I. A. Gritsenko

Quartz tuning-fork oscillations in He II and drag coefficient

The temperature dependencies of drag coefficient for quartz tuning forks of various geometric dimensions, immersed in the He II, were determined experimentally in the temperature range 0.1–3 K. It is identified, that these dependencies are similar, but the values of drag coefficient are different for tuning forks with different geometric dimensions. It is shown, that the obtained specific drag coefficient depends only on the temperature and frequency of vibrations, when the value of drag coefficient is normalized to the surface area of moving tuning-fork prong. The temperature dependencies of normalized drag coefficient for the tuning forks of various dimensions, wire, and microsphere, oscillating in the Не II, are compared. It is shown, that in the ballistic regime of scattering of quasiparticles, these dependencies are identical and have a slope proportional to T4, which is determined by the density of thermal excitations. In the hydrodynamic regime at T > 0.5 K, the behavior of the temperature dependen...

Mechanisms of dissipation of an oscillating quartz tuning fork immersed in He II at high pressures

The dissipative processes that occur with immersing a vibrating tuning fork in superfluid helium are investigated. The tuning forks resonance width Δf of frequencies from 32 to 97 kHz was measured in the temperature range from 0.2 to 2.5 K and He II pressure from SVP to 24.9 atm. Some of the tuning forks were in the original can (closed tuning fork), and for some tuning forks the can was either completely or partially removed (opened fork). We found that for the open tuning forks two dissipation mechanisms are clearly revealed in the temperature dependence of Δf, namely, acoustic radiation and scattering of ballistic thermal excitations at low temperatures, and viscous friction at high temperatures. At low temperatures (below ∼ 0.8 K) acoustic dissipation dominates, and the model of quadrupole oscillator for a tuning fork can be applied. We found that acoustic radiation for closed tuning forks is less effective and appears at lower temperatures. The first experimental data on dissipative processes in the ...

Dissipation of the kinetic energy of a tuning fork immersed in superfluid helium at different oscillation frequencies

The drag coefficient characterizing the dissipation of the energy of oscillating tuning forks immersed in liquid helium is studied experimentally. The experiments are done at temperatures from 0.1 to 3.5 K, a range that covers both hydrodynamic flow and the ballistic transport of thermal excitations in superfluid helium below 0.6 K. It is found that a frequency dependence of the drag coefficient exists in the hydrodynamic limit, where the main dissipation mechanism is viscous friction of the liquid against the surface of the oscillating object at temperatures above 0.7 K. In this case, the drag coefficient is proportional to the square root of the oscillation frequency and its temperature dependence in He II is determined by the corresponding relationships between the density of the normal component and the viscosity of the liquid. At lower temperatures, there is no frequency dependence of the drag coefficient and the magnitude of the dissipative losses is determined only by the temperature dependence of ...

Features of quasistable laminar flows of He II and an additional dissipative process

Quasistable laminar flow of He II at a temperature of 140 mK is studied experimentally. The liquid flow was excited by a vibrating quartz tuning fork with a resonance frequency of about 24 kHz. It was found that for velocities of the tuning fork oscillations from 0.046 to 0.16 m/s, the He II flow can be both quasistable laminar and turbulent. Transitions between these flow regimes were observed. When the velocity of the tuning fork oscillations increases more rapidly, the velocity at which the quasistable flow becomes unstable and undergoes a transition to a turbulent flow is higher. Mechanisms for the dissipation of the energy of the oscillating tines of the tuning fork in the quasistable laminar flow regime are analyzed. It is found that there is an additional mechanism for dissipation of the energy of the oscillating tuning fork beyond internal friction in the quartz. This mechanism is associated with mutual friction owing to scattering of thermal excitations of He II on quantized vortices and leads to...

Effect of phase separation kinetics on the homogenization kinetics of solid solution of 4He in 3He

New features of the kinetics of phase separation and the subsequent homogenization of solid solutions of 4He in 3He are observed at temperatures of 0.1–0.5 K. Separation was initiated by rapid cooling of a uniform solution, as well as by cooling it in small temperature steps. Information on the kinetics of the phase transitions was obtained using precision measurements of the pressure of the solid helium at constant volume. The homogenization kinetics are found to depend substantially on the kinetics of the preceding separation. The time constants for homogenization in the separation region are smaller by more than a factor of 5 with rapid cooling, than with slow cooling. These results are consistent with a model of homogeneous nucleation.

Nonlinear phenomena in the vibrations of a quartz tuning fork immersed in superfluid helium

Experiments involving the excitation of hydrodynamic flows in superfluid helium subjected to forced vibrations of a liquid-immersed quartz tuning fork are conducted. An investigation of the nonlinear vibrations that arise with an increase in the driving force and are manifested by the distortion in the shape of the resonant amplitude-frequency characteristic, in comparison to the Lorentz curves for an extremely small force, is carried out. The nonlinear resonance curves are described using the Duffing equation, the parameters of which are found by comparing the theoretical calculations against the experimental data. The vibration velocity of the tuning fork legs as a function of the driving force, established using the Duffing equation, is similar to the dependence that was obtained earlier for the quasi-laminar He II flow and contains a cubic velocity contribution caused by the mutual friction due to phonon scattering on the quantized vortices in a turbulent flow.

Additional mass from a quartz tuning fork vibrating in He II

The additional mass caused by vibrations of tuning forks in He II with resonant frequencies of 6.7, 8.5, 12.1, 25, and 33 kHz is studied experimentally. The additional mass coefficient, given by the ratio of the additional mass to the mass of the fluid displaced by the tuning fork, is determined from the measured temperature and pressure dependences of the resonance frequencies of the tuning forks. These studies were made at temperatures from 2.2 to 0.1 K and the pressure dependences of the tuning forks were found for pressures from 1 to 24.8 atm at a constant temperature of 0.365 K. It is shown that for temperatures below 0.7 K, where the viscosity of He II is negligible, the resonance frequency of the tuning fork oscillations is determined by the additional mass of the fluid. It is found that measurements of the resonance frequencies as a function of pressure can be used to determine the additional mass coefficient with an accuracy that is almost an order of magnitude higher than when the temperature de...