No abstract

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 of the oscillating tuning fork is estimated. Kinks attesting to a transition to nonlinear flow are found in the dependence of the electric response on the excitation voltage for high velocities of the tuning fork. At low temperatures this regime is accompanied by the appearance of flat sections near the maximum in the resonance curves. This behavior can be explained by a transition from laminar to turbulent flow of the liquid, which likewise is accompanied by the appearance of additional dissipation due to the creation of quantum vortices. The critical velocity of the transition to the turbulent regime, whose temperature dependence qualitatively agrees with measurements performed previously using oscillating bodies of a different shape, is determined.

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.