EARLY IN THE SPRING of 2006, Professor Stanley A. Cheyne and Associate Professor Walter C. (Mike) McDermott III, both of the Department Physics and Astronomy, with physics major Mark A. Goodin ’06, visited the Naval Research Laboratory (NRL) in Washington, D.C., to use the laboratory’s large bubble tank. Cheyne has been conducting research on the acoustical properties of bubbles for fifteen years, and Goodin went as part of his honors research project.

Bubbles play an important role in the propagation of sound in water. Depending on the frequency, bubbles absorb and radiate sound, thus affecting underwater sound waves. One effect of particular interest is how a group of bubbles (bubble cloud) collectively oscillate to change the speed of sound in a particular medium. For example, the speed of sound of pure air at room temperature is approximately 345m/s. The speed of sound of pure water at the same temperature is approximately 1500m/s. The speed of sound of air/water mixture consisting of only 1% air is approximately110m/s, significantly lower than either of the pure states.

Cheyne and others have successfully measured sound speeds of bubbly liquids with a variety of techniques over the years. Recently their results were included in Suspension Acoustics: An Introduction to the Physics of Suspensions by Samuel Temkin (Cambridge University Press, 2005). The experimental results to date are in a frequency regime outside the single bubble acoustic resonance. The current effort is to measure the sound speed at the single bubble acoustic resonance, a difficult task because the acoustic absorption is very strong. Simply trying to measure the speed of sound with traditional techniques is not possible because the wave gets strongly attenuated, or absorbed.

Cheyne and his colleagues are now trying to measure it indirectly through an effort to detect the loading on a piston inserted in the fluid. The acoustic impedance, or resistance, is the product of the density of the fluid and the sound speed. As the sound speed changes, the impedance and hence the loading on a piston should change. They coupled a piezoelectric transducer, which detects the motion of the piston, between the piston and the driver (the device that moves the piston). The is system was placed in a liquid-filled tube and easily detected the standing wave resonances. The effect that they are trying to see is small, much smaller than tube resonances.

They concluded that they needed to get rid of the large tube resonances, so they needed a large tank (to avoid standing waves) and one that had the capability to produce bubbles. A former colleague of Cheyne, Michael Nicholas of NRL, invited them to use the large bubble tank (20’ X 20’ X 13’) at NRL, and after a long day the Hampden-Sydney team came home with no significant data. Their conclusions were that the effect is very small and one that will require more precise measurements.

After returning to Hampden-Sydney, they decided that they needed a smaller version of the NRL tank. Irvin M. Robertson, the department’s laboratory technician, built a 4’ X 4’ X 4’ tank, complete with over 1300 hyperdermic needles. Compressed air is pumped into air chambers and forced through the needles to produce a uniform cloud of bubbles. Cheyne’s group, now including Robert H. Hembree ’09, is trying to make the measurements at the College.

McDermott’s research interests are in experimental solid-state physics. His current research utilizes Mössbauer spectroscopy, which is the recoilless absorption and emission of gamma radiation. This spectroscopic technique can be used to study a variety of iron-containing materials from corrosion products to the minerals found on Mars. Currently the research is focused on applying the tool to measure the interaction of nitric oxide with sickle-cell hemoglobin.

Last spring four members of the department—Cheyne, McDermott, Assistant Professor of Physics Hugh O. Th urman III, and Robertson—visited Leesville Road Elementary School in Lynchburg. They conducted science demonstrations for two groups of students, kindergarten through 2nd grade and 3rd through 5th grades. McDermott attended Leesville Road Elementary School. Cheyne has been at the College since 1990 and was promoted to the rank of professor in 2003. He received his B.A. from Hendrix College and his M.A. and Ph.D. from the University of Mississippi. McDermott, who is on leave this year, holds the B.S.S.E., M.S., and Ph.D., all from Old Dominion University. He began teaching at Hampden-Sydney in 1998. Th urman, who has been at the College since 2002, received his B.S. and Ph.D. degrees from Old Dominion University.

BEYOND THE Classroom FOR THE Classroom
Hampden-Sydney College Faculty Scholarship 2005-2008
A report by the Office of the Dean of the Faculty