Going without the Flow: Fluids in Space
Adam Frank for McGraw-Hill
Poised on the edge of becoming a space-faring race we are finding it's the little things which can matter most. How, for example, do you boil water in Space? Without gravity there is no buoyancy and a heated fluid has no reason to rise. How do you put out a fire in Space? Without gravity neither foam nor fluid can fall onto the flames and smother them. How to do you lubricate surfaces in Space? Without gravity surface tension draws the lubricant into beads leaving large areas dry, subjecting them to frictional heating and potentially catastrophic mechanical failure.
From boiling water to blood flow we rely on fluids acting in a certain way, but our expectations for fluid flow depend completely on gravity. In the absence of a strong gravitational field fluids behave in strange ways that remain, for the most part, unexplored. Why should anyone care? The International Space Station is about to become a mile-long multi-billion dollar reality, that is why. Terrestrial plumbing may have been perfected years ago but space plumbing on a large-scale is an entirely different ballgame. Designing plumbing, fire-control and safety systems on a free-floating structure as large as the Space Station requires an understanding of gravity-free fluid behavior that we just don't have yet.
Without gravity, surface tension becomes the dominant force in fluid flows. It is surface tension that controls the free-floating blobs of Tang seen drifting across the Space Shuttle in countless NASA telecasts. On the Earth our experience with surface tension is mostly confined to water bugs sliding across the surface of ponds. To study surface tension dominated flows in a way that will be useful for astro-engineering, scientists have to fake the conditions in space by drawing fluids into ultra-thin thin sheets. Anyone who has blown soap bubbles knows thin liquid films are not easy to manipulate and control.
Harry Swinney and Graham Carey are two University of Texas researchers who have figured out how to bring the heavens down to Earth at least when it comes to fluid motion. Swinney is an experimentalist who has found a bizarre but innovative solution to creating the needed conditions for studying micro-gravity fluid dynamics. By drawing fluid between the ends of two cylinders they can create a thin "liquid bridge". The bridge exists as a thin tenuous film between the caps of the cylinders and, being thin, is dominated by surface tension. By heating one of the cylinders Swinney can make his liquid bridge 'boil'' in the same way heating the bottom of tea kettle brings the water to boil. Carey, a computational physicist uses powerful supercomputers to simulate the liquid bridge experiments and understand the physics they reveal.
The liquid bridge may seem like a pretty unrealistic way to study fluids in space but there is a deep and profound lesson in these experiments about the basic way science works. Its true, a free floating drop of tang in the space shuttle bears little resemblance to a thin film of fluid in a Texas laboratory. What matters though is not appearance but fundamental physics. Both the Tang droplet and the thin film are controlled by surface tension. If Swinney and Carey can understand how the liquid bridge ``boils'' they can use that knowledge to design everything from space fire-hoses to orbital micro-chip and miracle drug manufacturing processes. That is a connection worth exploring.
In space everything behaves differently. If we are to learn to live in this high frontier then we will have to think differently as well. We will have to be clever with the laws of physics in ways we have yet to imagine. It's a challenge that frontiers have always demanded.
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