be observed in the sample on Earth, because sedimentation would cause the colloids to fall to
the bottom of the cell faster than the de-mixing process could occur. Knowledge gained from
these runs was used to develop the BCAT-3 later operated on ISS.
Fractal gels: Fractal gels may form when charged colloids have their electrostatic repulsions
screened out by the addition of a salt solution, permitting aggregation. These can be formed at
very low volume fractions and form highly tenuous aggregates that exhibit a remarkable scaling
property—their structure appears the same on all length scales up to a cluster size, and so can
be described as a fractal. It was thought that the samples studied (colloidal polystyrene and
silica gel) would, in the absence of sedimentation effects, ultimately form a continuous network
of fractal aggregate; the polystyrene fractal sample never fully gelled as expected. Initial
indications are that the volume fraction tested was too low. Large fractal clusters did grow
(larger than they do on Earth), allowing measurement of the internal vibration modes of these
structures. The silica gel is thought to have gelled, and is currently being evaluated.
Colloidal glass: These samples are still under evaluation. Comparison to samples formed in one-
g in the laboratory were needed to understand whether the crystallization observed was due to
poor mixing or was a true microgravity phenomena.
PUBLICATION(S)
Bailey AE, Poon WC, Christianson RJ, et al. Spinodal decomposition in a model colloid-polymer
mixture in microgravity. Physical Review Letters. 2007;99(20):205701. doi:
10.1103/PhysRevLett.99.205701.
Manley S, Davidovitch B, Davies NR, et al. Time-dependent strength of colloidal gels. American
Physical Society - Physical Review Letters. 2005;95(4):048302(4). doi:
10.1103/PhysRevLett.95.048302.
Manley S, Cipelletti L, Trappe V, et al. Limits to gelation in colloidal aggregation. Physical Review
Letters. 2004;93(10):108302-1 - 108302-4. doi: 10.1103/PhysRevLett.93.108302.
Doherty MP, Bailey AE, Jankovsky AL, Lorik T. Physics of colloids in space: Flight hardware
operations on ISS. AIAA 2002-0762, 40th Aerospace Sciences meeting, Reno, NV; January 14-17,
2002.
Weitz DA, Bailey A, Manley S, et al. Results from the physics of colloids experiment on ISS. NASA
Technical Publication; 2002.
Sankaran S, Gasser U, Manly S, et al. Physics of colloids in Space-2 (PCS-2). Conference and
Exhibit on International Space Station Utilization, Cape Canaveral, FL; October 2001.
This investigation is complete and all results are published.