Purpose
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Carousel Cells |
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PCS in Express Rack |
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The PCS experiment will further the study the basic
properties of colloidal particles in an overall goal to establish the
fundamental physical principles involved in engineering colloidal materials.
In particular, the PCS experiment will study the nucleation, growth
and properties of binary colloids, the structure, stability and equilibrium
properties of polymer colloids, and the mechanical properties of large-scale
colloid aggregates.
General Experiment Summary
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| PCS On-Orbit Images |
Laser light scattering is the primary method
used to obtain measurements of the colloid particles. Eight
sample cells will be mounted in a carousel, which is used
to rotate the sample cells to three different test stations.
One diagnostic station provides for full sample color imaging,
which is accomplished via a CCD camera. Also at this station
is the fluid combination system used for combining two-part
aqueous solutions (two of the flight samples). There is a
second imaging station that provides for 10X magnification
view of a sample region. The primary test station will be
used to perform light scattering measurements; Bragg, Low
Angle Static, Low Angle Dynamic, Dynamic and Static on all
the samples. For Bragg and Low Angle scattering measurements,
a laser beam is passed through the sample cell and is scattered
by the PMMA spheres in the sample and imaged onto a spherical
screen and a small mirror or imaged on low angle optics.
For high scattering angles, Bragg reflection data are gathered
with a black and white camera. Low angle scattering data,
known as “speckles”, will be imaged by a high
resolution CCD camera. Dynamic and Static light scattering
experiments are performed in the primary test section using
a laser beam launched through a set of bulk optics to the
center of the sample cell. Scattered light is collected by
variable position fiber-optic leads and routed to avalanche
photodiodes. The photodiode outputs are sent to a digital
correlator card for further on-board processing. At the primary
test station, the sample cell is rotated via a belt-motor
system. Cell rotation at this site along with the laser and
photodiodes provides for dynamic light scattering measurement
and rheology measurements. Rheology is performed by performing
dynamic light scattering measurements while oscillating the
sample cell at different frequencies and amplitudes. The
mix motor system also is used to perform the homogenization
mixes of the non-aqueous samples (six of the samples), and
to eliminate the sedimentation that occurs while in 1-g.
Each sample will be mixed on-orbit to homogenize the sample
and initiate growth. The Bragg (high angle static ), low
angle dynamic and low angle static scattering data will be
taken on each sample right after mixing. As the growth rate
reduces the measurements are taken less often and other samples
are initiated and studied. Each sample has a growth period
of three to seven weeks. At equilibrium conditions, additional
static and dynamic light scattering data is obtained as well
as rheology measurements are performed. Throughout the growth
and equilibrium periods, color images will be taken of each
sample.
Samples
click image to enlarge
click image to enlarge
click image to enlarge
Experiment/Payload Description Research
Summary
The International Space Station provides a long-term
laboratory for understanding the behavior of colloidal
mixtures in a microgravity environment. Some colloids (a
system of fine particles suspended in a fluid) have the
ability to act like a gas, liquid, solid, or even glass,
depending on the relative concentration between the suspended
material and the solution they are suspended in, and/or
the presence or absence of gravity.
The behavior of a densely packed colloid here on earth
mimics glass in the distribution of its particles, while
in space, the same density colloid acts more closely like
a solid. This results in a highly organized, lattice-like
arrangement of particles in the colloid. The crystalline
particle arrangement within the colloidal suspension creates
the maximum amount of particle spacing, which allows for
laser-based measurements of the particle structures. The
manipulation of a colloid to alter its physical properties
is termed colloidal engineering.
As the concentration of uniformly sized hard spheres
suspended in a fluid is increased, the particle-fluid mixture
changes from a disordered fluid state in which the spheres
are moving haphazardly to an ordered crystalline state
in which they are arranged periodically. Like atoms, the
thermal energy of the spheres causes them to bump into
each other until they form ordered arrays, or crystals,
which gives each sphere the most room to move around.
On earth, at even higher concentrations, these hard sphere
systems behave like glass. Their true nature and growth
manifests itself in microgravity. This has been pleasantly
surprising and will be studied with EXPPCS hardware.
Description
Colloids can be defined as fluids with other
particles dispersed in them, particularly particles of sizes
approximately between 1 nanometer and 1 micrometer. Since
colloids have widespread uses in nature and industry, understanding
of the underlying physics that controls their behavior is
important. Under the proper conditions, colloidal particles
can self-assemble to form ordered arrays, or crystals. On
Earth, the ordering of these particles is mostly directed
by gravitational effects, sedimentation, and buoyancy. Self-assembly
does not occur. Thus, the weightlessness of low Earth orbit
is an important element in the study of colloids.
Physics of Colloids in Space (PCS) focused on the growth,
dynamics, and basic physical properties of four classes of
colloids: binary colloidal crystals, colloid-polymer mixtures,
fractal gels, and glass. These were studied using static
light scattering (for size or positions of the colloids or
structures formed), dynamic light scattering (to measure
motions of particles or structures), rheological (flow) measurement,
and still imaging. |
