Purpose

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

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

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.