Properties of superfluid fog produced by an ultrasonic transducer

A dense fog consisting of superfluid He-4 droplets can be generated an heli um vapor up to a height of a few cm from the helium surface by driving a pi ezo transducer plate immersed under the liquid. The average droplet size is measured with a long-distance microscope and CCD camera, and ranges front about 100 mum at a drive frequency of 1 kHz to 10 mum at 100 kHz. The depen dence of the droplet size on frequency is quite consistent with the capilla ry-wave dispersion relation of the helium surface, with the droplets ejecte d from the surface being about one wavelength in diameter. The initial vert ical velocity of the droplets emerging from the surface is found to increas e linearly with the driving voltage being applied to the piezo.