Robert E. Apfel (203-432-4246) and Yuren Tian
Yale University
New Haven, CT
R. Glynn Holt
Jet Propulsion Laboratory
Pasadena, CA
Popular version of paper 5aPAb12
presented Friday, December 1,
Acoustical Society of America meeting, St. Louis, Mo.
The oscillation, fission, and coalescence of drops have fascinated scientists for centuries. Models for understanding the behavior of the sun or the atomic nucleus, for example, have often begun with simple drop models. Idealized experiments for studying drop behavior are afforded by microgravity, where gravitational forces are minimal and the sphericity of the drop can be preserved. In a recently completed mission of space shuttle Columbia, a water drop about the size of ping pong ball and saturated with the surfactant, Triton X100, a popular detergent additive and mixing agent, was deformed by sound waves in an air-containing resonant chamber called the Drop Physics Module (DPM). When this deforming force was suddenly reduced, the drop executed nearly free and axisymmetric oscillations for several cycles, demonstrating remarkable elongation and contraction. The study of this unprecedented observation should help to test models of nonlinear fluid dynamics and mode coupling important in fields ranging from nuclear structure to fluid mechanics. These experiments were part of research initiated by the Yale Engineering group led by Robert E. Apfel.
Drops that are squeezed flat to form a disk are termed "superdeformed". Although such superdeformation can be observed in the 1G environment of Earth, the drops are much smaller (by a factor of 1000 in volume), the drops are flattened by the forces required to balance gravity, and the frequencies of oscillation are approximately 100 times higher than were possible in the microgravity experiments.
The payload team performing experiments on the shuttle Columbia included two Ph.D. scientist-astronauts (Kathyrn C. Thornton, Payload Commander, and Catherine G. Coleman) and two non-astronaut Ph.D. Payload Specialists (Albert Sacco Jr. and Fred W. Leslie). They were supported on the ground by the Payload Operations team of the Marshall Space Flight Center (Huntsville, Al.) and the DPM science teams from Yale University, Vanderbilt University, and the Jet Propulsion Laboratory. This second United States Microgravity Laboratory (USML) mission, like its predecessor USML-1 flown in the summer of 1992, broke new ground in permitting real-time communications between the Spacelab team and the ground team, via voice, data, and video links. These links allowed for real-time adjustments to the module that were absolutely essential for the carefully timed sequences that led to the observations.
The drop oscillation sequence presented in this report involved the non-ionic surfactant Triton-X100. It was saturated in the water, at the critical micelle concentration-that is, the concentration beyond which small aggregates of the material will form in the bulk of the liquid and negligible lowering of the surface tension will occur. The approximate static surface tension of this aqueous solution is less than half that of pure water, assuring improved wetting capabilities as is desired in cleaning applications.
The one-inch-diameter drop was slowly squeezed so that in the top view the circular outline grew and the side view appeared as a narrowing ellipse. When the drop was almost totally flattened, the ground team gave the go-ahead to the Spacelab scientist (Dr. Sacco) to release the squeezing force by suddenly reducing the loudspeaker drive level. The sequence of images that followed was the dramatic result of this operation. One of the most remarkable and unprecedented features of this oscillation sequence is its maintenance of axisymmetry for several cycles as the drop oscillated back and forth from a disk to a cigar shape. A second remarkable feature is that drop did not come apart. The lower surface tension that allowed for the large deformations also probably prevented the elongated sections from pinching off. The surface active agent, Triton, damped out disturbances that might have destroyed the perfect symmetry required for this sequence to be observed.
The analysis of the superoscillation sequence will provide important tests of theories of nonlinear fluid hydrodynamics, as well as become a reference for maximal drop deformation in a number of application areas.