[Colloidal Clusters]
Study on Phase Behavior of Multi-component Colloidal Clusters under Microgravity

RESEARCH OVERVIEW

• Photonic materials (PM) are innovative materials Tetrahedral colloidal clusters are key parts for the diamond-lattice structure that is known to act as a PM. In order to understand the crystallization mechanisms and determine optimum conditions for fabricating tetrahedral clusters, the Japan Aerospace Exploration Agency Multicomponent Colloidal Clusters Experiments (JAXA Colloidal Clusters) investigaiton is performed on the International Space Station (ISS).
• A goal is to use the information obtained from the experiment to create an equilibrium phase diagram of tetrahedral colloidal clusters. Also, this experiment serves to verify a new simulation program for predicting the clusterization process.
• In the future, diamond-like PM’s are constructed from these tetrahedral colloidal clusters. The PM’s are used in the future development of innovative optical devices: including optical memories, optical resonators, optical wave guides, and possibly cloaking devices.

DESCRIPTION

The Japan Aerospace Exploration Agency Multicomponent Colloidal Clusters Experiments (JAXA Colloidal Clusters) investigates the equilibrium phase diagram of colloidal clusterization, in particular, for that of tetrahedral clusters. First, positively and negatively charged colloidal suspensions are mixed and stirred into a homogenized suspension. This process requires about two to three days to form the clusters and reach the equilibrium state. The suspensions contain ultraviolet-curable gelling agents for gelation of the suspensions. The suspension is then irradiated by ultraviolet light to gelate them for about 30 minutes. After the sample cells are returned to Earth, the gels are sliced and observed by confocal microscope and neutron scattering in order to characterize the clusters’ structures. The data obtained from the analysis is used to determine the phase diagram for the colloidal clusters.

SPACE APPLICATIONS

The phase diagram for particles having high refractive index, but large specific gravity (e.g., titania particles), is obtained in these experiments aboard the International Space Station (ISS). Carrying out this experiment in the microgravity environment of space demonstrates effectiveness of carrying out these types of experiments on the ISS.

EARTH APPLICATIONS

The experiment contributes to understanding of fundamental properties of tetrahedral colloidal clusters, which may have applications in future optical devices, including possible cloaking devices.

OPERATIONAL REQUIREMENTS AND PROTOCOLS

The Cargo Transfer Bags (CTBs) storing the sample cells and equipment are launched to the ISS aboard a Space-X Dragon cargo vehicle. After berthing to the ISS, the CTBs are stored in the ‘Kibo’ pressurized module. At the experimental preparation, the sample cells and equipment are taken out of the CTBs. The sample cells consist of two kinds of colloidal dispersions of 1 to 2.5 milliliter, each are held in plastic luer-lock syringes. Both dispersions are mixtures of:

1. highly pure water,
2. fluorescent-dye labeled positively charged or negatively charged mono-disperse fine particles around 1 micrometer in diameter,
3. ultraviolet-curable gelling agents,
4. ion additives
5. additives to make aggregation more fragile.

The top cover of equipment is removed, and the sample cells are placed inside the equipment. The equipment is restored in the CTBs for two to three days. The equipment is removed from the CTB and is affixed to the door of the work volume (WV) of Multi-purpose Small Payload Rack (MSPR), or MSPR2, to supply electricity to the equipment for 60 minutes.

At the completion of this step, RUN#1 is completed. The sample cells are removed and changed to the next sample cells for RUN#2. After RUN#2 is finished, the last experiment RUN#3 is carried out. The sample cells from RUN#1 to RUN#3 are stored in the sample containers and are returned to Earth via a Space-X Dragon cargo vehicle.

YAMANAKA Junpei [Nagoya City University]