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...
