Using Image Processing Techniques to Investigate Thermal and Concentration Fluctuations

Critical fluids have diverging compressibility, which hampers the study of critical phenomena on Earth. Using DECLIC instrumentation, experiments with SF6 were performed aboard the International Space Station (ISS). We used a novel approach to separate the temporal scales of fluctuations of SF6 experimental data by using the Bidimensional Empirical Mode Decomposition (BEMD) method, which decomposes the spatial frequency components into a set of Intrinsic Mode Functions (IMFs), followed by a Dynamic Differential Algorithm (DDA) to extract the Structure factor, ST, and the correlation time, tau. The first IMF for the shortest spatial scale is related directly to critical point fluctuations. The second and third IMF reveal a coarser structure determined by the initial stage of cluster formation.

We also investigated concentration fluctuations during free diffusion of a colloidal suspension of iron oxide and our data show that the Structure factor has the general shape both for original and IMF1, and IMF2. The peak relaxation time at qc decreases with the IMF order. This suggests a faster relaxation of concentration fluctuations at larger spatial scales.