The acoustic coefficients of reflection, transmission, and transformation of first-, second-, and gas sound waves incident normally on the free surface of liquid helium have been determined. The experimental results are compared with the theory developed in Part I of this paper. It is shown that equilibrium theories of boundary conditions disagree with the experiments. Nonequilibrium theories, however, which take into account that mass and energy transport processes occur at the liquid-vapor interface of helium II, lead to a very good description of the experimental data below 2 K. Within experimental accuracy the measurements fully confirm several kinetic models of evaporation and energy transport processes. Near the λ point, deviations from the theory have been found which might be attributed to influences of critical phenomena. The surface absorption coefficients of first-, second-, and gas sound waves could be deduced from the experimental data on the acoustic coefficients, and it is shown that, in agreement with the theory, appreciable energy dissipation occurs at the free surface of helium II when a second-sound or a gas sound wave is incident.

Sound conversion phenomena at the free surface of liquid helium. II. Experimental determination of acoustic coefficients and surface absorption coefficients

The attenuation of superfluid two-phase sound in4He is calculated and compared with experimental results. The dispersion relation of this mode (which consists of coupled vapor sound and second sound) is determined by boundary conditions at the free liquid surface. The primary source of attenuation is found to be the evaporation-condensation process at the free surface. A calculation using kinetic models for the nonequilibrium Onsager coefficients yields an attenuation in general agreement with experimental measurements.

Attenuation of superfluid two-phase sound

Experimental study of transient evaporation of superfluid helium induced by incidence of second sound thermal pulse onto free surface

We report the results of the first systematic study of the dynamical properties of the interface between the phase-separated 3
 He-
 4
 He liquid mixtures. With the acousic sound wave of 10MHz, we determined the kinetic growth coefficient of the interface in the temperature range from 150mK down to 7mK. Below about 70mK, it starts increasing with the decrease of temperature and shows an approximate temperature dependence of 1/T
 2
 . The magnitude and the temperature dependence cannot be explained with the existing theory of the hydrodynamic regime.

Transmission of Sound Through the Interface of Phase-Separated 3He-4He Liquid Mixtures Kinetic Growth Coefficient of the Interface

The critical behaviour of the transport coefficients of the free surface of liquid helium above Tlambda is studied by using the renormalisation group results of the semi-infinite model E, and a possible explanation for the experiment of Wiechert and Buchholz (see J. Low Temp. Phys., vol.51, p.291, 1983) on sound conversion at the free surface of He II is given by extrapolating these results to the region below Tlambda .

https://iopscience.iop.org/article/10.1088/0953-8984/1/44/037/pdf

A possible explanation of sound conversion at the free surface of superfluid helium near Tλ

On the basis of a set of boundary conditions describing quite generally mass and energy transport processes across the free surface of helium II, the acoustic coefficients of reflection, transmission, and transformation of first sound, second sound, and the sound wave propagating in the vapor are calculated in the case of perpendicular incidence of sound waves against the liquid-vapor phase boundary. Considering rigorously the influences of the Onsager surface coefficients, the isobaric thermal expansion coefficients, and the thermal conductivities of the liquid and the vapor, we derive sets of equations from which the acoustic coefficients are determined numerically. For estimations, simple explicit formulas of the acoustic coefficients are given. It is shown that the evaporation and energy transport processes occurring at the free surface of helium II due to the incidence of sound waves may be connected with appreciable energy dissipation. The surface absorption coefficients of first, second, and gas sound waves are deduced.

Sound conversion phenomena at the free surface of liquid helium. I. Calculation of the coefficients of reflection, transmission, and transformation of sound waves incident on the liquid-vapor interface of helium

F. I. Buchholz

Papers

Sound conversion phenomena at the free surface of liquid helium. I. Calculation of the coefficients of reflection, transmission, and transformation of sound waves incident on the liquid-vapor interface of helium

Sound conversion phenomena at the free surface of liquid helium. II. Experimental determination of acoustic coefficients and surface absorption coefficients