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Researchers Develop Virtual Walls for Oil

A created polymer surface with millions of micropores each less than 20 microns in diameter
Researchers at the University of Missouri have created a polymer surface with millions of micropores each less than 20 microns in diameter that stops oil from spreading laterally on surfaces. Courtesy of Jae W. Kwon

Seeking to control the flow of oil in inkjet printer heads, researchers developed a micropore surface that has potential applications in a variety of settings.

January 7, 2014—Researchers at the University of Missouri have developed a microstructured surface that can be deployed to control the lateral spreading of oil. Although still in the early stages of development, this surface has the potential to control the flow of many different oils on a variety of surfaces.

The research was led by Jae W. Kwon, Ph.D., an associate professor in the Department of Electrical and Computer Engineering. Kwon’s team was working on a project funded by the National Science Foundation to develop a solution for clogging nozzles on inkjet printer heads. The ink dries quickly when exposed to air, forming particles and crystals that clog the nozzles.

“The way we dealt with the problem was to utilize a thin layer of oil for covering the nozzle,” Kwon says. The oil “blocks the direct air exposure of the ink so that it can avoid the immediate evaporation problem. But that requires precision control of the movement of the oil layer in a tiny area.”

To accomplish this control, Kwon reviewed research into “virtual walls” that have been created to control the movement of water-based solutions. These virtual walls are created by deploying hydrophobic and hydrophilic areas that dictate where the water-based solutions will flow.

“We searched for a water-based solution approach and then we tried to adapt the idea for oil control, but all failed,” Kwon recalls. “We initially made oleophobic surfaces, which work just fine for vertically approaching oil drops. But all of the oleophobic surfaces we tried failed for confining laterally spreading oils.

“We learned that oil behaves really strangely and it’s really hard to control oil’s speading very precisely,” Kwon says. “Oil sticks everywhere and is very difficult to maneuver as we wish. We had to work on this to solve the controllability of this oil confinement to move our research project forward.”

The solution the team devised was a polymer surface with millions of micropores each less than 20 microns in diameter. Each micropore holds a small air pocket. As oil approaches these micropores, the oil cannot displace the air pockets and the flow is blocked.

“The oil tries to wet the surface, but cannot move farther because of high fluidic resistance generated by the air pockets at the boundary,” Kwon says. “The oil is locked in there. The oil cannot proceed.”

Although the researchers used a polymer surface, Kwon says the technology would work on any surface that can accommodate a coating with the micropores. “The key here is we need to have tiny holes that produce the reentrance structures.”

The team recently announced their findings in a letter titled “Virtual Walls Based on Oil-Repellent Surfaces for Low-Surface-Tension Liquids” published by the journal Langmuir. The letter is coauthored by Riberet Almeida, a research assistant at the Micro-nano Devices and Systems (MIDAS) Lab at the University.

Kwon explains that more testing is required but he believes the solution will prove to be effective in controlling the movement of oil in a variety of situations.

“We tested this on small devices in lab conditions only,” Kwon says. “We haven’t tested this in harsh conditions yet. There must be some issues there, probably. But as far as I believe, any surface that you don’t want to hold oil, this will do a good job.”

Although the team tested a variety of oils with a wide range of viscosities and surface tension, all were clean oils, free from particle contamination. Kwon says further testing needs to be done to determine the reliability of the system with contaminants.

The technology will likely find applications in areas where oil spills can be expected and must be controlled. Kwon gives the examples of heavy industry and commercial automotive garages.

“Petroleum companies ... are dealing with a lot of oil every day. They probably want to keep oils within certain areas only. They may have some spillage problems here and there, especially in areas they don’t want to clean every day,” Kwon says. “If there is a chance of oil spillage, we can define certain paths for the oil spillage to travel, so that it doesn’t go all over the surface. It will only travel along the predefined paths and you can simply collect it.

“That might be solving some problems in the field,” Kwon says.



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