A variation on the typical optical chromatography system was used to measure optical force
differentials of complex micro-particles that have been assembled or fabricated using bead
chemistries, bio-molecule tethers, or biological bead coatings. A number of bio-inspired particle
types have been created to help elucidate the origin of optical force differentials that are known
or suspected in biological systems such as bacterial cells / spores, and mammalian cells. A
number of optical force measurements will be presented for a variety of micro-fabricated
particles and the results and capabilities discussed.
Optical Chromatography involves the elegant combination of opposing optical and
fluid drag forces on colloidal samples within microfluidic environments to both
measure analytical differences and fractionate injected samples. Particles that
encounter the focused laser beam are trapped axially along the beam and are pushed
upstream from the laser focal point to rest at a point where the optical and fluid
forces on the particle balance. In our recent devices particles are pushed into a
region of lower microfluidic flow, where they can be retained and fractionated.
Because optical and fluid forces are sensitive to differences in the physical and
chemical properties of a sample, differences between samples and thus separations
are possible. An optical chromatography beam focused to completely fill a fluid
channel is operated as an optically tunable filter for the separation of
polymeric/colloidal and biological samples. We demonstrate this technique coupled
with an advanced microfluidic platform and show how it can be used as an effective
method to fractionate particles in an injected multi-component sample. Our
advanced microfluidic design accommodates three lasers simultaneously to
effectively create a sequential cascade optical chromatographic separation system.
In our experiments, microscopic polymer and glass spheres in microfluidic devices
are manipulated using pressure generated by a high power laser beam. The effect of
the laser on the particles and the manipulations are imaged using a microscope lens
connect to a CCD camera. Differential forces on particles of varying physical and
chemical composition have been measured. The goal is to measure the optical forces
on chemically different particles and catalog the associated chemical and physical
differences to understand which properties and mechanisms result in force
differentials. The aim is to better understand the range of optical separations that may
be possible and the extent to which the instrument can differentiate between similar
microspheres in terms of size and/or chemical composition.
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