Colloidal crystals are known as a particle system which forms ordered arrays in deionized water. Colloidal crystals offer an important opportunity to study the dynamics of crystallization. In atomic or molecular fluids, the rate of attachment of particles to a growing interface is the order of picoseconds, whereas in a colloidal system it corresponds to the rate of diffusion of particles in the order of seconds. This slows growth rates significantly and allows detailed mechanism of nucleation and growth of crystals to be studied on a convenient timescale. We aimed to clarify the effects of microgravity on nucleation and growth processes using colloidal crystals as a model material. The merit of studying colloidal crystals lies in in-situ observation of growth dynamics with visible light. Using reflection spectrum method, lattice constants Do were determined based on the Bragg reflection. The intensity measurements of Bragg reflections were executed to evaluate the
appearance of small nucleates during the crystal growth under low-gravity. Light scattering methods were also applied to low-gravity experiments. Low-angle light scattering method and dynamic light scattering method were used to measure the size of grown crystallites and to evaluate diffusion coefficients of latex particles during the formation of colloidal crystals, respectively. Low-gravity experiments were executed during parabolic flights of MU-300 rear-jet airplane.
In order to clarify the influence of gravity on solidifying the metal, a copper is solidified under rapid cooling in µg by using a drop shift. A cooling system consisting of carbon crucible and copper block inserting with tin powder is
constructed for solidifying rapidly the copper sample in the crucible. The macro-structure of the solidified copper in 1 g shows that the grains grow toward the upside of center of the sample by particular orientation, whereas the orientation of the grains in µg is random. Cooling curves are measured at 0, 2 and 5 mm from the sample bottom in µ g. The shapes of them are different from the curves measured in 1 g. The temperature at 5 mm drops gradually without a constant period. The results in µg suggest that the convection in a liquid copper sample is little, and the gravity affects the solidification process even under the rapid cooling.
We carried out a microgravity experiment -Mixing of Melt of Multicomponent of Compound Semiconductor- conducted aboard the Second International Microgravity Laboratory (IML-2) Space Shuttle mission in 1994. Six types of samples, which are composed ofln-Sb (M-1, D-1) and In-GaSb-Sb (M-2, M-3, M-3', D-2), were melted and solidified. M-samples (M-1, M-2, M-3, M-3') and D-samples (D-1, D-2) were mixed by Marangoni convection due to concentration gradient and molecular diffusion only, respectively. The concentration profiles were measured by Electron Probe for Micro Analysis (EPMA) . IML-2 samples show more uniform distribution than ground samples because the buoyancy convection and gravity segregation can be suppressed under microgravity. Moreover, the experimental fact that the M-samples were more uniform than D-samples reveals that the Marangoni convection enhanced the mixing of melt. Numerical simulations also justified the quick mixing by the solutal Marangoni convection.
Thermally Driven Flow Unit (TDFU) experiment was performed as a mission of IML-2 Project. TDFU is a thermal accumulator model for Two-Phase Fluid Loop. One of TDFU experiment's purposes was to verify Liquid/Vapor phase-separation and liquid positioning at core section in TDFU vessels, and was satisfactorily achieved in IML-2 Project. Be sides, in NASDA, another vessel experiments abroad airplane was performed as preflight estimation.
This paper presents numerical simulation analysis on above exprimenal results basis, and the method of modeling the surface tension and the wall adhesion force in the absence of gravity.
The experiments of CDU of VIBES were conducted as scheduled. The first one was executed while running the VIBES system and the second without running the system. G-jitter acceleration was measured by two sets of tri-axial accelerometers on board the VIBES and the CDU . Diffusion of OH- was measured by color of phenolphtalein by using the fact that phenolphtalein turns color between pH 8 and pH 10. Measured diffusion showed a more than 20% faster rate com pared to theoretically calculated one. This means that the apparent diffusion coefficient of OH- was D*~2D in thefirst experiment and D*~3D in the second, where Dis diffusion coefficient of OH-. Measured g-jitter by the accelerometer on-board the CDU showed a stronger g-jitter level in thesecond experiment than that in the first. This corresponds that the diffusion in the second was larger
than that in the first. This suggests that there exist some convection in the liquid due to g-jitter. With regards to measuring natural convection due to residual gravity, this failed because we could not obtained clear distribution of color. The measured flow from the distribution of color, however, was opposite to that predicted.
A new phase shift interferometry was developed for precise real time observation of crystal growth both in ground and microgravity conditions. The new signal processor employs an intelligent phase unwrapping technique not only for spatial axis but also time axis. This enables the two dimensional phase distribution and three dimensional surface profile of a crystal to be obtained in real time even in the case that dust or small crystal adsorbed on the surface disturb the phase unwrapping in the previous signal processor.