Page 95 - Optofluidics Fundamentals, Devices, and Applications

76 Cha pte r F i v e

F drag F prop

F trap

Optofluidic transport
• Light in a waveguide
exerts 2 forces on a
particle. A trapping force
that pulls it down and a
scattering force that
pushes it along.

Waveguide

FIGURE 5-1 Schematic of the optofl uidic transport of particles on a solid-
core waveguide. The particles are trapped and then pushed along the
waveguide surface via radiation pressure forces.

waveguiding devices rather than traditional free-space laser tweezing.
Mechanistically, optofluidic transport is the combination of two
unique phenomena: near-field optical trapping to attract a particle to
the waveguide and radiation pressure to perform all forms of species
handling including transport, concentration, and separation. The use
of dielectric waveguides eliminates axial dispersion of the optical
field, allowing us to apply the optical impulse over indefinitely long
distances, as opposed to free-space systems, which are limited by the
depth of focus of the objective lens. As we describe in detail in this
chapter, optofluidic transport has a number of unique properties that
give it several advantages over traditional microfluidic transport
techniques, like pressure-driven flow and electrokinetics. The three
most significant of these are
1. Favorable transport scaling laws: As the size of the device gets
smaller, the propulsive velocity can increase.
2. Extremely strong velocity dependence on particle size: The propul-
sive velocity has as much as a fifth power dependence on
particle size, which exceeds the state of the art in separation
techniques by at least 600%.