SLIP IN QUANTUM FLUIDS

In this review we describe theoretical and Experimental investigations of general slip phenomena in context with the flow of the quantum liq uids He-3, He-4 and their mixtures at low temperatures. The phenomenon of slip is related to a boundary effect. It occurs when sufficiently dilute gases flow along the wall of an experimental cell. A fluid is s aid to exhibit slip when the fluid velocity at the wall is not equal t o the wall's velocity. Such a situation occurs whenever the wall refle cts the fluid particles in a specular-like manner, and/or if the fluid is describable in terms of a dilute ordinary gas (classical fluid) or a dilute gas of thermal excitations (quantum fluid). The slip effect in quantum fluids is discussed theoretically on the basis of generaliz ed Landau-Boltzmann transport equations and generalized to apply to a regime of ballistic motion of the quasiparticles in the fluid. The cen tral result is that the transport coefficient of bulk shear viscosity, which typically enters in the Poiseuille flow resistance and the tran sverse acoustic impendance, has to be replaced by geometry dependent e ffective viscosity, which depends on the details of the interaction of the fluid particles with the cell walls. The theoretical results are compared with various experimental data obtained in different geometri es and for both Bose and Fermi quantum fluids. Good agreement between experiment and theory is found particularly in the case of pure normal and superfluid He-3, with discrepancies probably arising because of d eficiencies in characterization of the experimental surfaces.