Ring vortices in point-plane gaps at sub-breakdown fields

The electrical breakdown of liquid dielectrics involves the generation, growth and distortion of a vapor cavity in the liquid as a precursor of the breakdown process. In high viscosity fluids other hydrodynamic growth processes can be observed at sub-breakdown fields, and in 100 cs and 1000 cs DC-200 silicone fluids, a region of hot liquid forms near the point cathode, and this develops into a vortex ring which grows as it moves a short distance toward the anode, before it decays. The decay time constant is of order 50 to 60 /spl mu/sec, and we interpret this as the thermal decay of a structure of alternating hot and cold layers in the ring, the hot liquid layers being about 5 /spl mu/m thick. (Such layered structures are well-known from work on macroscopic vortex rings in dyed fluids.) We find that vortex rings can also be generated in cyclohexane, but on a smaller scale than in viscous silicone fluids, and the thermal relaxation time is much shorter (of order 4 or 5 /spl mu/s). The proposed model of alternating hot and cold liquid layers in the toroidal ring, which accounts for the short time constant, can also account for the unusual optical properties of the decaying vortex rings, which scatter light in a very directional manner.