Spiky surfaces steer different fluids in opposite directions

A new surface carved with neatly arranged rows of angled teeth steers different liquids in different directions without the need for pumps (Science 2021, DOI: 10.1126/science.abg7552). The 3D surface can propel liquids at high speed. This simple way to maneuver liquids without using energy could be important for microfluidic lab-on-a-chip systems and for wicking heat-transfer liquids inside the miniature pipes used to cool electronics. To transport fluids without pumping, researchers typically rely on surfaces with slowly changing curvature or varying abilities to repel liquids. But with such designs, all liquids move in the same direction, says Zuankai Wang, a mechanical engineer at the City University of Hong Kong. He and his colleagues wanted to make a system that directs liquids in different directions according to their intrinsic properties and not those of the surface. Their inspiration was the Araucaria plant, which sports periodically arranged pointed, curved leaves. Liquids with different surface tensions move in opposite directions along these leaves. The team used 3D printing to make rows of ratchet-like arrays on a polymer surface. Liquids with high surface tension, like water, moved against the tilting direction of the teeth, while low-surface-tension fluids, like ethanol, moved in the opposite direction. The researchers also demonstrated that this selective liquid steering can be used to separate water-oil mixtures. When they put a 1:1 water-oil mixture on the surface, the fluids moved in opposite directions, separating completely in seconds.