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John O | March 2017

Researchers show bubble-recoil could cool microchips in space


Scientists at the University of Illinois at Chicago, funded by NASA, have demonstrated how to utilize the minuscule force that is created by forming bubbles on a heated surface to mix liquid coolant around high-powered electronics, even in space, for more efficient thermal management.

 

bubble

UIC researchers showed that the tiny force from the creation of bubbles could circulate liquid
and cause bubbles to leave the surface of a chip. (UIC)

 

According to a report from the university, scientists explored pool-boiling as a solution for the thermal needs of high-powered electronics, a need that has increased in recent years due to the miniaturization of the components and higher component-density within systems.

 

The problem with this technique in space applications is that the bubbles do not rise in space. Instead, the bubbles remain on the surface of the component and form an insulating vapor layer that interrupts the dissipation of heat.

 

To fix this problem and continue the circulation of liquid and vapor, motors and electrical impulses were tried but only added to the power needs of the system and created their own heat as well.

 

The article explained, UIC researchers “sandwiched two heat-generating circuit chips back-to-back. By alternating the voltage to the two chips, they were able to cause the apparatus to swing back and forth through the coolant at about 1 centimeter per second.”

 

When one chip was operating, it produced bubbles and the bubble-recoil force. This pushed back on the other chip enough to cause the bubbles to leave the chip surface.

 

“The researchers also showed that the force is greater when the bubbles are smaller and more numerous, resulting in a swing of greater arc and velocity,” the article continued. “Nanofibers made of polymer were supersonically blown onto the chips, creating a nanotexture for increased bubble nucleation.”

 

The work was recently published in Nature Microgravity. The abstract from the report read:

 

“Here, we demonstrate that heat removed in pool boiling from a heater mimicking high-power microelectronics could be used to facilitate a swing-like motion of the heater before being finally dissipated. This swing-like motion could be beneficial for shedding a large vapor bubble that encapsulates high-power heaters in microgravity where buoyancy force is unavailable for vapor bubble removal.

 

“The swing-like motion is propelled by vapor bubble recoil, the force which exists irrespective of gravity and buoyancy. We also demonstrate that this force could be significantly enhanced by depositing on the heater surface supersonically blown polymer nanofibers with cross-sectional diameters below 100 nm.

 

“These nanofibers provide additional nucleation sites, resulting in much more frequent bubble nucleation and departure, and thus a higher overall vapor recoil force propelling the heater motion. Such nanofibers strongly adhere to the heater surface and withstand prolonged harsh pool boiling. The measured velocity of the model swing-like heater in Novec 7300 fluid is about 1 cm/s.”

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