Fluid Physics Research

The third European-provided microgravity facility was the Bubble, Drop and Particle Unit (BDPU), which had previously been on board IML-2, and was used during STS-78 to observe and record the behaviour of fluids under differing temperature levels and concentrations. Early in the mission, on 22 June, it was utilised for one fluid physics investigation which it was hoped could lead to a greater understanding of the processes controlling evaporation and condensation. According to Johannes Straub of the Technical University of Munich, who designed the experiment, the results were expected to refine a long-standing physics study.

In its specially designed test cell within the BDPU, a small heater emitted an electrical charge into liquid freon, supersaturated with gas, which produced a single bubble by boiling. Straub's investigation measured its development and then reversed the process by increasing pressure to cause condensation in which the gas bubble was redissolved into the liquid. ''Eventual applications of what is learned from this basic study,'' said Straub, ''will benefit designers of power and chemical plants and of air conditioners, among others.''

Despite requiring some minor in-flight maintenance after suffering a blown fuse, the BDPU performed very well and successfully processed all nine of its test containers, including a spare. The blown fuse was repaired on 24 June by Favier and Kregel, using a credit card-sized piece of plastic from the cover of their flight manual between two layers of wires. This insulated the short-circuiting wire from the unit's metallic housing.

One experiment, provided by Antonio Viviani of the Second University at Naples, examined surface tension and interactions of gases and liquids in microgravity. Various sizes of air bubbles were injected into a water-and-alcohol solution with temperature gradients ranging from 'hot' to 'cold'. Another study, devised by R.S. Subramanian of Clarkson University in New York, examined the behaviour of inert gas bubbles within silicone oil. From such studies, it was hoped new insights might be gained into controlling defects in many aspects of materials processing, possibly leading to the production of stronger and more resilient metals, alloys, ceramics and glasses.

A second glitch with the BDPU led to a remarkable repair procedure on 28 June, that had actually been uplinked to the crew in the form of a video from Mission Control; Favier and Kregel were once again able to resuscitate the unit. Their efforts proved successful when they activated the Electrohydrodynamics of Liquid Bridges experiment, which focused on changes occurring in 'bridges' of fluids suspended

Fluid physics research 273

Fluid physics research 273

Jean-Jacques Favier (left) and Kevin Kregel repair the Bubble, Drop and Particle Unit (BDPU) in the Spacelab module.

between a pair of electrodes. It was hoped that such research could aid industrial processes in which control of liquid sprays is vitally important, such as inkjet printing and polymer fibre spinning.

Fluids under study in the experiment, which had been designed by Dudley Saville's team from Princeton University in New Jersey, included castor oil, olive oil, eugenol and silicone oil. Another, two-part investigation was conducted by Rodolfo Monti of the University of Naples; its first segment looking at the interaction of moving, pre-formed bubbles and the melting and solidifying 'edge' of a solid, and its second segment examining the ways in which droplets were captured by, or pushed away from, a moving solidification front.

The experiment called for the injection of water droplets of differing diameters into a liquid alloy, in order to study their behaviour during the application of heat. Unfortunately, difficulties were encountered with the injector, which prompted the crew to perform another experiment and defer Monti's study until 4 July. A repair procedure, performed by Kregel and Helms, revealed that the water injector would not retract from its 'deployed' position and the sample cartridge had to be removed. However, ground-based engineers developed a solution to what was conceded to be a minor electrical problem and the experiment was later run satisfactorily.

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