Forces in the flow of liquid He II

Flowing liquid He II through a capillary is studied, for types of flow in which the velocities v_n and v_n, of the normal fluid and the superfluid respectively, can be varied independently. A survey of the measurements from all types of flow is given in diagrams of the (v_n, v_n) plane, in which curves of constant ρS ΔT, ρΔμ, and ΔP are given. These diagrams include curves for isothermal flow (ΔT = 0), flow without a chemical-potential difference (Δμ = 0), and isobaric flow (ΔP = 0). The results indicate that the normal fluid is moving laminarly both in subcritical flow and supercritical flow. In supercritical flow apart of the mutual friction force F_sn, a frictional force F_s, connected with the superfluid, is observed. A general relation between the ratio F_sn/F_s and the relative velocity (v_n-v_s) is found, which is more complicated than the pattern obtained with pure superfluid flow (paper II, Physica 61 (1972) 566).