| Experiments investigating vortex motion in 2-3 nm films were performed using capacitive motion detection of third sound waves in a circular resonator. The Doppler shifts of the three lowest detectable modes of oscillation were used to detect changes in the circulation state of the film.;The flow field v(r) and the distribution of vortices n(r) in the cell were found. A program integrated the equations of motion for our system, given a flow field and a mode, and generated a frequency shift. Linear combinations of these shifts for at least three modes and proposed flow fields were fit to the experimental frequency to find the flow field in the experiment. The vortex distribution was found from v(r).;The motion of a vortex in an oscillating fluid flow was modeled. The hills of the gold substrate in our cell acted as energy wells that pinned vortices. If the superfluid flowed faster than the critical velocity, the vortices were unpinned and moved with a frictional drag force. A program integrated the motion of a vortex and calculated its equilibrium position at each radius r in the cell. These calculations generated v(r), but the program results did not duplicate the experimental flow fields. Another approach was developed to further test the pinning model. The power needed to change the flow in the cell was compared to the power used by theoretical vortices in an oscillating flow. The theoretical results were of the same order of magnitude as the experimental results.;We have determined that vortex motion must involve other forces than the simple friction model we examined, but the power model we used has contributed other important results. We can directly calculate the viscous drag force on the vortices, which sheds some light on the motion of vortices after depinning. General conclusions about swirling behavior can be drawn as well. In particular, swirling occurs in two ways for different film thicknesses and modes, and different mechanisms appear to be responsible for each. |
