ABSTRACT
This chapter considers the behavior of the superfluid Bose liquid at finite temperatures. The thermally-excited quasi-particles are identified with the normal fluid. In general, their motion will be irreversible and may be described, in suitable limit, with the aid of a coefficient of viscosity. Consideration of the rotating bucket experiment provides such a prescription, both experimentally and theoretically. Since condensate motion is irrotational, the superfluid component will not respond to rotation of its container. Collisions with the walls thus represent the physical mechanism by which the normal fluid is singled out, the superfluid component being unaffected by the presence of the pipe. According to the two-fluid model, the velocity of the normal fluid vanishes, since it is "attached" to the walls of the pipe by viscous effects. As a consequence of superfluid flow, the thermal quasi-particle distribution is asymmetric in momentum space.
