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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-4, 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 2008 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-1. Thus the method is useful well below the range where small
angle light scattering is typically impossible because of stray light
and other effects. The resulting data are the product of S(q) and
the shadowgraph transfer function T(q) = Sin2 (q2z/2ko).
Objective
- Study gradient driven density and concentration
fluctuations that are strongly enhanced in fluids by the absence
of gravity.
- Achieve a quantitative understanding of gradient
driven fluctuations, both on Earth and in the microgravity environment
provided during a Foton-M3 mission.
Relevance / Impact
- In reduced gravity, gradients drive giant fluctuations that may
impact processes such as crystal growth.
- This experiment was featured on the front-cover of the April
1, 2006 issue of Applied Optics.
Development Approach
- ESA / ESTEC is funding the flight hardware and
provides ground-based support in Italy.
- NASA funding allowed the development of essential
prototype hardware and provides ground-based support in the U.S.
Contacts at NASA Glenn Research
Center
Project Manager: Dr.William V. Meyer
NCSER at NASA GRC
william.v.meyer@nasa.gov
216-433-5011
Project Scientist:Dr.William
V. Meyer
NCSER at NASA GRC
william.v.meyer@nasa.gov
216-433-5011
Principal Investigator: Professor
David Cannell, UCSB |
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GRADFLEX
on frront-cover of Applied Optics |
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Foton-M3
satellite |
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Gradient
driven fluctuations visible with a shadowgraph |
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GRADFLEX
Sample Degassing Configuration |
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GRADFLEX
Sample Filling |
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