How do you make a bubble buoyant in space? For an astronaut, the answer could be a matter of life or death
“NOBODY really understands how bubbles behave,” says Eugene Trinh, an astronaut and applied physicist based at the Jet Propulsion Laboratory in Pasadena, California. But there are good reasons for finding out. In space, gas bubbles affect everything from rocket fuel and life-support systems to human digestion and excretion.
Here on Earth, one of gravity’s more curious effects is to make gas bubbles rise to the surface of a liquid. But in space, bubbles do not rise and so can block tubes that carry liquid. They can’t be bled away, like bubbles in a central-heating system, because there is not enough gravity to separate gas from liquid. When NASA astronauts on the shuttle recently asked for the amount of gas in fizzy drinks to be reduced, the problem was merely discomfort: they had trouble burping. But if bubbles were to block pipes carrying liquid in a life-support system, the problem could be disastrous.
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The Russians, for example, are designing a gas turbine power system that will keep equipment working and astronauts alive on the International Space Station. Solar rays will boil a liquid and the vapour used to power a turbine to generate electricity. The system will be far more energy-efficient than photovoltaic cells, but its success, and ultimately the astronauts’ lives, will depend on understanding bubble behaviour.
So the race is on to work out how to control bubbles in space. The work focuses on replacing gravity and the buoyancy it creates with another force. The present candidates are acoustic pressure waves and electric fields, both of which can push bubbles around. The difficulty is in working out exactly how these influences affect bubbles.
Such knowledge could net another major prize: the ability to use bubbles to move heat efficiently from one place to another in the absence of gravity. “Today, heat generation is a major limiting factor in the design of space systems,” says Andrea Prosperetti, a physicist at Johns Hopkins University in Baltimore and one of the world’s leading theorists on bubble behaviour. All spacecraft have to be carefully designed so that heat generated by onboard equipment does not exceed the amount that can be transported away and dissipated into space, otherwise computers and other instruments would rapidly overheat and break down.
On Earth, convection is a particularly efficient way of transferring heat. Warm fluid rises, carrying heat away from the source and allowing cooler fluid to replace it. This process becomes even more efficient when heat changes the coolant from a liquid to a vapour, since bubbles rise even more quickly than warm liquids. “Boiling is the most efficient form of heat transport that we know,” says Prosperetti.
In space, boiling a liquid is one thing. But with no gravity, getting the bubbles – and hence the heat – away from a heat source is another. On Earth, bubbles generally grow to the size of marbles before their buoyancy detaches them from a heated surface. But in space, without this buoyancy, bubbles just get bigger and bigger. “They can reach the size of footballs,” says Prosperetti.
To dislodge these growing spheres, Prosperetti plans to use the forces experienced by bubbles in a “sound field of a few kilohertz”. His idea is to set up a standing sound wave in a liquid near a hot surface and use it to pull the bubbles away to where a slow flow of fluid will pick them up. Behind the technique is the fact that bubbles tend to move towards areas of low pressure. And the bigger the bubbles, the bigger the force they experience.
On Earth, bubbles rise because the pressure in a liquid is lowest at the surface. In a standing sound wave, which is stationary in space, the pressure at certain points – called nodes – does not vary. But in between, the pressure alternates between compression and expansion, with the greatest swing taking place at what are called the antinodes. “You might expect that this effect averages out and the bubbles move nowhere,” says Prosperetti. “But you’d be wrong.”
The reason is that the bubbles get larger when the pressure falls, so the force of attraction is greater than the force of repulsion during compression, when the bubbles are smaller. The net effect is that bubbles are pulled towards the points of lowest pressure – the antinodes. By positioning the antinodes away from a heated surface, the bubbles should be pulled free.
Prosperetti has shown that his idea works on Earth with bubbles floating in a liquid, but he cannot be sure how bubbles attached to a surface will behave in space. “The high risk is that acoustic waves may not be able to dislodge them,” he says. Another problem is that bubbles in a sound field tend to repel one another because they pulsate and set up their own pressure fields which push other bubbles away. Nobody is quite sure how this will affect the process. With a grant from NASA, Prosperetti has teamed up with Trinh to test the idea in microgravity.
Murky border
Proving his technique will mean finding room on a space shuttle or on aircraft flights that create microgravity conditions for short periods. Another option is to design computer simulations, but this presents its own problems. Describing the behaviour of bubbles mathematically is a tough task. It involves interactions between solids (the heating element), liquids and gases, and is highly complex. Buoyancy, heat flow, fluid flow and surface tension are only a few of the factors that play a part. “When a bubble moves, we simply do not know how the fluid in the vicinity behaves,” says Trinh, who flew on the shuttle in 1992 to study the behaviour of fluids in space.
