H. van Beelen

On a balance equation for superfluid vorticity in capillary flow of helium II

Abstract It is demonstrated that a simple, “one-dimensional” balance equation for the superfluid vorticity can already account for the occurence of the various types of nonuniform distributions of superfluid turbulence observed in capillary flow of helium II. Expressions for the rates of creation and annihilation of superfluid vortices similar to those suggested by Vinen for the case of thermal counterflow would already be adequate.

Dissipation in the flow of He II

Abstract Measurements are reported on superfluid flow through capillaries of different dimensions. The energy dissipation, the chemical potential difference and the temperature difference over the capillary are measured as a function of the steady state mass velocity, generated by a plunger system. The results can be described by the Gorter-Mellink mutual interaction force and an additional superfluid frictional force; no critical velocity is found.

Thermal effects in adiabatic flow of He II: I. the role of mutual friction and the influence of pressure on the limit for cooling

Abstract In continuation of our investigations on the cooling effect which occurs when He II is forced to flow under adiabatic conditions through a system consisting of a superleak, a chamber and a capillary in series measurements on the flow of He II under high overall pressures are reported. It is found that as expected the limiting temperature for cooling of about 0.75 K does indeed decrease with increasing overall pressure, occurring at a fixed roton density. The occurence of this limit appears to be a complicated phenomenon, in which the roton density as well as dissipation by the flow, either in the chamber or in the capillary, seems to play an essential role. In the present apparatus the velocities of the normal and superfluid components could be varied independently in a wide range of velocities, enabling us to study the nature of the all-important mutual-friction force. It is found that a mutual-friction force of the Gorter-Mellink type, i.e., Fsn = Aρsρn (vs - vn)3 gives an adequate description of the behaviour of the chamber temperature at sufficiently low velocities.

Persistent currents in the He-II film

Abstract A persistent current in the He-II film is observed in a closed circuit with a length of 136 m. Within the measuring accuracy no decay of the flow is measured up to time intervals of more than 18 hours.

Experimental proof that a moving He II film can be thinner than a static film

Abstract It is shown that under certain experimental conditions it is possible to detect a reduction in film thickness of a moving He II film in quantitative agreement with the prediction of Kontorovich.

Cooling by convective heat transport in superfluid helium

Abstract Further experiments are described concerning the cooling effect which occurs when liquid He II is forced to flow through a system consisting of a superleak and fine capillary in series. The nature of this cooling process and the origin of a lower limit which occurs at about 0.75 K, independently of the initial conditions, are the subjects of the present research. In carrying out the experiments at temperatures below 0.75 K, no cooling was detected; instead a warming occurred from which we were able to calculate the dissipative energy rate. It is found that the “mutual friction” force plays an essential role in the cooling process and that cooling is most likely due to interaction between rotons and moving vortices.

Van der Waals Potentials of 4He Derived from Adsorption Isotherms

Van der Waals potentials for thin 4He films on glass and on diamond are deduced from helium adsorption measurements. Significant deviations from the expected cubic dependence on the areal mass density are found. The commonly used values for glass appear to be much too small.

On the motion of plugs of turbulence in the flow of Helium II

Abstract An extensive experimental study of the flow properties of helium II in tubes of ∼0.2 mm inner diameter is reported. The emphasis is on flow states of which the temperature profile along the tubes is nonuniform. Such flow states apparently correspond to nonuniform distributions of superfluid turbulence in the tube. Steady nonuniform profiles are found to occur at almost pure normal fluid flow ( v s ≈ 0), whereas a strongly fluctuating behaviour of the temperature along the tube shows up when the relative velocity v n − v s reaches a certain, only temperature dependent, value (of e.g. ∼9 cm/s at T 0 = 1.4 K). The fluctuations are interpreted to result from the motion of plugs of turbulence through the tube. The analogy with the similar phenomena observed for ordinary liquids is discussed.

The rotating He II film

Abstract Measurements of the He II film thickness in a rotating vessel are reported. In section 2 the results obtained by using a spinning top are described, showing that contrary to the findings of Little and Atkins the thickness of a film in a rotating system behaves as if the film as a whole participates in the rotation. In section 3 similar attempts are described to measure the change in film thickness due to a possible persistent current, by using a multiply connected top. No changes in thickness are observed which shows that either no persistent current was created at all or that, in agreement with Kellers results, the superfluid velocity does not affect the film thickness.

Investigations on the saturated moving He II film with a length of up to 200 m

Abstract Film flow in long and narrow capillaries has been studied by monitoring bulk-liquid levels. The observed oscillations, the damping and third-sound effects are discussed. Changes in film thickness, due to the superfluid velocity as well as due to acceleration, are observed and discussed together with the vapour-film equilibrium. Our earlier reported observations of the Kontorovich effect are confirmed using a longer capillary (200 m) and for different types of flow.