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Tiny bubbles…

Photo: C.L. Chen, Simon Chen and Sheng Wang

C.L. Chen, a professor in the Mechanical and Aerospace Engineering Department, Assistant Research Professor Simon Chen and postdoc Sheng Wang have worked together to demonstrate both numerically and experimentally that bubble departure can be potentially promoted with electrowetting. Photo by Hannah Sturtecky

The increased integration density of electronic components and subsystems, including the nascent commercialization of 3-D chip stack technology, has intensified the thermal management challenges facing electronic system developers. And current cooling technologies have not kept pace with the emerging technologies.

C.L. Chen has been at the forefront of thermal management and microfluidic technology for “hot” electronic devices for years. It was part of his research during his time as department manager and founder of Applied Computational Physics and Thermal and Flow Physics at Teledyne Scientific and Imaging. And it has remained at the core of his work since he joined the MU Mechanical and Aerospace Engineering Department faculty as a professor and director of the Aero, Thermal, Fluid and Energy Laboratory in 2011.

Graphic: Illustration of water vapor film.

This graphic illustrates the forming of a layer of film caused by water vapor when water is boiled at extreme temperatures, as well as how it keeps bubbles from escaping.

“I’m doing all this cooling for high-powered-density electronics, and that’s one of my core research interests — trying to cool something very hot in a very small spot,” Chen said.

The electronics that are Chen’s focus aren’t limited to in-home or handheld items, though his work does have applications in those devices. Rather, think military and space-grade devices. His thermal management and microfluidics research has been funded by Defense Advanced Research Projects Agency (DARPA), NASA, the Advanced Research Projects Agency-Energy (ARPA-E), the Office of Naval Research and others.

It makes sense. The military and other science-based governmental agencies use of high-powered devices with dense electronics devices on such things as fighter jets, spacecraft, aircraft carriers, tanks and many others. These agencies can greatly benefit from advanced cooling technologies to keep up with advances in electronic technology in cases where overheating could be catastrophic.

Photos and graphics: Diagrams of how electrowetting causes bubbles to detach

The top set of photos illustrates how electrowetting can cause bubbles to detach from a body of water. The bottom set shows the same event graphically.

One of the ways Chen combats high temperatures in tight quarters is to control a process called electrowetting, which is the modification of surface wetting properties with an applied electric field. Electrowetting allows large numbers of droplets to be independently manipulated under direct electrical control without the use of external pumps, valves or even fixed channels.

MU Assistant Research Professor Simon Chen, who works closely with C.L. Chen, said creating a pattern of hydrophobic and hydrophilic surfaces allows for the potential to move water droplets to the heated areas in electronics.

“Have you ever seen liquid droplets climbing uphill?” he said. “With microscopic manipulations on the surface, you can actually make that happen. With a specific pattern, you can actually see the water start climbing. That’s part of the reason people are so interested in surface modification.

Photo: Close up of microscope

C.L. Chen and his research team use a
high-powered video microscope to get an
up-close look at the movement of bubbles using
electrowetting — the modification of surface
wetting properties with an applied electric field. Photo by Hannah Sturtecky

Recently, Sheng Wang and Sean Shi in Chen’s research group have demonstrated experimentally and numerically that bubble departure can be potentially promoted with electrowetting.

“When you boil a pot of water, you’ll see bubbles form on the bottom and detach. The new water has to fill the void in order to continuously generate the bubbles. In order to have the water fill the void, you have to have some kind of hydrophilic surface,” Simon said. “But if you have too much of a hydrophilic surface, then your bubble will not form easily because water vapor doesn’t like the surface.”

The manipulation of hydrophobic and hydrophilic surfaces can be set up for the removal of warmer liquids and bubbles. At extreme temperatures, the water used to regulate the electronics’ temperature can boil to the degree that the bubbles begin to cause a film, which holds in the warm vapor while preventing cooler liquid to penetrate. Electrowetting potentially can speed the removal of the bubbles to avoid such a film, allowing the cool water to penetrate and regulate temperature.

“If you can facilitate the quick departure, you may keep the film from creating,” C.L. said. “Hopefully we can accelerate the departure speed to keep the cool liquid contacting the surface.”

Cooling technologies must evolve as electronics continue to simultaneously grow in power and decrease in size. And as those technologies continue to develop, expect to see C.L. Chen hard at work to make it possible.

“We’re looking at all surfaces in micro- and nano-scale structures to see how to design something at a micro-level scale as well as to make an impact on the chip temperature and to improve [existing technology],” he said.