The Strange Behaviour Of Fluids In Microgravity

Sacco also demonstrated 'surface tension' in space on 2 November, by squeezing orange juice out of a tube. It promptly formed a ball, thus highlighting how surface tension makes fluids assume spherical shapes when they no longer have to contend with gravity. The six-channel, simultaneous video footage from STDCE was only possible thanks to the new High-Packed Digital Television Technical Demonstration, nicknamed 'HI-PAC'. Prior to USML-2, video from Spacelab could only be downlinked on a single channel. As part of the tests of the new system, the crew acquired live images of colleagues sitting at their consoles in Mission Control.

Other fluid physics investigations were conducted in the Drop Physics Module (DPM), another facility carried over from USML-1, which sought to explore future ways of conducting 'containerless' materials processing and encapsulation of living cells for pharmaceutical research. One investigation was the Drop Dynamics Experiment (DDE), provided by Spacelab-3 veteran Taylor Wang and his team from Vanderbilt University in Nashville, Tennessee, which gathered high-quality data in support of such pharmaceutical applications. During USML-2, the experiment investigated the breaking-apart of distorted drops under a fluid of varying viscosity, as well as attempting to encapsulate and create 'compound' drops.

It was hoped that such 'encapsulation' tests would someday allow methods to be routinely exercised to insert living cells for the treatment of hormonal disorders into polymer shells to protect them from immunological attack and provide timed releases. Instances where such techniques might be useful include the treatment of diabetes - perhaps by injecting a pancreatic cell that secretes insulin into the body. Normally, the foreign cell would be attacked by the patient's immune system, but if

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encapsulated in a shell strong enough to withstand attack, yet porous enough to excrete the insulin, it could provide an effective treatment.

A second DPM experiment was provided by Robert Apfel of Yale University and examined the influence of 'surfactants' - substances which alter a fluid's properties by aiding or inhibiting the way it adheres to, or mixes with, other substances - on the behaviour of drops. On Earth, surfactants are routinely used: soap and water interact in dishwashers, for example, and cosmetics manufacturing, the cleaning-up of oil spills and the dissolution of proteins in synthetic drugs also rely heavily upon them. Apfel's study focused on the oscillation of single drops and the coalescence of several drops with different concentrations of surfactants.

On 22 October, the facility's Project Scientist, Arvid Croonquist, and his team watched live downlinked video of the first liquid drop deployment as Thornton released a 2-cm-diameter glob of water and used precisely controlled soundwaves from a pair of loudspeakers to manipulate its movements. She reported that its performance was as expected. The other members of the science crew also worked with the DPM over the following days, with Sacco commenting ''I think it's beautiful'' after deploying a 2.5-cm-diameter drop of water on 24 October.

The next day, a cheer rose from Taylor Wang's team when Leslie succeeded in the delicate process of encapsulating an air bubble inside a floating water droplet. ''There are actually two factors at work in liquid spherical shells: fluid physics and chemical reactions,'' Wang told journalists. ''With these experiments, we are able to separate them.'' Sacco later manipulated drops treated with an organic surfactant known as bovine serum albumin for Apfel's study. He deployed two drops and brought them perfectly together until they coalesced into a single blob.

''These are the first and best drop coalescences we've ever had,'' said DPM Co-Investigator (and USML-1 veteran) Gene Trinh of JPL, adding that ''it went very nicely.'' Cady Coleman, a rookie astronaut, was enjoying not only her first experience of microgravity, but also its unusual effects on fluids: on 31 October, she stretched a 'bridge' of liquid between the facility's two injector tips and later precisely centred a drop of water inside a glob of silicone oil. Moreover, she achieved this while the interfaces of both liquids were moving in opposite directions, in a condition known as 'slosh mode'.

Both Croonquist and Wang praised her efforts, calling them ''very important'' and having ''major significance.'' From Columbia, Coleman seemed pleased that after a year and a half in the Spacelab simulator practising encapsulating drops, ''it's sure nice to see it for real.'' She also worked with Croonquist, who uplinked instructions to her to use soundwaves to manipulate and finally split apart rotating drops of silicone oil.

Later, Sacco succeeded in forming a polymer membrane between two different chemicals, which coalesced into a single drop. ''A picture is worth a thousand words - we'd seen it, now we've done it!'' exulted Wang. After deploying the drops, Sacco had allowed them to coalesce and a membrane formed as a consequence of the ensuing chemical reaction - which proved to be an important step in the encapsulation studies.

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