RESULTS
Results are discussed by class of colloid material studied.
Binary colloidal crystals: These alloy samples are dispersions of 2 differently sized particles in
an index-matching fluid. Two samples were studied: an AB 13 crystal structure and an AB 6 crystal
structure. Due to a hardware failure late in Expedition 4, the AB 6 experiment was not
completed. Unexpected “power law” growth behavior that is still under investigation was
observed in the AB 13 crystal structure sample.
Colloid-polymer mixtures: These mixtures induce a
weak attractive interaction that allows precise tuning
of the phase behavior of the mixtures, and
approximate the phase separation below the critical
point of a gas-liquid mixture. The phase behavior is
controlled by the concentration of the colloid, the
concentration of the polymer, and the relative size of
the colloid and the polymer. The results from the ISS
experiments studied the spinodal decomposition, or
phase separation near the critical point,
unencumbered by density differences of the phases.
The growth of the phase separation was studied using
both light scattering and imaging. Without gravity, the
phase separation took 30 times longer than on Earth.
The sample was mixed, then phase separation began,
gradually coarsening until the container walls
interacted with the mixture (at 42 hours) and the
colloid-rich phase wet the container wall, completely
coating it after 60 hours. Because the results follow
very similar time evolution as a shallow quench of a
binary liquid, they provide insight into the importance
of the length scale of colloidal gels; separation
depends more on coarsening rates than initial colloid
size (Bailey 2007).
Colloid-polymer gels: This sample was expected to be in a fluid-cluster state, but unexpectedly
formed a solid gel. The elastic modulus, which was estimated using the experiment’s rheology
capabilities, will be compared to ground samples. Aging characteristics of this gel were found to
be similar to those formed on Earth.
Colloid-polymer critical point: Immediately after mixing, the colloid-polymer critical point
sample began to separate into 2 phases—1 that resembled a gas and 1 that resembled a liquid,
except that the particles were colloids and not atoms. The colloid-poor regions (the colloidal
“gas” phase) grew bigger until, finally, complete phase separation was achieved and there was
just 1 region of each—a colloid-rich phase and a colloid-poor phase. None of this behavior can
This photo was taken with the 1X color
camera of the AB 6 sample after
crystallization had occurred. The different
colors are the result of different
wavelengths of the white light illumination
satisfying the Bragg condition at different
angles relative to the lights. Bright spots
are large crystallites. Diffuse color occurs
are due to small crystallites. This image
was taken prior to launch. The circular
outline is the 2 cm outside diameter of the
sample cell. NASA’s Glenn Research
Center image.