Bubble-Recoil Facilitates Cooling of Microchips Even in Space

The bubbles forming on a heated surface tend to create a tiny recoil when they leave it, similar to the kick from a gun shooting blanks.

Recently, a research team from the University of Illinois at Chicago has demonstrated how this tiny force can be harnessed to mix liquid coolant around high-power microelectronics - on Earth or in space. Their research was funded by NASA.

The vapor-recoil force “is not well-studied, and has never been applied, to my knowledge,” says Alexander Yarin, UIC Distinguished Professor of Mechanical Engineering and the study’s senior author. Details of the research have been published in the Nature Microgravity journal.

In flights to Mars or the moon, equipment like computers generate a lot of heat.

Alexander Yarin, Professor, University of Illinois

As chips and computers become smaller and are packed tighter, the generation of heat becomes a limitation on computing power.

Researchers have looked to “pool-boiling,” which can be defined as liquid-cooling at a temperature close to the boiling point of the fluid. During boiling, the heat is absorbed in converting the liquid to vapor, without any additional increase in temperature until the phase change is complete.

However, there is a unique issue in performing pool-boiling in space as there is no gravity in space, which means the bubbles do not have buoyancy.

On Earth, the bubbles rise, and cold coolant comes in. But in space, the bubbles don’t rise. They stay on the submerged surface, and can merge together to form an insulating vapor layer, and the heat-removal process is interrupted. You can try mechanical mixing, but a motor also creates heat. You can try a strong electric field, but that also produces heat and creates other problems.

Alexander Yarin, Professor, University of Illinois

Both techniques occupy space and need power.

Two heat-generating circuit chips were sandwiched back-to-back by Yarin and his coworkers. They alternated the voltage to the two chips, which caused the apparatus to swing back and forth through the coolant at approximately 1 cm/second.

When one chip operates, it produces bubbles and a recoil force. Then the other, and it pushes back — enough to swing the chips in the cooling fluid and shed the bubbles. It works with or without gravity – in space, exactly as on Earth.

Alexander Yarin, Professor, University of Illinois

The researchers also demonstrated that the force is greater when the bubbles are smaller and in a bigger quantity, resulting in a swing of higher velocity and arc. Nanofibers created using polymer were supersonically blown onto the chips, forming a nanotexture for enhanced bubble nucleation.

Each single bubble works like jet propulsion. When a bubble leaves a submerged surface, it pushes the surface back. You don’t see it, because the bubbles are tiny and the surface is big. But we organized the bubbles to get the chip swinging.

Sumit Sinha-Ray, Doctoral Student, University of Illinois

Other authors on the research, all current or former students in Yarin’s UIC laboratory, are Suman Sinha-Ray, adjunct professor of mechanical and industrial engineering; Wenshuo Zhang; Rakesh Sahu; and undergraduate student Barak Stoltz, who is presently spending a year at SpaceX on an internship.