But the difficulties are not confined to the liquid. The vapour also presents problems. For starters, even its temperature near the liquid surface is a matter of debate. At the tiny scale over which the temperature is calculated, the macroscopic laws of physics break down and the system is too complex to use quantum mechanics. This region straddles the murky border between classical and quantum physics. “The kinetic theory of matter is not sufficiently good to explain it,” says Prosperetti.
Despite these problems, Prosperetti has had some success using supercomputers to simulate bubbles. By calculating what is happening at a large number of points in a fluid, he can simulate an isolated bubble in a sound field and even model how it behaves near a flat surface. “But several bubbles and complicated geometries are too complex, even for the most powerful computers,” he says.
The problem here is that a computer can simulate what is happening only at a limited number of points. To simulate a bubble, these points must be distributed around the bubble surface and in the surrounding liquid. The resolution of these simulations is still woefully inadequate. Worse still, computer approximations introduce errors so that tiny capillary waves that ripple across the bubble’s surface rapidly turn into raging tsunamis that ruin the simulation. Prosperetti has avoided this problem by developing artificial methods for damping down unwanted capillary waves. In future, he hopes to fine-tune the simulations by comparing them with the results of real experiments.
One of these experiments is housed in the basement beneath Prosperetti’s office. Here, a small team of researchers is setting up equipment that can measure the shape and size of bubbles and the temperature of the fluid around them. With the lights down low, Cila Herman, who leads the group, talks enthusiastically about her experiment. She throws a switch and a laser flickers into action, sending a green beam bouncing around a forest of mirrors and lenses arranged on an optical bench the size of a large office desk. “Don’t touch it,” she warns. “It’s very sensitive to vibration.” Herman’s instrument is a holographic interferometer and it produces moving images of the temperature changes in a fluid as it flows over a heated surface. “Look,” she holds a white card in front of the beam. “This is where the image forms.”
The image consists of the shadow of the heated surface and a series of fringes where the laser light that has passed through the sample interferes with a reference beam. These fringes represent isotherms or lines of constant temperature. With a high-speed camera, Herman can film the isotherms’ movement, which represents the way the liquid is carrying heat away from the surface. This is already a hugely intricate experiment, but Herman has an even more ambitious goal: to turn up the heat until the liquid starts to boil and film the temperature changes around the bubbles as they form and break loose.
The results should help to improve computer simulations of bubbles in a sound field. But Herman also wants to test another way of controlling bubbles – with an electric field. Researchers have studied this idea on Earth since the early 1960s but, once again, not in microgravity. High-speed films from experiments show how bubbles in an electric field sweep violently around a heated surface and split up to create more bubbles in the process.
Electric fields act on bubbles in a number of ways. The major effect is similar to that responsible for the movement of a piece of paper when it is placed in an electric field created by a pen which has been rubbed. The pen becomes charged and induces an opposite charge on the closest part of the paper. The paper is then attracted to the pen. In a similar way, the charges on fluid molecules can become polarised and are pulled to where the electric field is strongest. This creates buoyancy in the same way as a gravitational field: with the liquid pulled in one direction, the less dense bubbles are pushed in the other.
Cold chips
The force created in this way has two main components. The first presses the bubble against the heated surface, which traps a thin film of liquid between the bubble and the element. Since heat transfer is far more efficient across a thin film, this hugely increases the rate of heat transfer from the surface.
The second component of the force acts parallel to the surface, sending the bubble whizzing around on a seemingly random path. In space, it is this motion that would replace convection, by stirring the coolant and bringing a constant supply of cool liquid to the heated surface. Once it has left the surface, the bubble would be carried away by a gentle flow of liquid. Herman plans to use her interferometer in microgravity to study the impact of the electric field on bubbles. Last year she received a grant from NASA to develop her ideas.
One spin-off of an improved understanding of boiling could be a new generation of cooling devices for microchips. Today, chips are cooled by blowing air over them, but faster chips generate more heat and it will take more than a breeze to keep the chips of the future cool. “But chip manufacturers are afraid of boiling,” says Herman “because it is difficult to predict the temperature of a surface cooled this way.” A temperature overshoot of just 10 °C can ruin a chip.
The problem is that bubbles can only cool the areas where they form. To cool the entire surface area of a chip, Herman has suggested stimulating bubble formation by injecting microbubbles. She envisages a future in which microchips are housed in sealed baths of a coolant such as a liquid metal or an organic coolant. The liquid would be pumped around the chip to carry away heat in the form of vapour bubbles. These would later condense and the fluid would be recycled. With the addition of an electric or sound field to the fluid, these powerful chips could also be used in space.
Trinh agrees that improved heat transfer would be one of the most important gains to emerge from a better understanding of bubbles. Of course, that is not to belittle the importance of the other potential benefits. With improved knowledge, perhaps even burping in space could become more comfortable.
