We present measurements of the drag forces on quartz tuning forks oscillating at low velocities in normal and superfluid 4 He. We have investigated the dissipative drag over a wide range of frequencies, from 6.5 to 600 kHz, by using arrays of forks with varying prong lengths and by exciting the forks in their fundamental and first overtone modes. At low frequencies the behavior is dominated by laminar hydrodynamic drag, governed by the fluid viscosity. At higher frequencies acoustic drag is dominant and is described well by a three-dimensional model of sound emission.

We report that at low drives, the resonant frequencies and linewidths of nominally 32-kHz quartz tuning forks oscillating in isotopically pure superfluid 4 He at ∼10 mK are dependent on the dimensions of their environment. We confirm the importance of coupling between forks and acoustic modes within the cell, and develop a theory of their coupled dynamics to account for the observations. The frequencies and linewidths are reproducible on a time scale of tens of minutes, but pronounced drifts are seen over longer intervals. We suggest that the drifts are attributable to changes in the velocity of sound due to tiny pressure changes. In studies at high drives, we observe two critical velocities: υ c1 ≈ 0.6 cm/s, where the drag may either increase or decrease, depending on the linewidth; and υ c2 ≈ 10 cm/s, above which there seems to be fully turbulent flow. At high drives, the behavior of the drag differs markedly between forks that appear otherwise to be very similar.

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We have investigated a copper-mesh grid oscillating at its fundamental (0,1) Bessel mode in isotopically pure superfluid 4 He for temperatures 10 < T < 1500 mK at a pressure of P = 5 bar. The high quality factor (Q ∼ 10 5) of the oscillator allowed us to observe new features of its response to a periodic drive which, at the lowest T , was found to depend on the prehistory of the helium. The experiments have confirmed the existence of two critical velocities, believed to be associated quantized vortices. The observed phenomena are discussed.