Setting droplets on a one-way street has huge implications
By creating specific kinds of tiny structures on a material’s surface researchers can make a liquid spread only in a single direction. While this may not appear to be a momentous breakthrough it has important implications for a wide variety of technologies, including microarrays for medical research, inkjet printers and digital lab-on-a-chip systems. Up until now the designers of such devices could only control how much the liquid would spread out over a surface, not which way it would go. This new system changes that.
The new system developed by a team at MIT is completely passive, based on producing a textured surface with tiny pillars shaped in specific ways to propel liquid in one direction and restrict its movement in others. Once the surface is prepared, no mechanical or electrical controls are needed to propel the liquid in the desired direction, and a droplet placed at any point on the surface will always spread the same way.
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To test the system the researchers etched the surface of a silicon wafer to produce a grid of tiny pillars, which were then selectively coated with gold on one side to make the pillars bend in one direction. To prove that the effect was caused just by the bent shapes rather than some chemical process involving the silicon and gold, the researchers then coated the surface with a thin layer of a polymer so that the water would only come in contact with a single type of material. The result was all the pillars curving in one direction, which caused the liquid to move in that direction.
The researchers say that in principle such systems could provide new ways to manipulate biological molecules on the surface of a chip with minimal energy, for various testing and measurement systems. It might be used in desalination systems to help direct water that condenses on a surface toward a collection system. Or it might allow more precise control of cooling liquids on a microchip, directing the coolant toward specific hotspots rather than letting them spread out over the whole surface.
“It’s a big deal to be able to cool local hotspots on a chip,” says MIT Assistant Professor of Mechanical Engineering Evelyn N. Wang, especially as the components on a chip continue to get smaller and thermal management becomes ever more critical.
The MIT team’s research appears in a report published in the journal Nature Materials.