In the supercritical phase, pure fluids have great potential for industrial applications and are increasingly used by
industry as nonpolluting solvents of organic materials and media for high yield chemical reactions. The experimental
data were recorded in microgravity for sulfur hexafluoride (SF6) and on Earth for density-matching binary mixture of
methanol and partially deuterated cyclohexane (CC*-Me). We used small angle light scattering experiments to
investigate fluctuations in SF<sub>6</sub> near critical point and in density-matched binary mixture CC*-Me in the absence of
convective flows. For binary mixture, we used three different filtering methods: bright filed (BF - no filter), phase
contrast (PC - quarter wave plate at focal point) and dark field (DF - small opaque object at focal point). The power
spectrum of scattered light contains information about local inhomogeneities encountered by light traveling through the
sample cell unit (SCU). We found that the spatial correlations revealed by Fourier transforms follow power laws both for
SF<sub>6</sub> in microgravity and binary mixture on Earth. This is an indication of the universality of fluctuation mechanisms.
Temporal correlations of fluctuations were investigated using the correlation time.
Many experiments used light scattering to visualize the fluctuations of fluid's density. Fluids near the critical point are
affected by gravity because the compressibility of the fluid is very large near the critical point. Therefore, microgravity
experiments allowed new phenomena to be discovered by reducing convection, sedimentation and buoyancy
In order to study, fluctuation and phase separation processes near the critical point of pure fluids without the influence of
the Earth's gravity, a number of experiments were performed in microgravity. Our results refer to a set of experiments
that studied local density fluctuations by illuminating a cylindrical cell filled with sulfur hexafluoride, near its liquid-gas
critical point. Using image analysis, we estimated the temperature of the fluid in microgravity from the recorded images
showing fluctuations of the transmitted and scattered light. Our method has the advantage of avoiding any reference to
the spatial correlation of the pixels in the recorded images. We assumed that the variation of the scattered light intensity
is proportional to the average value of the gray levels. Furthermore, we also assumed that a small fluctuation of the fluid
density induces a change in the scattered light intensity that can be measured from average gray scale intensity of the
image. We found that the histogram of an image can be fitted to a Gaussian relationship and by determining its width we
were able to estimate the position of the critical point.