Over hundreds of millions of years of evolution, water-repellent skin has enabled tiny insects, called springtails, to breathe through their skin without suffocating in damp soil flooded by rainwater. More recently, such natural engineering has inspired a new approach to cooling new generations of miniaturized electronic devices.
The secret of the springtail’s skin is tiny surface compartments that contain sharp edges, a physical design that resists the advance of liquids and can help contain the flow of liquids. Researchers in the United States and South Korea adapted this idea in a “porous membrane” design that could someday keep electronic systems from overheating through evaporative cooling. The porous membrane consists of tiny liquid-filled pillars that rely upon the sharp-edged trick to keep the liquid contained, even as the open ends of the pillars allow for liquid evaporation to get rid of excess heat.
“This technology can significantly improve the cooling efficiency in a wide variety of applications such as data centers,” says Damena Agonafer, an assistant professor in mechanical engineering and materials science at Washington University, in St. Louis. “In addition to robust cooling, it will also decrease the electronic footprint as compared to traditional cooling technology.”
Many layers of such porous membranes could eventually be integrated within microelectronic stacks to provide cooling within laptops, Internet of Things devices, and data centers. Agonafer and his colleagues published their work in the 15 March 2018 online issue of the Journal of Colloid and Interface Science.
Better cooling is necessary for packing more electronics into a smaller amount of physical space. Such efforts lie at the heart of modern technological advancement with ordinary people expecting smaller and yet more powerful computing devices. Companies expect the same boost in computing power provided by warehouse-size data centers in order to harness a growing number of online applications and services.
One popular solution to meet the demand for more computing power has come from deploying 3D stacked chips in the high-end servers used by major data centers operated by Silicon Valley companies such as Apple, Facebook, and Google. But stacked chips struggle to rid themselves of all the waste heat being generated.
Mixing electronics and water can be a recipe for disaster. So instead of using water-based cooling, researchers turned to dielectric liquids that act as electrical insulators. Their work was backed by the U.S. Defense Advanced Research Projects Agency (DARPA) and the National Science Foundation.
“Such a method is particularly promising in interlayer two-phase cooling for heat removal in 3D stacked chips, where dielectric liquid is required to avoid risks associated with water in electronic components,” Agonafer says.
Previous attempts to use dielectric liquids in microelectronics cooling have only been able to handle heat flux—the amount of heat energy transfer per area—in cases below 1 kilowatt per square centimeter. The porous membrane solution engineered by Agonafer and his colleagues could help remove heat flux in extreme heat-flux cases beyond that point.
The sharp-edged design inspired by the springtail insects proved crucial in overcoming a key challenge: Dielectric liquids tend to easily wet any solid surface. That means the liquids could usually leak through the membrane and flood over the entire cooling package, resulting in a failure of the cooling system, Agonafer explains.
By showing how the sharp-edged micropillars could successfully contain the dielectric liquids under certain test conditions, the researchers found a possible way forward in realizing this evaporative cooling design. “We were the first to demonstrate retention of extreme low surface tension liquids behind a porous membrane based on a unique geometric feature with sharp solid edges,” Agonafer says.
The initial testing showed how the porous membrane could handle fairly tame liquid behavior involving a “very slow flow rate,” Agonafer says. But to become a truly practical cooling solution for electronics, the membrane will also need to be able to contain liquid under more challenging conditions such as those involving a “high mass flow rate.”
Another challenge: dealing with how the waste heat from hard-working electronics causes uneven liquid evaporation. Some pores may dry out quickly, while others remain overloaded with liquid. Solving that issue may require a system that ensures a consistent liquid level over the entire porous membrane.
Still, there is reason for optimism if the membrane proves successful under a wider variety of evaporative cooling conditions. Major semiconductor fab facilities should be able to produce the tiny geometric structures of the porous membrane on silicon wafers without raising the overall price tag for microelectronics much. “The proposed approach can definitely be scaled up for real-world production at reasonable cost,” Agonafer says.
Agonafer estimates that the design will require about three or four more years of work before it shows up in commercial technologies. He and his colleagues recently received a grant from Cisco to develop a microheat exchanger for 3D integrated circuits and similar stacked structures of microelectronic structures.
“Ultimately, we will develop a fully functional evaporative cooling prototype containing the porous silicon membrane and demonstrate its role in electronic cooling applications,” Agonafer says.
Editor’s note: The original version of this story quoted Agonafer as saying the new cooling technology would increase the electronic footprint. He has clarified that it would decrease the electronic footprint.