Thesis (Ph. D.)--University of Wisconsin--Madison, 1965. / Vita. Typescript. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1975. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 112-115).

Thesis (Ph.D.)--Brown University, 2005. / Vita. Thesis advisor: Humphrey Maris. Includes bibliographical references (leaves 88-92). Also available online.

Critical velocities have been measured in liquid helium at 1.40 K for counterflow, superflow and counterflow superimposed on superflow. The measurements were made in five pipes having rectangular cross-sections and ranging in size nominally from 0.2 x 1.2 x 10 cm to 1.2 x .1.2 x 10 cm. The turbulence was detected by means of negative ion currents perpendicular to the flow. The results are interpreted in terms of creation of quantized vortex lines. For superflow, the critical velocities were slightly higher but within the experimental range of those measured for counterflow. For the case of superflow-plus-counterflow a second critical point was observed, in addition to the one corresponding to superflow only. Considerable data was also collected regarding the nature of the ionic current attenuation effects for supercritical flow. The delay times between when the flow rate was raised to or above the critical velocity and when the current attenuations were first observed ranged from more than five minutes for the smallest pipe to three-quarters of a minute or less for the largest one. When the pipe width is greater than the ion source width the attenuations may be explained by Vinen's theory of vortex generation and decay but for smaller pipe widths the attenuation buildup and decay times are greatly increased. A method for increasing the sensitivity of the detection method for large pipes is also outlined. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

It is well known that most liquids exhibit a tensile strength which is much smaller in magnitude than the tensile strength predicted by homogeneous nucleation theory. This lack of agreement is usually attributed to the difficulty of preparing liquid samples free from foreign gases which act as heterogeneous nucleation sites. Liquid helium occupies a unique place among liquids for tensile strength measurements because all foreign gases are frozen out at liquid helium temperatures. Furthermore, superfluid 4He should fill all crevices on solid surfaces, eliminating the chance of heterogeneous nucleation on helium vapor pockets. Despite the quantum mechanical nature of liquid helium, Becker-Doring theory of nucleation of the vapor phase from the liquid phase should be valid down to 0.3 K in 4 He, yet previous results have been in stark disagreement with the theory. In this study, a piezoelectric transducer in the form of a hemispherical shell was used to focus high-intensity ultrasound into a small volume of 4He . The transducer was gated at its resonant frequency of 566 kHz with gate widths of less than 1 msec in order to minimize the effects of transducer heating and acoustic streaming. The onset of nucleation was detected from the absorption of acoustic energy and the scattering of laser light from microscopic bubbles. A new theory for the diffraction of light from the focal zone of a spherical converging sound wave was developed to confirm calculations of the acoustic pressure amplitude at the focus of the piezoelectric transducer, calculations which were based on the acoustic power radiated into the liquid and the nonlinear absorption of sound. The experimental results were in agreement with homogeneous nucleation theory for a nucleation rate of approximately 1015 critical size bubbles/sec-cm3. This is only the third liquid for which the theoretical tensile strength has been reached and it confirms homogeneous nucleation theory over a range three times greater than any other experiment. A noticeable decrease in the magnitude of the tensile strength was noted at temperatures near the lambda transition and a hypothesis that bubbles are being nucleated heterogeneously quantized vortices is presented.

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