Critical temperature shift in pure fluid SF6 caused by an electric field.

A systematic study of the effect of an electric field on the critical temperature of a pure fluid is made for the first time to our knowledge. An ac electric field is applied to a spherical capacitor filled with SF6 at its critical density, while the temperature is slowly ramped down through its critical temperature T(c). By continuously observing the light transmission through the fluid during the temperature ramp, a shift in T(c), DeltaT(c), is found at various electric fields. By shining the light vertically through the fluid, we utilize the density gradient induced by the fluid's weight to compensate for the effects of density changes from electrostriction. This technique effectively keeps the system at constant critical density with respect to the observation of T(c). We observe an increase in T(c) as expected from thermodynamic stability and renormalization group theory, but quantitatively larger by an order of magnitude.

Large-scale geophysical flows on a table top.

Results from two similar experimental systems that attempt to create laboratory geophysical analogs in spherical geometry are presented. In the first system, real time holographic interferometry and shadowgraph visualization are used to study convection in the fluid between two concentric spheres when two distinct buoyancy forces are applied to the fluid. The heated inner sphere has a constant temperature that is greater than the outer sphere's constant temperature by DeltaT. In addition to the usual gravitational buoyancy from temperature induced density differences, another radial buoyancy is imposed by applying an ac voltage difference, DeltaV, between the inner and outer spheres. The resulting electric field gradient in this spherical capacitor produces a central polarization force. The temperature dependence of the dielectric constant results in the second buoyancy force that is especially large near the inner sphere. The normal buoyancy is always present and, within the parameter range explored in our experiment, always results in a large-scale cell that is axisymmetric about the vertical axis. We have found that this flow becomes unstable to toroidal or spiral rolls that form near the inner sphere and travel vertically upward when DeltaT and DeltaV are sufficiently large. These rolls start near the center sphere's equator and travel upward toward its top. In the second experimental system, the central force is applied to a highly compressible near-critical fluid in weightlessness (parabolic flight) and normal gravity. Although a geophysically similar density distribution could not be obtained in the limited time of a parabolic flight, clear influences of the central force on the fluid were observed in both weightlessness and terrestrial experiments.