Page 119 - Optofluidics Fundamentals, Devices, and Applications

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as “critically unstable” represent regions where the drag force on the
particle is stronger than the calculated trapping force. In such regions,
particle trapping should not be possible, unless unrealistically pow-
erful lasers (greater than 100 W) are employed.
As can be seen, increasing the fluid velocity proves to be detrimental
to the trapping stability. It is interesting to note that successful particle
trapping depends upon maintaining fluid flow below some critical limit,
and passing the limit either renders the trapping ineffectual or enters the
critically unstable region. This barrier is significantly lower for polymer
waveguides, largely because the same amount of optical power trans-
lates into less trapping force than in a silicon waveguide. This limit can
be adjusted, however, by increasing the available power to the wave-
guide, which would result in a larger region containing a stability num-
ber greater than one. Tuning the laser power coupled in the waveguide
acts as one means of controllable release, where the trapping forces are
reduced by a decrease in the available optical power. Our model system,
as specified, provides another method of control, via tuning of fluidic
flow rates. Tuning of flow speeds directly alters the drag force on the
particle, allowing a nonoptical means of adjusting the ability of a particle
to overcome the transverse trapping field.

5-5 Optofluidic Chromatography
As the final section in this chapter we discuss the application of opto-
fluidic transport to particle chromatography. As alluded to in Sec. 5-1-2
one of the most interesting applications of optofluidic transport is in
the development of a practical optical chromatography system. The
basic idea behind such systems is that when an initially mixed group of
particles is subjected to an intense optical field, they can be separated
out (fractionated) into different homogeneous groups due to differ-
ences in the propulsion velocity based on size or dielectric constant.
The major limitation of current free-space systems is related to light-
particle interaction length problem discussed in Sec. 5-2-1. Optofluidic
transport, as mentioned, avoids this and could allow us to apply these
separation impulses over indefinitely long distances. The separation
velocity relations shown in Table 5-1 allow us to directly compare the
fractionalization resolution of this technique with the state of the art.
As can be seen in the a<<λ regime the fifth power dependence on the
particle size exceeds the nearest competitor (dielectrophoresis) by three
powers suggesting an optofluidic system should be able to achieve
higher resolution. We note, in the context of this table, that the reason
why techniques such as electrophoresis are successful is not because
they fundamentally are very size sensitive but rather that the impulse
can be applied over very long distances (electrophoresis for example is
carried out in capillaries that are tens of centimeters long).