Third sound: Where are the solitons

With nanometer thicknesses and millimeter wavelengths, third sound is unquestionably in the long-wavelength limit of the classical fluid-dynamic description. The validity of the linearized form, successfully used for decades to predict the third sound speed, depends on the size of the dispersive terms in that description as well as on the amplitude. If the usual capillary dispersion is used, most third sound measurements would be far from linear and should consequently show a dramatically amplitude-dependent speed. The authors report measurements of the frequency of a third sound resonator as a function of amplitude. Although there are uncertainties about the possible role of several dispersions, capillary dispersion appears to be negligibly small. A direct measure of the dispersion from the frequencies of higher resonant modes supports this.