When microelectronic components heat up, cycling fluid around them is a great way to cool them off (see Micro-cool in the Micro/nanoscale Science and Engineering area). To provide even more cooling, engineers exploit the large amounts of energy consumed in boiling. So much energy is required to break the bonds and rearrange the atoms from the liquid to the gaseous phase that even when energy continues to be added to the system, the temperature of the liquid stays constant. And because gas is less dense than liquid, the bubbles move away from the hot surface, cooling it.

Engineers routinely use boiling to cool equipment. But boiling only cools things when the buoyant bubbles leave the hot surface. As any hurried cook knows, putting an empty pot to boil ruins the pot. In a micro-gravity environment, bubbles don’t rise—the gas created by boiling remains trapped next to the hot surface, which often does not get cooled well. In order to use boiling in space, engineers must find another way to move the bubbles. Professor Cila Herman is pioneering one interesting possibility—using electric fields. She and post-doctoral researcher Estelle Iacona and visiting scholar István Földes modeled the fundamentals of this bubble formation problem and have begun testing their theory experimentally. As a first step, they are working with electric fields in the laboratory, looking at the behavior of single isothermal bubbles.

The next step is to see how electric fields affect bubble behavior in micro-gravity conditions. With funding from NASA, Prof. Herman conducted her experiments on a plane officially nicknamed the “Weightless Wonder” and fondly known by researchers as the “Vomit Comet.”

Early in the morning of one of these parabolic flights, a number of scientists (there are several experiments on each flight), a doctor and the NASA support crew climb aboard the KC135 military aircraft—the same one that was used to film “Apollo 13.” They are strongly encouraged to refrain from eating breakfast. The airborne plane goes into a series of steep rises and falls—25 seconds of microgravity while it heads up, and then—“feet down, coming around” —25 seconds of 2G while it plunges back down the parabola. Over the course of the 2 hour flight, this is repeated 40 to 60 times. During those 25 seconds of microgravity, the scientists have to concentrate on getting good data. During the transition, it’s important to be upright and away from the equipment, since falling on one’s head or getting bonked by something at 2G can be very painful. During the 25 seconds of 2G, researchers have to hustle their suddenly extremely cumbersome arms, legs, and hands to get the setup ready for the next “take.” Just reaching over and operating a valve is a huge effort under these conditions. To make things worse, almost everyone gets nauseous. It’s a shame to waste valuable research time throwing up (special bags for that purpose are stacked into pockets of the flight suit at the beginning of the flight and regularly collected by the NASA crew), but sometimes it can’t be helped. Prof. Herman was proud to report that she never succumbed. She was also impressed by Math Sciences Prof. and Chair Ed Scheinerman who managed to eat breakfast before a flight with no ill effects.

In 1999–2000, Prof. Herman went on three parabolic flights, accompanied at various times by graduate student Gorkem Suner, once by Prof. Scheinerman, and once by graduate student Steven Marra. Data collected from these experiments in micro-gravity indicate that an increased electric field causes the bubble to elongate and move away from the surface, bearing out their predictions. The remaining task is to digitally analyze the data to calculate the force components involved. At the moment, they are running more experiments back on the ground, and tentatively scheduled to fly an experiment in conjunction with a team from NASA on the space station in 2004.

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