Clustering of charged colloidal particles in the microgravity environment of space.

We conducted a charge-charge clustering experiment of positively and negatively charged colloidal particles in aqueous media under a microgravity environment at the International Space Station. A special setup was used to mix the colloid particles in microgravity and then these structures were immobilized in gel cured using ultraviolet (UV) light. The samples returned to the ground were observed by optical microscopy. The space sample of polystyrene particles with a specific gravity ρ (=1.05) close to the medium had an average association number of ~50% larger than the ground control and better structural symmetry. The effect of electrostatic interactions on the clustering was also confirmed for titania particles (ρ ~ 3), whose association structures were only possible in the microgravity environment without any sedimentation they generally suffer on the ground. This study suggests that even slight sedimentation and convection on the ground significantly affect the structure formation of colloids. Knowledge from this study will help us to develop a model which will be used to design photonic materials and better drugs.

Oscillations and accelerations of ice crystal growth rates in microgravity in presence of antifreeze glycoprotein impurity in supercooled water.

The free growth of ice crystals in supercooled bulk water containing an impurity of glycoprotein, a bio-macromolecule that functions as 'antifreeze' in living organisms in a subzero environment, was observed under microgravity conditions on the International Space Station. We observed the acceleration and oscillation of the normal growth rates as a result of the interfacial adsorption of these protein molecules, which is a newly discovered impurity effect for crystal growth. As the convection caused by gravity may mitigate or modify this effect, secure observations of this effect were first made possible by continuous measurements of normal growth rates under long-term microgravity condition realized only in the spacecraft. Our findings will lead to a better understanding of a novel kinetic process for growth oscillation in relation to growth promotion due to the adsorption of protein molecules and will shed light on the role that crystal growth kinetics has in the onset of the mysterious antifreeze effect in living organisms, namely, how this protein may prevent fish freezing.

Growth rate measurements of lysozyme crystals under microgravity conditions by laser interferometry.

The growth rate vs. supersaturation of a lysozyme crystal was successfully measured in situ together with the crystal surface observation and the concentration measurements onboard the International Space Station. A Michelson-type interferometer and a Mach-Zehnder interferometer were, respectively, employed for real-time growth rate measurements and concentration field measurements. The hardware development, sample preparation, operation, and analysis methods are described.

Stable growth mechanisms of ice disk crystals in heavy water.

Ice crystal growth experiments in heavy water were carried out under microgravity to investigate the morphological transition from a disk crystal to a dendrite. Surprisingly, however, no transition was observed, namely, the disk crystal or dendrite maintained its shape throughout the experiments, unlike the results obtained on the ground. Therefore, we introduce a growth model to understand disk growth. The Gibbs-Thomson effect is taken into account as a stabilization mechanism. The model is numerically solved by varying both an interfacial tension of the prism plane and supercooling so that the final sizes of the crystals can become almost the same to determine the interfacial tension. The results are compared with the typical experimental ones and thus the interfacial tension is estimated to be 20 mJ/m(2). Next, the model is solved under two supercooling conditions by using the estimated interfacial tension to understand stable growth. Comparisons between the numerical and experimental results show that our model explains well the microgravity experiments. It is also found that the experimental setup has the capability of controlling temperature on the order of 1/100 K.

Measurements of growth rates of an ice crystal from supercooled heavy water under microgravity conditions: basal face growth rate and tip velocity of a dendrite.

The growth of single ice crystals from supercooled heavy water was studied under microgravity conditions in the Japanese Experiment Module ''KIBO'' of the International Space Station (ISS). The velocities of dendrite tips parallel to the a axis and the growth rates of basal faces parallel to the c axis were both analyzed under supercooling ranging from 0.03 to 2.0 K. The velocities of dendrite tips agree with the theory for larger amounts of supercooling when the growth on the basal faces are not zero. At very low supercooling there is no growth on the basal faces. With increasing supercooling the basal faces start to grow, the growth rate changing as a function of supercooling with a power law with an exponent of about 2, with the exponent approaching 1 as supercooling increases further. We interpret the growth on the basal faces as being controlled by two-dimensional nucleation under low supercooling, with a change in the growth kinetics to spiral growth with the aid of screw dislocations with increasing supercooling then to a linear growth law. We discuss the combined effect of tip velocity and basal face kinetics on pattern formation during the growth of ice.