European Space Agency experiments on thermodiffusion of fluid mixtures in space.

This paper describes the European Space Agency (ESA) experiments devoted to study thermodiffusion of fluid mixtures in microgravity environment, where sedimentation and convection do not affect the mass flow induced by the Soret effect. First, the experiments performed on binary mixtures in the IVIDIL and GRADFLEX experiments are described. Then, further experiments on ternary mixtures and complex fluids performed in DCMIX and planned to be performed in the context of the NEUF-DIX project are presented. Finally, multi-component mixtures studied in the SCCO project are detailed.

Gradient-driven fluctuations in microgravity.

Equilibrium fluctuations of thermodynamic variables, such as density or concentration, are known to be small and typically occur at a molecular length scale. In contrast, theory predicts that non-equilibrium fluctuations grow very large both in amplitude and spatial size. On earth, the presence of gravity and buoyancy forces severely limits the development of the fluctuations. We will present the results of a 14-year long international collaboration on an experiment on non-equilibrium fluctuations in a single liquid and in a polymer solution under microgravity conditions. Non-equilibrium conditions are generated by applying a temperature gradient across millimetre-size liquid slabs. Phase modulations introduced by fluctuations are measured using a quantitative shadowgraph method, with the optical axis parallel to the temperature gradient. Thousands of images are analysed and their two-dimensional power spectra yield the fluctuation structure function S(q), once data are reduced accounting for the instrumental transfer function T(q). The mean-squared amplitude of the fluctuations exhibits an impressive power-law dependence at larger q and a crossover at low q showing that the fluctuation size is limited by the sample thickness. The shape of the structure function, its increase due to removing gravity, and its dependence on applied gradient are in reasonable agreement with available theoretical predictions.

Physics of Hard Spheres Experiment: a general-purpose light-scattering instrument.

A general-purpose, multifunction light-scattering instrument has been developed at the NASA Lewis Research Center for Space Shuttle and Space Station colloid crystallization and other microgravity experiments. For a single sample, the instrument can measure two-dimensional Bragg scattering from 0.5 degrees to 60 degrees , dynamic and static light scattering from 10 degrees to 170 degrees , the shear modulus of samples before and after crystallization, and digital color images of the sample. A carousel positions any one of eight 3-ml samples into the test position for separate experiments. Program challenges and flight results from the STS-83 Space Shuttle mission are discussed.

Multiple-scattering suppression by cross correlation.

We describe a new method for characterizing particles in turbid media by cross correlating the scattered intensity fluctuations at two nearby points in the far field. The cross-correlation function selectively emphasizes single scattering over multiple scattering. The usual dynamic light-scattering capability of inferring particle size from decay rate is thus extended to samples that are so turbid as to be visually opaque. The method relies on single-scattering speckle being physically larger than multiple-scattering speckle. With a suitable optical geometry to select nearby points in the far field or equivalently slightly different scattering wave vectors (of the same magnitude), the multiple-scattering contribution to the cross-correlation function may be reduced and in some cases rendered insignificant. Experimental results demonstrating the feasibility of this approach are presented.

Imaging crystals, polymers, and processes in water with the atomic force microscope.

The atomic force microscope (AFM) can be used to image the surface of both conductors and nonconductors even if they are covered with water or aqueous solutions. An AFM was used that combines microfabricated cantilevers with a previously described optical lever system to monitor deflection. Images of mica demonstrate that atomic resolution is possible on rigid materials, thus opening the possibility of atomic-scale corrosion experiments on nonconductors. Images of polyalanine, an amino acid polymer, show the potential of the AFM for revealing the structure of molecules important in biology and medicine. Finally, a series of ten images of the polymerization of fibrin, the basic component of blood clots, illustrate the potential of the AFM for revealing subtle details of biological processes as they occur in real time.

The effect of succinate on the translational diffusion coefficient of aspartate transcarbamylase.

Employing a differential optical mixing spectrometer, we have determined that the translational diffusion coefficient (DT) of aspartate transcarbamylase (AT-Case) decreases by (4.1 plus or minus 0.6)% in the presence of succinate and carbamyl phosphate. This result, combined with the change in the sedimentation coefficient determined by Gerhart and Schachman (1968) and repeated by us in the present work indicates that ATCase experiences an increase in frictional coefficient of approximately 4% due to succinate and carbamyl phosphate, and that any change in the enzyme's partial specific volume (v) under these conditions is less than about 0.3%. We have also measured (DT)20,w for ATCase as (3.75 plus of minus 0.11) x 10-7 cm2/sec. Combining this with our measured value of s20,2-o for ATCase of (11.7 plus or minus 0.2) x 10-13 sec and the calculated value of v of 0.738 cm3/g (Rosenbusch and Weber, 1971), we have determined the molecular weight of ATCase as (2.9 plus of minus 0.1) x 10-5. We have also observed the ATCase dimer and find that at a dimer concentration of 0.6 mg/ml the value of s20,w for the dimer is 15.8 x 10-13 sec and that this value decreases by (4.0 plus or minus 0.5)% upon the addition of succinate and carbamyl phosphate, a behavior essentially identical with that of the monomer.