Superfluid fog

A dense fog consisting of superfluid 4He droplets can be generated in helium vapor up to a height of 4–6 cm from the helium surface by driving a piezo transducer plate immersed under the liquid. The average droplet size ranges from about 100 μm at a drive frequency of 1 kHz to 10 μm at 100 kHz. The dependence of the droplet size on frequency is consistent with the capillary-wave dispersion relation of the helium surface, with the droplets ejected from the surface being about one wavelength in diameter. The initial vertical velocity of the emerging droplets is found to be directly proportional to the velocity of the oscillating liquid surface, above a threshold value. The dynamics of normal and superfluid fogs are studied using the technique of diffusing-wave spectroscopy For a water fog generated with a 1.75 MHz piezo driver below the liquid surface, the 7 μm diameter droplets are found to have diffusive dynamics, since the correlation times are long compared to the viscous time. For a fog of 10 μm diameter superfluid helium droplets in helium vapor at 1.5 K the motion appears to be ballistic for correlation times short compared to the viscous time. The relative velocity between the helium droplets is found to increase proportionally to the velocity that the droplets are being ejected from the helium surface.