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The Light Microscopy Module (LMM) is planned as a remotely controllable
on-orbit microscope subrack facility, allowing flexible scheduling
and control of physical science and biological science experiments
within the GRC Fluids Integrated Rack (FIR) on the International
Space Station.
Within
the FIR, an initial complement of four fluid physics experiments
will utilize an instrument built around a lightmicroscope. These
experiments are the "Constrained Vapor Bubble" experiment
(Peter C. Wayner of Rensselaer Polytechnic Institute), the "Physics
of Hard Spheres Experiment–2" (Paul M. Chaikin of Princeton
University), the "Physics of Colloids in Space–2" experiment
(David A. Weitz of Harvard University), and the "Low Volume
Fraction Entropically Driven Colloidal Assembly" experiment
(Arjun G. Yodh of the University of Pennsylvania). The first experiment
investigates heat conductance in microgravity as a function of liquid
volume and heat flow rate to determine, in detail, the transport
process characteristics in a curved liquid film. The other three
experiments investigate various complementary aspects of the nucleation,
growth, structure, and properties of colloidal crystals in microgravity
and theeffects of micromanipulation upon their properties. Key diagnostic
capabilities include video microscopy to observe sample features
including basic structures and dynamics, thin film interferometry,
laser tweezers for colloidal particle manipulation and patterning,
confocal microscopy to provide enhanced three-dimensional visualization
of colloidal crystal structures, and spectrophotometry to measure
colloidal crystal photonic properties. In addition to using the
confocal system, biological experiments can conduct fluorescence
imaging by using the fiber-coupled output of the Nd:YAG laser operating
at 532-nm, the 437-nm line of a mercury arc, or appropriate narrow-band
filtering of the FIR provided metal halide white light source.
The LMM concept is a modified commercial research
imaging light microscope with powerful laser-diagnostic hardware and
interfaces, creating a one-of-a-kind, state ofthe-art microscopic
research facility. The microscope will house several different objectives,
corresponding to magnifications of 10´, 40´, 50´,
63´, and 100´. Features of the LMM include high-resolution
color video microscopy, brightfield, darkfield, phase contrast, differential
interference contrast (DIC), spectrophotometry, and confocal microscopy
combined in a single configuration. Sample manipulation techniques
also integrated with the diagnostics are laser tweezers. The LMM provides
an enclosed workarea called the auxiliary fluids container (AFC) with
gloveports and an equipment transfer module (ETM) for transporting
experiment samples from stowage to the LMM. The multiport imaging
head on the top of the microscope provides a motorized slider to select
the sensor or sensors to which the images are directed. The AFC is
fastened to the microscope body and is sealed to provide a clean working
space and one level of containment. Gloveports allow access to the
sample area for cleaning before opening the box and experiment sample
changeout or reconfiguration. The ETM can be configured to support
various experiment modules and is located below the AFC which has
a pass-through for the samples. Materials are thus transferred without
the risk of contamination release. The ETM will be loaded with experiment
modules on the ground, and will provide contained storage until the
samples are utilized in the experiment.
Laser Tweezers
Laser tweezers will be implemented using
a custom-built system based upon a 1064-nm Nd:YAG laser, beam-focusing
optics, and two acousto-optic deflectors to steer the trap within
the field of view of the microscope. Laser tweezers simply is
the trapping of a colloidal particle using radiation pressure
by focusing a laser beam through a high-numerical aperture lens
and striking the particle. Laser tweezers will be used to measure
the viscosity and viscoelasticity of the fluid. A particle will
be trapped and oscillated at a fixed frequency. When this is done,
the centroid of the trap and particle will not coincide; the difference
in the two positions through the scan provides the driving force.
Using that information along with the motion, both linear and
nonlinear viscoelastic properties can be computed.
Confocal Microscopy
Confocal microscopy will be implemented
using a 532-nm frequency-doubled Nd:YAG laser, a confocal scanner,
and an 8-bit digital CCD camera. The scanner will allow 30 frames
per second of confocal images to the CCD camera. The crystal's
three-dimensional structure is reconstructed by assembling the
slices with an image analysis program, from which colloidal growth,
structure, and dynamics can be measured. The confocal module will
be attached and aligned to the side of the LMM and will access
the sample through an auxiliary port on the Leica RXA. The microscope’s
reflected light turret will contain a reflecting mirror to direct
the light to and from the sample.
The engineering, design, and development of the LMM is being
performed under NASA contract NAS3-99155 (Federal Data Corporation). |
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LMM/CVB
Qualification Model #2 |
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Brightfield
image of colloid particles manipulated by laser tweezers. |
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The particles
are dyed with rhodamine in order to make them visible for confocal
fluorescence microscopy. About 100 image slices are combined
to determine the particle positions in a volume. |
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