Page 116 - Optofluidics Fundamentals, Devices, and Applications

Optofluidic Trapping and Transport Using Planar Photonic Devices 97

Direction of flow Flow
field
600-nm particle

Streamlines
Waveguide

Optical
field

(a)

70

60
Propulsion force (pN) 40 F EM α a 2
50

30

20
10
0

0.5 1.0 1.5 2.0 2.5 3.0
Particle diameter (μm)
(b)
FIGURE 5-7 Forces on a particle trapped on a waveguide. (a) Cross section
of guided mode in the waveguide and streamlines for a particle trapped on
the waveguide and subject to a crossfl ow. (b) Propulsion force computed on
particles trapped on the waveguide. (See also color insert.)

affects the transport of a particle along a waveguide in a number of
ways. Most importantly it can serve to break the trap in that if the
random thermal energy delivered to the particle exceeds the strength
of the optical confinement, the trap is considered unstable and the
particle will diffuse away. In this section we focus on describing how
the conditions under which trap breaking will occur can be predicted,
allowing us to define the conditions under which stable optofluidic