Intrinsic critical velocities in superfluid {sup 4}He flow through 12-{mu}m diameter orifices near T{sub {lambda}}: Experiments on the effect of geometry

Journal Article · Wed Mar 01 04:00:00 EST 1995 · Journal of Low Temperature Physics

DOI:https://doi.org/10.1007/BF00752282· OSTI ID:70470

Shifflett, G M ^[1]; Hess, G B ^[1]

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1. Univ. of Virginia, Charlottesville, VA (United States)

The authors report superfluid {sup 4}He flow measurements at temperatures from 1.2 K up to T{sub {lambda}} {minus} 3 mK in three orifices of different mesoscopic geometry. Under conditions of these experiments, the flow usually reaches a temperature-dependent intrinsic critical velocity, where dissipation is believed to occur by thermal (or quantum) nucleation of individual quantized vortex rings or loops. The nucleation rate should be sensitive to the wall geometry of the flow channel and to any local velocity enhancement at the most favorable nucleation site. According to the Iordanskii-Langer-Fisher (ILF) theory, the radius of the {open_quotes}critical{close_quotes} vortex ring, the threshold size which can grow freely by extracting energy from the flow, increases inversely as the superfluid density on approach to the superfluid onset temperature, T{sub {lambda}}. Thus sufficiently near T{sub {lambda}} the critical ring should be large enough that the geometry relevant to the nucleation process and local velocity enhancement can be studied by scanning electron microscope (SEM). The authors examined their three orifices by SEM. One, a standard optical pinhole, has a relatively smooth taper on one side and a sharp lip on the other. The second is similar, but contains a 1-{mu}m flake perpendicular to the flow, which should provide additional velocity enhancement at its edge. In the third, the sharp lip is beveled to reduce the velocity enhancement at that site. Contrary to expectation, the intrinsic critical velocities are the same, within a relative calibration error of 10%, in all three cases. Thus, local sites of enhanced velocity do not appear to be active in nucleating vortices. This raises a question whether the classical two-fluid model which underlies the ILF calculation is adequate to describe the superfluid hydrodynamics near walls, as it affects the vortex nucleation process.

• Sponsoring Organization: USDOE
• OSTI ID: 70470
• Journal Information: Journal of Low Temperature Physics, Journal Name: Journal of Low Temperature Physics Journal Issue: 5-6 Vol. 98; ISSN JLTPAC; ISSN 0022-2291
• Country of Publication: United States
• Language: English

Related Subjects

66 PHYSICS
CRITICAL VELOCITY
HELIUM 4
ORIFICES
SUPERFLUIDITY
TEMPERATURE RANGE 0000-0013 K