Page 108 - Optofluidics Fundamentals, Devices, and Applications

Optofluidic Trapping and Transport Using Planar Photonic Devices 89

light) from the waveguide using an upright microscope, which observed
the chip from above.
When a flowing particle comes in contact with the optically
excited waveguide, it may be captured in the evanescent field and
begin moving in the direction of optical propagation. Figure 5-5d to
5-5f shows time step images of the particle becoming trapped on the
waveguide and propelled in the direction opposite the initial flow.
Using the system described here we observed particle trapping and
optical transport velocities along the waveguide as high as 30 μm/s
and capture particles flowing by as fast as 80 μm/s. We observed
approximately linear behavior of the optical transport velocity and
the guided optical power. This is roughly as expected from our quali-
tative description given earlier since the number of photon strikes
should be proportional to the optical power in the waveguide. Mov-
ies showing the transport and many more results are available from
Schmidt et al. [49].

Comments on Particle Capture Rate
As noted in the previous sections, the “capture rate” of particles
flowing over the waveguide is relatively low in this arrangement
at approximately 10% of all the particles, which overflow the
waveguide (this is better illustrated in the movies) [49]. Experi-
mentally, we observed that this capture rate increases as the flow
rate decreases and the optical power increases. The reason is that
in this experimental arrangement, a particle passing over a wave-
guide must be on a streamline that passes through a region of the
evanescent field which exerts a force on the particle greater than
the flow drag force in order to be captured (this is analogous to the
condition that a flowing particle must be on a streamline that
passes through the focal point of a free-space optical tweezer in
order to be trapped). In a low-Reynolds number microfluidic flow,
the only way in which a particle can hop streamlines is through
diffusion or when acted upon by an external impulse. Since the
average volume over which a particle will travel through diffusion
increases with the amount of time it is observed, the probability
that it will sample a streamline that passes through the evanescent
field increases with the amount of time it takes for it to flow over
it. As such the rate of capture can be increased by reducing the
flow rate as observed. Increasing the optical power increases the
strength of the trapping force at a given point in the evanescent
field and, therefore, also increases the number of streamlines that
pass through the “attraction basin.” If greater trapping probabili-
ties are desired, the simplest way of accomplishing this is to
decrease the channel size (here we use a 5-μm-tall channel). This
serves to physically confine the particles closer to the waveguide
effectively reducing the number of streamlines that do not pass
through the evanescent field.