New research from mechanical engineers at MIT has revealed a new approach that, by creating specific kinds of tiny structures on a material's surface, can make a droplet spread only in a single direction [Chu et al., nature mat. (2010), doi:10.1038/nmat2726].

The system 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.

The chips used for testing were made by etching a silicon wafer surface to produce a grid of tiny pillars, which then were 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 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 pillars are all curved in one direction, and cause the liquid to move in that direction.

The researchers comment, “while this work is still early-stage basic research, in principle such systems could be used for a wide variety of applications. For example, it could provide new ways to manipulate biological molecules on the surface of a chip, 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”.