapping potential on the particle properties, enabling much more
advanced and subtle operations. As an example, MacDonald et al.
[24] demonstrated an optical lattice technique where particles of dif-
ferent sizes were sorted into different streams depending on their
strength of attraction or repulsion to regions of high optical intensity.
In a series of papers, Imasaka and coworkers [25–28] provided the
initial foundations for optically driven separation techniques, which
they termed optical chromatography. In optical chromatography (see a
recent review by Zhao et al. [29]) a loosely focused laser beam is incident