Electrostriction of a near-critical fluid in microgravity

We used interferometry to measure the electric-field-induced (i.e., electrostrictive) increase of the density of sulfur hexafluoride (SF6) near its critical point. The results at three temperatures (T(c)+5.0 mK, T(c)+10.0 mK, T(c)+30.0 mK with T(c)=319 K) agree with a calculation based on the Clausius-Mossotti relation and the restricted cubic model equation of state. To measure electrostriction, an inhomogeneous electric field (< or =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 that the small electrostrictive density changes (< or =3.5% in this paper) would not be complicated by stratification of the fluid's density induced by the Earth's gravity. The predicted shifts of the critical temperature and density resulting from the electric field were too small to detect.