Electrostriction of a near-critical fluid in microgravity

We used interferometry to measure the electric-field-induced (i.e., electro strictive) increase of the density of sulfur hexafluoride(SF6) near its cri tical point. The results at three temperatures (T-c + 5.0mK, T-c + 10.0 mK, T-c + 30.0mK with T-c = 319 K) agree with a calculation based on the Claus ius-Mossotti relation and the restricted cubic model equation of state. To measure electrostriction, an inhomogeneous electric field (less than or equ al to 26 kV/cm) was applied to the SF6 sample by charging a fine wire that passed through it. These measurements were performed in microgravity so tha t the small electrostrictive density changes (less than or equal to 3.5% in this paper) would not be complicated by stratification of the fluid's dens ity induced by the Earth's gravity. The predicted shifts of the critical te mperature and density resulting fi-om the electric field were too small to detect. [S1063-651X(99)13405-6].