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Under its Undergraduate Opportunities Research Program, NASA accepts proposals from undergraduate students to conduct experiments on board its KC-135A, the military equivalent of a Boeing 707, used to train astronauts. During the experiments, the aircraft cycles between 0 g and 2 g in a parabolic trajectory. The high incidence of motion sickness has earned the aircraft the nickname "Vomit Comet."
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The "Vomit Comet" on its parabolic trajectory.
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Students carrying out experiments in 0 g.
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Under the supervision of APL-UW Senior Physicist John Wettlaufer, Lisa Couret and 1998 Hardisty Scholar Dorothy Caplow, both physics majors, conducted an experiment to determine what happens when a drop of fluid hits a fluid surface. The experiments were designed to investigate the effect of gravity on the transition between splashing and coalescing drops and the effect on the characteristic "crown of thorns" produced by a splashing drop. The characteristic shape and evolution of the resulting space depend on the balance of capillary, inertial, and gravitational forces. Under microgravity conditions, the effects of surface tension dominate, and the resulting interplay between capillary and inertial forces can create effects impossible in 1 g. Because of the complicated, nonlinear nature of the problem, these effects are difficult to model or predict beforehand.
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As the aircraft flew its roller-coaster profile, the students launched small droplets of slightly salinated water or milk into a container of the same fluid and captured the results on high speed video. In 0 g the rim of the pit produced by the drop of fluid often collapsed before occurrence of the classical Rayleigh instability that creates a crown of thorns at 1 g. The height of the crown was often much greater than its 1 g counterpart, and the jet of water produced in the middle of the crown was much shorter and thicker. At 2 g, the jet was much taller and thinner. The relationship between the Weber and Froude numbers, which governs the transition between coalescing and splashing, appeared to translate well into the microgravity regime; consequently, it was easier to generate a splash at low velocities in microgravity than at 1 g.
These results contribute to a substantial body of knowledge about splash behavior, which is important in applications from ink jet printing to predicting the impact of a meteor on a planetary surface.
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A Worthington jet is formed instead of a splash at high gravity.
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