An unprecedented microgravity observation of maximal shape oscillations of a surfactant-bearing water drop the size of a ping-pong ball was observed during a mission of Space Shuttle Columbia. The goal of the research, of which this observation is a part, was to study the rheological properties of liquid drop surfaces on which are adsorbed surfactant molecules under conditions not possible at 1g. Numerical computation of the evolution of the shape of greatly deformed drops using the boundary integral method has successfully predicted the observed drop shapes over a complete cycle of oscillation, thereby permitting the calculation of the dynamic surface tension under these unique conditions. [S0031-9007 (97)02484-8] PACS numbers: 47.55.Dz, 68.10.Cr, 68.10.EtLiquid drops have intrigued physicists for over a century. Absent any external force, the drop forms a sphere, which is conceptually elegant and mathematically tractable. Rayleigh [1] was the first to quantitatively investigate the modeling of drops, doing so from intrinsic as well as practical interest. Beginning with Kelvin, liquid drops, because of their simplicity and self containment, have also been conceived as paradigm models for other more complicated systems: the Earth, in the case of Kelvin's liquid "globes;" atomic nucleii, for those researchers interested in modeling questions of nuclear collisions and stability [2], and protostellar and stellar masses in astrophysics [3]. A body of theory has accumulated to describe the oscillations of such drops of pure liquid constituents, both in the linear normal mode limit [4] and in the limit of nonlinear, finite-amplitude, and mode-coupled oscillations [5].The ideal interface and low-amplitude modal oscillation structure of such spherical drops is so well understood that any observed deviations from theory can be exploited to infer properties of multicomponent, nonequilibrium solutions. Specifically, the effects of introduction of very small concentrations of soluble surfactants to a pure liquid drop can now for the first time be quantitatively studied by solving the coupled convection-diffusion problem [6,7]. But the validation of such a theory requires both an ideal spherical drop equilibrium and a congruence of physical time scales (primarily those for mass diffusion and modal oscillation) such that the multiple effects of surface-active species dominate the fluid flow and hence the drop dynamics. This is impossible on Earth, and we have recently [8] completed a series of experiments which satisfy these idealized conditions by taking advantage of the minimal acceleration environment afforded by low Earth orbit aboard the Space Shuttle Columbia.In the course of these experiments, an unprecedented maximal shape oscillation of a surfactant-bearing water drop the size of a ping-pong ball was observed (see Fig. 1) during the second United States Microgravity Laboratory, USML-2 (STS-73, 20 October-5 November, 1995). The observation was precipitated by the action of an intense sound field which produced a deform...

Large liquid drops were deformed by an acoustic standing wave in a resonant air chamber called the Drop Physics Module, which was carried on Space Shuttle Columbia as part of the second United States Microgravity Laboratory mission. When this deforming force was suddenly reduced, the drops executed free oscillations about a perfect spherical equilibrium. The complex frequency of these oscillations was measured. The first results from a series of experiments are presented here. Results for pure water compare favorably with fundamental theoretical predictions. Drops containing aqueous solutions of soluble surfactants exhibited remarkable differences in both frequency and damping, and these are explained phenomenologically by a discussion of the multiple time scales introduced by the addition of surfactants.

The presence of bubbles has been shown to change the compressibility and complex sound speed in a liquid. In the ocean, acoustically compact bubbly mixtures manifest themselves as highly compressible regions that effectively scatter low-frequency sound. To study low-frequency sound scattering, multifrequency backscattering experiments have been performed in a tank using partially voided three-quarter-inch diameter polyurethane spheres as targets. Target strengths (2-20 kHz) were measured for four spheres with void fractions of 0%, 3.4%, 4.2%, and 6%. Measured target strengths for the voided spheres were on the order of −40 to −60 dB (re 1 m). The frequency response exhibited modal structure, with peaks shifting to lower frequencies for higher void fractions. No backscatter signal was detected for the solid polyurethane sphere. Target strength was also measured for a hollow polyurethane sphere containing a suspension of bubbles in polymer gel. The void fraction was determined by fitting the scattering theory and low-frequency bubbly fluid compressibility model to the measured data. [Work supported by ONR.]

The attenuation and phase speed of bubbly fluids have been shown to be highly dispersive due in part to the resonance response of individual bubbles. Surface-active materials, or surfactants, are known to dramatically alter the viscoelastic dynamics of an interface without affecting the bulk fluid properties. In this theoretical model the effects of surface viscosity on the propagation of linear pressure waves through a bubbly fluid are examined. The attenuation and phase speed are calculated for varying radii, void fraction, and interfacial viscosity for air bubbles in water. The results show an increase in attenuation for frequencies far removed from the bubble resonance for radii less than 100 microns. There is no significant increase in attenuation for larger bubbles (order 1 mm). Interestingly, near resonance, the interfacial viscous case has a lower attenuation than the clean interface, most likely due to the reduced radiation damping associated with smaller radial excursions. Likewise, resonance effects on the phase speed dispersion curve are also significantly diminished due to the interfacial viscosity. These results could be important for understanding propagation near the surface of the ocean, where surface-active materials can dramatically affect the dynamics of micron-sized bubbles and bubble distributions. [Work supported by ONR.]

An experimental apparatus has been designed to measure the physical properties of low-frequency sonoluminescence (SL) (15 kHz). Radial motion of the bubble has been measured by Mie scattering and also by a new method, which images the bubble through a magnifying CCD camera illuminated by a strobing LED lamp. The light emission of SL is measured by a photomultiplier tube. An automated gas handling system has been constructed in order to measure the effects of various gas mixtures and concentrations on SL. The system consists of voltage actuated valves, a mixing vessel, and pressure sensors, which are interfaced to a computer and controlled using the LabVIEW programming environment. With the lower frequencies and the resulting larger bubbles, it is hoped that the parameter space for this region of SL can be mapped out. [Work supported by NASA through JPL Contract No. 958722.]