GRADFLEX: Fluctuations in Microgravity

We present the results of experimental investigations of gradient driven fluctuations induced in a liquid mixture with a concentration gradient and in a single-component fluid with a temperature gradient. We also describe the experimental apparatus being developed to carry out similar measurement under microgravity conditions. The GRAdient Driven FLuctuation EXperiment (GRADFLEX) involves the investigation of fluctuations induced in simple fluids and in binary mixtures by imposing a macroscopic temperature or concentration gradient under microgravity conditions. Recent experiments have shown that giant nonequilibrium fluctuations are present during diffusion processes in liquid mixtures and in the presence of a heat flux through a fluid. These fluctuations occur at all length scales between the microscopic and a macroscopic scale set by the sample dimensions. The fluctuations are due to corrugations in the diffusing front, whose fractal properties explain the presence of fluctuations involving all length scales. The fluctuations are generated by coupling between velocity fluctuations and the macroscopic gradient (concentration or temperature) which drives the flux. The amplitude of these fluctuations diverges as q , where q is the wave vector of the fluctuation. Long wavelength fluctuations are stabilized by gravity, which quenches the q -4 divergence at the smallest wave vectors. On Earth, gravity suppresses the long wavelength fluctuation below a typical cutoff wave vector. The aim of the GRADFLEX project is to investigate these fluctuations in the absence of gravity, where the long wavelength fluctuations are no longer predicted to be stabilized by gravity, and to compare the results with those obtained on Earth. Many materials science processes (for example, crystallization and growth of materials) are performed in microgravity because of advantages expected from the absence of convection. However, the presence of nonequilibrium fluctuations could lead to the unexpected presence of large scale inhomogeneities that could impair processing under microgravity conditions. Two prototype systems to guide the engineering of flight hardware have been developed, one in the Optics and Microgravity Laboratory at the University of Milan by the Istituto Nazionale per la Fisica della Materia (INFM) and one in the Physics Department at the University of California at Santa Barbara (UCSB). Both systems use the shadowgraph method to measure the fluctuations. The system developed at INFM is devoted to the investigation of concentration fluctuations occurring during a Soret induced mass diffusion process, while that developed at UCSB is designed to investigate fluctuations induced by a thermal gradient in a single-component fluid. The project is scheduled for flight in 2006 onboard the Russian satellite capsule FOTON M3. The current sensitivity of the shadowgraph method is now sufficiently developed to measure the scattering from the fluctuations, both on Earth and in microgravity. Samples are contained between parallel sapphire windows to provide the necessary thermal boundary conditions. The fluctuations give rise to phase perturbations in the wavefronts of a beam of light passing through the sample, resulting in measurable intensity modulation a sufficient propagation distance beyond the sample. This intensity modulation is time-dependent, and it can be analyzed to obtain both the mean squared amplitude of the fluctuations S(q), and their power spectrum S(q,ω), for wave vectors as small as 20 cm . Thus the method is useful well below the range where small angle light scattering is typically NASA/CP—2004-213207 50