Dynamic structure factor of density fluctuations from direct imaging very near (both above and below) the critical point of SF6

作者：Oprisan Ana; Oprisan Sorinel A; Bayley Brittany; Hegseth John J; Garrabos Yves; Lecoutre Chabot Carole; Beysens Daniel

来源：Physical Review E, 2012, 86(6): 061501.

DOI：10.1103/PhysRevE.86.061501

Large density fluctuations were observed by illuminating a cylindrical cell filled with sulfur hexafluoride (SF6), very near its liquid-gas critical point (vertical bar T - T-c vertical bar %26lt; 300 mu K) and recorded using a microscope with 3 mu m spatial resolution. Using a dynamic structure factor algorithm, we determined from the recorded images the structure factor (SF), which measures the spatial distribution of fluctuations at different moments, and the correlation time of fluctuations. This method authorizes local measurements in contrast to the classical scattering techniques that average fluctuations over the illuminating beam. We found that during the very early stages of phase separation the SF scales with the wave vector q according to the Lorentzian q(-2), which shows that the liquid and vapor domains are just emerging. The critical wave number, which is related to the characteristic length of fluctuations, steadily decreases over time, supporting a sustained increase in the spatial scale of the fluctuating domains. The scaled evolution of the critical wave number obeys the universal evolution for the interconnected domains at high volume fraction with an apparent power law exponent of -0.35 +/- 0.02. We also determined the correlation time of the fluctuations and inferred values for thermal diffusivity coefficient very near the critical point, above and below. The values were used to pinpoint the crossing of T-c within 13 mu K.

• 出版日期2012-12-3