G. A. Sheshin

Characteristics of the transition to turbulence in superfluid He4 at low temperatures

A piezoquartz oscillator (tuning fork) immersed in liquid is used to study the kinetic and dissipative processes in He II experimentally. The electrical response of the tuning fork near its resonance frequency is measured with different exciting voltages at temperatures ranging from 0.1Kto4.2K. The measured values of the half-width of the resonance curves made it possible to determine the viscosity of the normal component of He II in a wide temperature range. A maximum of the effective viscosity is found at temperature ∼0.5K; this maximum is due to a transition from the hydrodynamic to the ballistic regime in the phonon gas in He II. It is established that for low velocities of oscillation of the tuning fork the velocities are a linear function of the excitation force; this corresponds to laminar flow of the liquid in the boundary layer near the oscillating surface. The main dissipative process is associated with the viscosity of the normal component. The thickness of the boundary layer near the surface o...

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...

Viscosity and relaxation processes in the phonon-roton system of He II

The viscosity of He4 in the temperature range 0.1–2.2K has been measured using a vibrating quartz tuning fork. A quantitative comparison is made of the experimental data and the conclusions of the modern theory of the phonon-roton system of superfluid helium. The complex hierarchy of relaxation processes is analyzed and the role and contribution of each process to the coefficient of viscosity are determined. Agreement between the experiments and theory is obtained in the hydrodynamic region. The transition from the hydrodynamic to the ballistic regime of phonon flow is analyzed and the effective viscosity of He II at such temperatures is found. It is shown that the position of the maximum of the temperature dependence of the effective viscosity, obtained using different methods of measurement, correlates with the characteristic size of the measuring apparatus.

Relation between the temperature gradient and concentration gradient in superfluid 3He–4He mixtures

The temperature and concentration gradients ∇T and ∇x created by a heat flow in a superfluid 3He–4He mixture with a concentration of 9.8% 3He are measured in the steady state in the temperature range T=70–500 mK. It is found that the ratio ∇x/∇T obtained in the experiment can be described on the basis of a dependence of the osmotic pressure on the temperature and concentration. The data are used to find the thermodiffusion ratio and, hence, the thermodiffusion coefficient of this mixture.

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 ...

Convective instability of 3He–4He superfluid mixtures heated from below

The convective thermal instability of 3He–4He superfluid mixtures on heating from below is studied in the temperature interval 100–500 mK in mixtures with an initial concentration of 9.8% 3He. It is found that at certain critical values of the power of the heat flux in the mixture there is a change in character of the heat transfer, indicating the onset of convective heat transfer. The critical temperature gradients associated with the onset of convection correspond to Rayleigh numbers many orders of magnitude greater than the Rayleigh numbers in the heating of the mixtures from above. The results are analyzed in the framework of the theory of the convective instability of binary mixtures.

Attainable Supersaturation of Superfluid 3He-4He Solutions and the Role of Quantized Vortices

The earlier systematic measurements of attainable supersaturations of superfliud3He-4He solutions depending on temperature and pressure have been continued. The experiments were carried out at continuously varied solution concentration and at constant temperature and pressure. The results obtained are analysed using various models of nucleation in supersaturated liquid solutions. The fast nucleation line which is constructed from experimental results is in strong contrast with the kinetic theory of homogeneous nucleation for supersaturated3He-4He solutions. It is shown that reasonable agreement with experimental results can be obtained assuming that completed centres of nucleation generated on quantized vortices in superfluid liquid are available.

Concentration dependence of the attenuation of first sound in supersaturated superfluid 3He–4He solutions under pressure

The concentration dependence of the attenuation coefficient of first sound in superfluid 3He–4He solutions in the saturation and supersaturation regions is investigated experimentally at pressures of 0–10 atm. An original technique of continuous variation of the concentration in situ by variation of the osmotic and thermomechanical pressures is used, permitting measurements to be made in the long-lived metastable phase of the superfluid solutions. It is shown that the data obtained are described well in terms of the theory of sound propagation in a gas of Fermi excitations without taking the Fermi-liquid corrections into account. The corresponding values of the effective mass and relaxation time of 3He quasiparticles are taken from an analysis of the existing experimental data. Within the experimental error, no excess sound attenuation was found in the region of supersaturated solutions.

Mutual interactions of oscillating quartz tuning forks in superfluid 4He

The quartz tuning fork has recently become a popular experimental tool for investigations of both classical and quantum turbulence in cryogenic helium. Its increased use in low-temperature experiments and a number of puzzling results obtained in the past have led to many questions concerning the interaction of multiple tuning forks or the interaction of tuning forks with other oscillators. We report measurements performed in He II at low temperatures around 360 mK, on the mutual interaction of tuning forks placed in the same volume of fluid, and examine the responsible mechanisms in an effort to discriminate between acoustic coupling and interaction via quantized vortices. To this end, the interaction of two tuning forks is investigated by analyzing their recorded resonance curves, looking for any nonelectrical crosstalk. Further, the force-velocity characteristics of a detector tuning fork are measured for different operating velocities of a generator tuning fork. As a complementary measurement, the inte...